Biasing Double-Ended Amplifier Output Tubes, Part 2: Demonstration and Discussion

[@catfish573]

Thank you for this series of lessons. I learned a lot and enjoyed every minute of your teaching. I went to school in Air conditioning and refrigeration, where by you learn a lot about electricity. Even at 63, I am trying to expand my knowledge of electric and teaching myself how to play the guitar. Never to old to learn. You make it a lot easier to learn.👍👍👍✌️✌️✌️

[@UncleDoug]

You're welcome, CF. I agree completely re the "never too old to learn" philosophy. I personally got started in tube electronic about 10 or 12 years ago, completely self-taught from internet sources.

[@catfish573]

@@UncleDoug awesome!!! You do an awesome job at not just fixing electronics but teaching it while doing the same. Not everyone can teach, but you sure have a nack for it. I appreciate the time you take to do it also. Merry Christmas and Happy New Year. Let me know if you ever cross over and start fixing Ham Radio Transceivers.

[@UncleDoug]

Thanks, CF. We appreciate the kind words :) Merry Christmas to you all :)@@catfish573

[@johngeddes7894]

Uncle Doug- Whenever I might think of adding some substitutions in any song I might be playing, I think of all the lessons Rusty worked so hard to prepare as far as cicuitry and whatnot, it’s as if it made no difference to Rusty. Electrical theory was as natural to him as his level of jazz theory and his being brilliantly able to play on any guitars, although I think he preferred his electric instruments eventually. I don’t compare Jack to Rusty, but they both came prepared to give us videos that would last a lifetime. For this we can thank them both and acknowledge you too, as you were under their auspices from an early start. Great QC managers both in their individual styles!

[@UncleDoug]

Thanks so much for your lucid analysis, John. Like all great artists, they each have their individual styles.....but what really matters is their ability to resonate with the audience.

[@johngeddes7894]

I dont have pets of my own, but that doesnt mean I dont get a big kick out of seeing others’ pets and their personalities. Very humerous at times. There are deer where I live that freely come and go thru my yard, and not many, but a few I got to be friends with; including a couple of bucks that pal around together. One neighbor even cuts up apples and puts them in the deer bowl for their enjoyment. They have become neighborhood treasures.

[@johngeddes7894]

I dont have pets of my own, but that doesnt mean I dont get a big kick out of seeing others’ pets and their personalities. Very humerous at times. There are deer where I live that freely come and go thru my yard, and not many, but a few I got to be friends with; including a couple of bucks that pal around together. One neighbor even cuts up apples and puts them in the deer bowl for their enjoyment. They have become neighborhood treasures.

[@johngeddes7894]

I dont have pets of my own, but that doesnt mean I dont get a big kick out of seeing others’ pets and their personalities. Very humerous at times. There are deer where I live that freely come and go thru my yard, and not many, but a few I got to be friends with; including a couple of bucks that pal around together. One neighbor even cuts up apples and puts them in the deer bowl for their enjoyment. They have become neighborhood treasures.

[@UncleDoug]

Let's hope that no "sportsmen" see them. I'm very nervous about getting attached to any "huntable" animals.......their loss would be really hard to accept. Hopefully, these guys will live to be old timers....... like me :)

[@WvHomestead]

Uncle Doug, I know quite a bit about tube amps but you are excellent at putting the info into perspective for the common man. I commend you sir. You are doing a great service to the younger generation which this technology is becoming increasingly scarce. Thank you.

[@UncleDoug]

+wvhomestead You're welcome, WV, and I sincerely appreciate your kind appraisal. Such nice comments make it all worthwhile :)

[@patprop74]

Doug your videos are a gift of knowledge that keeps on giving! I hate to sound like a broken Parrot, like so many, I have to thank you yet again!

[@UncleDoug]

You're quite welcome, Pat. Thanks :)

[@michaelbranch1010]

I just finished putting new tubes in my 66 Fender Super Reverb, and used your plate dissipation method exactly as shown here in your video. I have to admit, it took some time for it all to sink in, but once I started making adjustments and recalculating plate dissipation, it became obvious how useful this method is. My amp was running very cold and no amount of bias adjust could get the plate dissipation up beyond 13-14 watts per tube. It should be 20-21 watts each. I realized that we are operating with higher AC voltages these days compared to 1966, so experimented with changing the bias bleed off resistor to a lower value, from 27K to 22K. this did the trick! I was able to make the bias voltage less negative (-40 to -48 VDC), and this allowed adjustment of the pot that I needed. After several swings through adjustment and calculation, it really started to make sense how the bias voltage affects plate current, plate voltage, transformer voltage drop and plate watts each time you change it. I started to get a good feel for where it needed to be, and in the final adjustment, the output was the specified 20 watts per tube, for 40 watts total. The negative bias level to achieve this result turned out to be -48 VDC, and that's fine. It is definitely worth the extra time and trouble to go through this exercise, and I think I learned more about how power tubes work than I have in my entire career! It's one thing to know the theory and try making small adjustments, but when you have to change the circuit to get the right result, that's pretty exciting! I'm very happy with the sound now, and wanted to thank you for putting this great information out there! Keep 'em coming Uncle Doug! Mike in Texas.

[@UncleDoug]

What great news, Mike. I'm really glad to hear that the video was helpful. As you discovered, the absolute best way to learn is to use your basic knowledge in a practical way, gaining experience and increased knowledge along the way. I have only one observation. In most circuits, by lowering the bleed-off resistance, more of your -DC bias voltage went to ground, so less was available for the tube grids. This would have made the grids less negative, as you said, allowing for higher plate current......but -48VDC is more negative than -40VDC and would result in lower PC. I would have thought that your grid bias would have gone in the -40VDC direction, not the other way.

[@royspeakman1157]

Such a wealth of knowledge ! You are an incredible Teacher Doug !

[@jameswatkins7263]

Thanks for all the info in your videos. Today I was called in by my favorite music center to bias a Marshall JVM210H. I felt comfortable enough to accept the job, only after watching most of your videos. You are the Man Uncle Doug... Thank You.

[@UncleDoug]

That's great news, James. I'm glad the videos have been helpful.

[@burjordastur9654]

Hello Uncle Doug, just finished your 3 videos on Biasing output tubes. Once again your videos very helpful in clarifying biasing output tubes. Biasing output tubes no longer seems like such a mystery. Thanks Again.

[@UncleDoug]

You're welcome, Burjor. I'm glad the videos were helpful :)

[@Soest7556]

Doug , I just remembered that this is probably the only "pay" you get for all your work ! Thanks again for a real thorough explanation , it really makes it so much easier to understand all this ! As the last time I wrote you , my yellow Lab Bello sends his regards to Rusty !

[@UncleDoug]

You're right, SS. Nice comments from viewers are the "pay" I receive for the videos, and I must say that the payment has been quite generous. Thanks so much for your kind words, and best (beast) regards from Rusty to Bello :)

[@cdnsatgate]

Wonderful lesson and demonstration. Your series of videos gave me the confidence to repair my 60's vintage Riviera 725 amplifier. After replacing one Filter Capacitor Can and all the tubes, the amp has never sounded better. With the new matched set of 6V6s, the Plate Dissipation were 13.2 and 13.3 watts :-) Thank you.

[@UncleDoug]

Thanks, Harmen. Is the circuit cathode biased? If so, then your PD values are a bit high. If it is fixed bias (grounded cathodes) the PD is way too high. Please double check to avoid harming your new tube set.

[@cdnsatgate]

@@UncleDoug Thanks for your reply. The circuit is cathode biased with a 400 ohm resistor. I assumed that maximium PD was 14 W on the 6V6 and thinking 94% was high but still acceptable since I seldom go past 3 on the volume. Should I be getting it closer to 70 % as you mentioned in another video?

[@UncleDoug]

@@cdnsatgate Cathode-biased tubes can be biased at 100% of max PD, Harmen. Fixed bias are restricted to 70% of max.

[@markbteeps]

I just biased my Fender Prosonic properly for the first time thanks to this video. Thank you Uncle Doug!

[@UncleDoug]

You're welcome, Mark. It's great to hear that the video was helpful and that your biasing job was successful. Thanks for the positive input :)

[@starlingave]

Hi Uncle Doug: Your biasing video is perfect. I built a Tweed Deluxe about a year ago, which came out great for a customer of mine. The issue with the tweed with the bias using the stock 250 ohm resistor caused the amp to breakup too fast. Using your described method allowed me to figure out that 330 ohms put the amp at 13.9 and 14.2. The amp for my customers taste and mine is just right now, thanks again, Bob

[@UncleDoug]

+starlingave You're welcome, Bob. It's great to hear that the video was helpful. Thanks for sharing this experience with us.

[@diegocastro5114]

This videos is a treasure of knowledge. Thanks Uncle Doug!

[@UncleDoug]

You're welcome, Diego :)

[@whatyoumakeofit6635]

I never noticed this about national amps until now. If you have a look at the grill cloth on the front of the national amp, the cloth has several different patterns on it. All the patterns have something in common. They all can be related to how we draw out sound on paper. Every pattern. Have a look at it again and I think you will see what I'm trying too say. Have a good one Uncle doug. And thanks for the great videos !

[@UncleDoug]

I'll have to check that out, Bryan. Thanks for the tip. Likewise. You're welcome for the videos :)

[@3MonkeysGarage]

Awesome. I have watched this one about 8 times now. Project #1 electronically complete and happy. Couldn't have done it without your videos. So happy I drank my coffee black.

[@UncleDoug]

Glad it was helpful, AZ :)

[@theguitarguy777]

I just recently found your videos and have to say " thank god you teach in a way i can understand. I have wanted to know this stuff forever and will be cramming until it all sinks in" Tonight i will attempt your OT bias method on my cathode biased valco supro 6400 circut. Thanks for sharing. Your a blessing.

[@UncleDoug]

Thanks for the very nice comments, Marc. Best of luck with your bias project......be very careful.

[@jessferrera3155]

Thank you Uncle Doug & Rusty for sharing these outstanding instructional videos

[@UncleDoug]

You're welcome, Jess.

[@cnsengineer]

Thank you for taking the time to produce this thorough and clear video series. The well drawn and explicit diagrams mixed with hands on examples help to reinforce the concepts.

[@UncleDoug]

Thanks so much, CN. I really appreciate your time, interest, and kind appraisal. I have several other related videos which may also be of interest. You are always welcome :)

[@AlanKinsella-nu4gm]

Excellent instruction Uncle Doug.

[@UncleDoug]

Thanks, Alan. Glad to hear it :)

[@DjVinceALot]

I just wanted to thank you for this video Uncle Doug. I was just able to bias my Fender Deluxe Reverb Reissue using this method and thank goodness I did. After putting the new Tung Sol 6v6 power tubes in and taking my measurements, I concluded that the tubes were running at approximately 11.5 watts of plate dissipation! .5 watts away from hitting the max power for 6v6 tubes! After a few trial and error episodes of turning the bias pot and taking the measurements again, I was able to get the tubes down to an average of 7.64 watts of plate dissipation which, obviously, is much more acceptable. What I was curious about was what exactly would change when adjusting the bias pot on a grid based bias circuit. It appears that the output transformer resistance remains constant. Turning the bias pot didn't change the resistance from the center tap to pin 3 of the power tubes. What did change was the voltage drop. Before any adjustments of the bias pot, I was getting approximately 3.4 volts of voltage drop which resulted in approximately 29ma of current. In the end, I was able to get the voltage drop down to 1.9 volts which made everything acceptable. Here is my final chart: i.imgur.com/jyK0bRn.jpg Anyway, thanks again for all of your help.

[@UncleDoug]

You're welcome, DJ. The maximum PD for grid-biased 6V6's is 70% of the 12 to 13W max for the tube, or around 8.4 to 9.0W, so your PD is now about right. When you adjust the bias pot, you are altering the negative bias voltage being sent to the tube grids. Increasing the resistance to ground will increase the negative bias voltage on the grids, reducing the plate voltage and plate current....and vice-versa.

[@sawmill21]

Great video by a great teacher!! Almost makes me want to go back to school!! Thanks

[@UncleDoug]

You're welcome, SM. No need to go back to school.....just grab a beer and watch our videos on YT :)

[@trevisanabc]

I have been working on a 'Plush" PRB1000S Amp, this is one of the closer Schematics that I have Found, Besides, 'Earth G2000', Fender AA864, and Fender Black Face, and You of course! I have been able to get the Bias Right, with 4 6l6GC tubes. Running Hot at 26.8 Plate Dissipation Wattage Each. To Verify for myself, I connected amp meter in line with the Plate (60 MA each)....Trying to test on Ohms Law...it was still correct! works out to 80% of Max on the 6l6 GC. Guess you really don't need to break the Pin #3 connection for PI reading, but I just had to! Using your Videos, I have been able to reverse engineer, utilize your knowledge, to Plate Bias the Plush that has no Known Schematic. The Measured Values are different, But Ohms Law Never changes. Thanks!

[@UncleDoug]

You're welcome, Trevis. I'm glad the videos are helpful :)

[@erikjohnson2976]

Fantastic overview of biasing. This will greatly help me as I delve into amp restoration! Thank you.

[@UncleDoug]

Glad it was helpful, Erik.

[@marktierney3938]

Thanks so much for these videos, they're really well explained and I managed to take bias readings for my 5e3 clone. I now have the confidence I can measure the bias accurately on my Bandmaster Reverb!

[@UncleDoug]

That's good news, Mark. Just be very careful.

[@marktierney3938]

@@UncleDoug will do!

[@MrBobwells48]

Doug, I'd just like to say, I've watched all your tube (valve UK) biasing video's and found them very useful. So thank you very much for the excellent video's and information. Bob

[@UncleDoug]

You're welcome, Bob. We're glad the videos were helpful.

[@marthesemercieca5197]

thanks uncle Doug for your reply I've learnt so much from your videos you have great talent and thanks Rusty

[@UncleDoug]

You're welcome, MM. We're glad the videos are helpful.

[@mchavez56]

these three videos were Sweet! thanks for these. I'm turning a Wurlitzer 700 Jukebox amp into a Guitar amp and now that it is up and running.....bias checking (and other) is the next step!

[@UncleDoug]

Thanks, McC. Good luck with your project.

[@rctxtd5]

Hi Uncle Doug. Second post on your channel. Just wanted to say thanks again. In the few months I've been bingeing your videos - I've gone from a person who vaguely understood the way tube amp circuits work - to someone who feel confident enough to tackle my first amp build which I'll kick off during the holiday season (very probably a Champ, from scratch). I've built a few pedals in the meantime to brush up on my soldering and wiring skills. Reason for the post - I thought you might be interested in the design of a British amp maker who is growing in popularity over here, and globally - Victory Amps. Extremely well-made amps, but with one feature you may find neat - a rear panel mounted pair of probe ports and bias adjustment pot to allow basic bias measurement and adjustment without opening the chassis. For me, this is a good approach to getting valve amp users used to the idea of checking the bias and health of their amp, without introducing too much risk or danger. While I haven’t seen a schematic, this approach appears to approximate a measure for the plate current (by using a 1 Ohm resistor and reading the voltage across it). While you don't take a direct measurement of Plate Voltage and ultimately calculate Plate Dissipation - it seems to me a safe and convenient approach for quick health checks by the user between proper sessions at the tech. I hope this is of interest, and thank you once again for sharing your knowledge with the world - your hands on and practical approach is surely the best resource I've come across. Link to a picture of the rear panel and a link to the manual - which describes the process on page 6 - tinyurl.com/yczk88rz tinyurl.com/ycxygq6z

[@UncleDoug]

The amp appears to be very nicely designed and built. I think the idea of readily accessible bias ports is a wonderful one......safe, convenient, and economical since you can do it yourself. I hope this catches on with other amp manufacturers. One thing puzzled me. I wonder why the bias is labeled in mV instead of mA? Perhaps the method of measurement and calculation are different from what I am accustomed to. Thanks so much for sharing this with us. I look forward to seeing one of these gems in person some day :)

[@rctxtd5]

Uncle Doug I believe they list the value in mV to simplify the process. Getting a reading of 22mV on the multimeter equates to a current of 22mA, because the reading is taken over a 1 Ohm resistor. Less to explain for those who don't care about the detail I suppose.

[@towerman75]

Very knowledgeable, wish I would have had some professors like this when I was in technical college....

[@UncleDoug]

Thanks so much, James.

[@maggit13]

Uncle Doug, Thanks again for these awesome videos, you have a ton of great information. I just replaced the caps on my Fender 75 and it does sounds a lot better, less hum and everything. I decided to check the bias on it as well. It's a fixed (grid) biased amp with a bridged rectifier, as you mentioned in another video, it has the 4 diodes and caps. The center tap from the OT goes to a hi/lo power switch before the rectifier so I measured resistance from there to the tube plates. I'm pretty sure that is the center tap, there are 2 other leads outside of this one that go to the other grids of the tubes. These are the numbers I got on low power (15 watt) mode, the other tube is very similar: Resistance: 50.8 ohms, Vdrop: 0.72V, PC: 14.2mA, PV: 216, PD: 3.07W This is Hi power (75 watt) mode: Resistance: 50.8 ohms, Vdrop: 0.62V, PC: 12.2mA, PV: 420, PD: 5.124W These numbers seem very low to me compared to the numbers you usually get. I'm pretty sure I'm measuring in the right spots and i double-checked the scale I was using so these are the right values. There is also an output tube matching pot which I'm thinking of converting into a real bias pot to see if that changes anything. What do you think? Oh and thank god I made a current limiter like the one you have. That bulb lit up a few times, i'll be more careful next time...

[@UncleDoug]

+maggit13 I looked up a schematic, and it appears that the resistance of the OPT primary windings would be measured (meter set to "Resistance") from between the (red) wire attachment to the center of the Hi/Lo Power switch to pin 3 (Plate) of each 6L6 (blue wire and brown wire)....amp "OFF". Then you would switch to "DC Voltage" and measure the voltage drop across each winding and at both power settings....amp "ON" (be careful). Your voltage drop measurements are quite low.....I would expect them to be around 5 times greater. Your measurement look more like Screen Current than Plate Current.

[@maggit13]

+Uncle Doug ok thanks for the info. I'm 99% sure I measured from the plates, pin 3 on the tubes but I'll check again. if those are the right numbers, what should I do?

[@UncleDoug]

I would think that a high-powered 6L6 amp that is biased at 3W to 5W would sound awful. 6L6GC's can handle up to 30W of PD. If the amp sounds good, then there has to be some error in the method of measurement or calculation of PD.

[@maggit13]

+Uncle Doug I don't doubt that there is some user error in there :). I'll check again but it does sound pretty good, have to turn it way up to get overdrive though, very loud.

[@UncleDoug]

The hotter the bias, the lower the headroom. If your output tubes are biased excessively cold, then the headroom would be huge and the tone should be rather weak and brittle (to use purely subjective, abstract terms).

[@andyk295]

Hi Uncle Doug- I just came across a '65 Princeton Non-Reverb that's Pre-CBS and an absolute cream puff. It came with 6L6's installed; I'll try to bring it back to factory specs with a pair of 6V6's, using your methods on how to bias them; if that doesn't do it I'll add a vintage Alnico speaker. The 6L6's don't have that "creamy" blues sound the amp is known for and seem to break up too much ('muddy out') on the low frequencies. Thanks so much for the great videos!

[@UncleDoug]

Congratulations on a great acquisition, Andy. By all means, convert the amp back to 6V6's. The high current draw of the 6L6's can damage your power transformer.

[@autumn2749]

Thank you so so much for explaining biasing, this is the first explanation on youtube ive seen that accurately shows how to do this without buying a probe. Just biased my 74' deluxe reverb and it sounds better than ever!

[@UncleDoug]

You're welcome CD. I'm really glad to hear that you were able to bias your own amp, and that it turned out so well. It's good to know that the video was helpful :)

[@jfranciskay]

Thanks so much for all of these videos. I know how much time they take and trust me it is appreciated! I tried your method on my JTM45 RI and thought I was doing something was wrong. I could only push 6.6 Plate Dissipation on my 5881's at max bias. Turns out I had a weak rectifier tube. While that helped a bit, I still couldn't get them up to 70%. I replaced the power tubes and was in business. Maybe that is why I never really liked the sound of this amp. I also compared the results with a kit based bias probe I made and the numbers match almost exactly. Your method is accurate and inexpensive. Thanks for your insights!

[@UncleDoug]

You're quite welcome, Jimmy. It's great to hear that the video(s) were helpful. Thanks for the kind words :)

[@edwardhannigan6324]

Wow, such in depth instruction on biasing Doug. I was a bit foggy with it all from a few of the other videos of yours, but these 2 on biasing has helped enormously. Cool stuff and so informative & safe..Thanks for sharing once again..Ed..U.K.

[@UncleDoug]

You're quite welcome, Ed. Thanks !!!

[@jorge195501]

Doug , thank you for this, I get more than what I asked before , I will check if you already have , One of these tutorials talks about how all those Control volume ext , read the Schematics . Thank you very much sir.👍

[@UncleDoug]

You're welcome, Jorge.

[@dardaraveiga6512]

I wish I had a teacher like you when I was young 😃🙏🏻👍

[@UncleDoug]

Thanks. I wish I could have been there for you, D.

[@marceugeni]

You really rule the universe and some parts of other foreign multiverse universes out there man.

[@UncleDoug]

Thanks, Marc. I guess it's time to start charging some high taxes from my "subjects" :) :)

[@marceugeni]

Uncle Doug really an invaluable info. The best of it is the way you explain so maybe an online course on the subject will be nicer than the TUT books and the Radiotrons

[@UncleDoug]

Think of our video series as a free online course, Marc.

[@raymondbailey7045]

Thank you Uncle Doug for being an amazing teacher ! Keep up the great work.

[@UncleDoug]

You're welcome, Raymond. Thanks !!!

[@joeycigar63]

Thanks for another great series. I'll no doubt study all 3 again, probably several times each. More things are starting to come together now, little by little. Many thanks, as always. JC

[@UncleDoug]

It was our pleasure, Joey.

[@frankgeeraerts6243]

HI Uncle Doug.......just a small comment . Plate dissipation on a valve is indeed calculated by plate V and plate Current , so far OK. But plate voltage is measured between cathode an plate , not between plate and over the common cathode resistor.to ground.... . It was common practice to use a common cathode resistor.....cheaper and as they said sounding better ...( because no tubes are really identical and so the PP produces more second harmonics which are normally annulated by the PP annulation of these even pair harmonics ) but it is much SAFER to use individual cathode resistors ( one per tube ) .... tubes getting out of balance on the long run and run away of one of them is the often seen red plating of one of the tubes carrying almost all of the current. Another nice solution instead of individual cathode resistors is a partly bias with one common cathode resistor and a complementary neg voltage bias for each tube with a trimmer - pot control adjustment on the grid of each tube.. it works very well , easy and good balancing of the tubes for extended bass and avoiding premature transformer core saturation , is quite safe in case of bias failure ....and sounds good...

[@UncleDoug]

Thanks for your very informative input, Frank. This was an early video, kept basic for beginners. I have since posted quite a few videos demonstrating bias techniques which do rely on plate-to-cathode PV measurement for cathode-biased output tubes.

[@brandonlamora4987]

Awesome videos, learning a lot. One thing you didn’t mention is that if you are doing this procedure on an amp with a “standby” switch, you will need to turn the switch on or you will show no resistance between pin 8 and pin 3.

[@UncleDoug]

Thanks, Brandon. Yes, you're definitely right.

[@gearathon]

Thanks Doug, now I am up and running! Exciting to measure and see how things changes with different tubesets in the same amp

[@UncleDoug]

You're welcome, G. Good luck with your experiments.

[@LestonDr]

Uncle Doug, I cannot thank you enough I have been studying bias for 6 months and finely get it. Hope to restore my late brother's home built mono amp as a remembrance (no schematic of course). Since bias has alluded me up until now I can move forward. Thanks again. Looking forward to each video.

[@UncleDoug]

You're quite welcome, LD. It's great to hear that the video(s) were helpful. Good luck with your project. Be careful.

[@anton4069]

Hello! Thank You for the informative videos! Good health to You!

[@UncleDoug]

You're welcome, Anton :) Likewise.

[@thaddgrossi5959]

Hi Uncle Doug, First of all Thanks so much for your videos. Youve taught me alot already. I have a Fender 140 that I have performed your bias analasys on. There is not a tube rectifier so i am using the red wire from the OPT to the HI LO Power switch to as th CT connection. My readings are as follows. switch is in the high power position. Tubes are 6l6 GC. Pin 3 to CT resistance = 38 pin 3 to CT voltage drop = 3.12 calculated voltage drop = .0821 Pin 3 to chassis Plate voltage = 515 calculated Plate dissapation = 42.285 I've only included specs for 1 tube as they are all really close. This amp does have a schematic online and does have a output tubes matching pot. Bias seems to hot to me but my attempts to lower the bias via the matching pot are failing. pot was originally all the way turned down and had a 10k resistor on it. i changed the resistor value to 33k then 100k. pot still has to be in the fully turned down position or the bias goes up and I'm still in the 40's for watts. Please help me figure out how to get this bias down. Thanks, Thadd

[@thaddgrossi5959]

Also could you clarify how that matching pot should be wired. it looks different to me than the schematic. mine has: red from hi lo power switch on the left tab wires to grid on the wiper center tab and resistor grounded to pot on the right tab

[@UncleDoug]

I'm afraid I can't help on this, Thadd. If your amp is unmodified, then I would accept its wiring as is.

[@UncleDoug]

Your red-to-blue resistance (OPT primary) is only 38 Ohms? This seems way to low. With a Vdrop of 3.12V this would indicate a PC of 82mA, which is very unlikely, and a PD of 42.3W.....also very unlikely. You'd better check your resistances again, red-to-blue and red-to-brown.

[@thaddgrossi5959]

+Uncle Doug Thanks for your quick response. I just re-checked reading ohms red to brown 36.6 red to blue 38.6 Idk? am I supposed to divide by 2 anywhere in this equation since there are 4 tubes total. 2 tubes per side.

[@UncleDoug]

The schematic I looked at did not show 4 output tubes www.emerald-green.de/images/fender-140-schem.jpg. Yes, the PC should be divided by two, so the PC for one 6L6 is 41mA, which is more believable. The PV is best measured from plate to cathode (not ground). Let's say it's 495V, then the PD would be a little over 20W, which is just right for fixed bias 6L6GC's (70% of 30W).

[@boctok55]

I really appreciate the fact that you demonstrate various methods to determine the bias of the output tubes. I have recently had fits with a cathode biased amp that I have told you about. I was explaining my problem to another person and they told me that the resistance across the output transformer center tap to pin 3 of the output tubes was irrelevant and then wanted to know how I obtained the plate current. They suggested that I go to a certain website and use a bias calculator. I was thinking to myself, how do you think these things we're done before online calculators? That's the beauty of the way you explain these things. I want to know the hows and whys as well the answers.

[@UncleDoug]

That's great to hear, B55. You're right of course. It's always best to know the fundamentals.

[@ronwarren4173]

thanks again Doug great explanation as always. like the reminding folks about the hand in pocket, method to stay safe, and discharge res caps. thanks again looking forward to your next.

[@UncleDoug]

You're welcome, Ron. I'm glad you enjoyed the video. I try to emphasize safe practices to take care of my subscribers ;))))

[@bagoodale]

Best videos ever on youtube!!!

[@UncleDoug]

Wow, Brandon, what a nice compliment. Thanks !!!

[@bagoodale]

Uncle Doug You sure do know how to teach complicated subjects. Great job!

[@UncleDoug]

Thanks again, Brandon... :))

[@anthonycalia1317]

Nice job as always. I do have one comment general to all of your wonderful videos. You take the time to explain theory targeting individuals who, if they lack such basic theoretical knowledge of Ohm's Law and high voltage circuity, probably have no business with their hands inside a tube amplifier. Kinda like having books titled "nuclear reactor operation for dummies." probably not a good idea...

[@UncleDoug]

How else will they learn it, Anthony? I understand your point, but based on the significant number of viewer comments thanking us for helping them learn how to fix their own amps and actually scratch-build fairly elaborate circuits, it would appear that our approach is quite successful.

[@AliKiani7887]

Thanks uncle, another great lesson from you, i really admire you...

[@UncleDoug]

You're welcome, Ali :)

[@SuperCarver2011]

Uncle Doug. That bias adjustment for the 6V6 pair is called BIAS MODULATION. I printed off the Princeton blackface AA1164 schematic to see it a bit better of the values with a magnifying glass. The negative bias diode goes to one side of the 220k (intensity control) but the intensity control also goes to ground via 22k resistor, so the dc bias component or this dual purpose control has a minimum value of 22K. Now the other side of the intensity control is AC coupled to the plate to isolate the tremelo oscillator via a 1 meg resistor and 0.02 caps. (more) This is called Bias Modulation One cap to ground and the other to 250K (L = Linear) intensity pot. "The Bias Modulation design, conversely leaves the preamp gain alone, but instead plays with the power tubes gain and consequently, their clipping characteristics. The circuit also introduces a type of distortion called “crossover distortion”. It is a nasty, odd-harmonic distortion caused by the fact that the signal is not being completely reproduced. It ALSO CAN'T BE REALLY USED IN HIGH POWER AMPS. Therefore, this inherently less costly, less complex circuit is usually reserved for LOW POWER inexpensive amps. Good examples are the Princeton, Deluxe and Tremolux. So the way I interpret it. The 250K pot just moves the negative bias (as supplied from that diode) to EITHER MORE NEGATIVE to decrease conduction of the OSCILLATOR signal from the the oscillator, or LESS NEGaTIVE to increase conduction of the oscillator signal. and of course some inherent distortion goes with it. The bias is modulated by the oscillator signal and the intensity pot setting. Don't know why Fender decided to do it this way, but they may have had their reasons to save on tubes and components in manufacture. Because of the capacitor isolation, only the AC signal from the oscillator is present THROUGH THE 250k pot. The dc negative bias is determined by the centre tap of the intensity control. Intensity in this case is the , amplitude of the signal. Basically a design requiring fewer components. If you want to read more..go to " 300guitarsdotcom " and search for " Tremelo-How it actually works in you amp."

[@UncleDoug]

+SuperCarver2011 Thanks for this very detailed analysis, SC.

[@michel333100]

Hi Doug, thank you very much for your advice. Like I said , I hope you never stop doing videos. My main question also is why is the plate voltage so high at 485. I have also used 6l6's. I recently had a problem with red plating in this amp.it turned out that I had to replace the bias potentiometer. Thanks again mate. Michael

[@UncleDoug]

You're welcome, Michael. Perhaps the power transformer had been replaced with one that produces too much voltage. Another possibility is that the plate voltage is too high because the plate current is too low. They are indirectly related.

[@GB-iy3wt]

Thank you again for another very informative video series

[@UncleDoug]

You're welcome, Garrett :)

[@bullthrush]

I cannot imagine a better explanation, thank you.

[@UncleDoug]

You're welcome, BT. I'm glad you enjoyed it.

[@joepublic4069]

Excellent. Great explanation.

[@UncleDoug]

Thanks, Joe.

[@taylorc315]

Thanks Uncle Doug, Your the Best! Love your videos. I watch all your videos and have learned a great deal, Keep up the great work!

[@UncleDoug]

taylorc315 You're welcome, Taylor. Rusty and I really appreciate your kind works and wish you the very best :)

[@c.hundley9714]

Uncle Doug, do they ever use pots in the cathode bias.?

[@UncleDoug]

You can, as long as the pot can withstand the current level.

[@harmindersingh7833]

Great sir very deeply knowledge

[@UncleDoug]

Thanks, HS.....glad you liked it.

[@BAMPSANDPEDALS]

Hello Uncle Doug, Rusty. I did a test, using your measuring method, with two 6V6's who were not bought as a match pair. The outer tube has a plate dissipation of 9,88 watt, the other 11,25 watt. Average value is 10,55 watt. I think this is not a bad combination, following your advise to stay under the 12 watt. Many thanks, Benny

[@UncleDoug]

BAMPSANDPEDALS Benny, you're right, they are not ideally matched, but should work just fine at your settings. I have operated amps with similar PD disparity and have been satisfied with the result.

[@SiliconSet]

Great video! So many thanks. Thanks to Rusty for his input too.

[@UncleDoug]

+SiliconSet Thanks so much, SS. Rusty really worked hard on this video, as usual. We're glad you enjoyed it.

[@dortoh1]

Very good video, but one thing to consider. When using a light bulb current limiter you can get a input voltage drop under load. Mine with a 200 watt bulb drops the loaded input voltage from 117.6 to 107.3 which can really affect your test results.

[@UncleDoug]

With a 250W bulb, the voltage drop and reduction of PV and PC is minimal for low-wattage circuits. For medium to high-wattage circuits, final bias should be with wall AC, not through the CL

[@jonnybeck6723]

As per usual, another great, easy to understand instructional vid... Thanx so much Uncle Doug, you bring a flicker of light into my imagined sea of ignoramousness (and this is really appreciated, BTW) ...so, er cheers

[@UncleDoug]

You're welcome, Jonny :)

[@oneofthemodels]

Once again thank you so much I could die for posting this so helpful God bless you and your family

[@UncleDoug]

You're welcome, MM. Please don't die.....we want all our viewers to live long, happy lives :)

[@MyFirstGen]

Thank you for sharing your knowledge.

[@prtallis]

What a great contribution!!! Thanks.

[@UncleDoug]

+prtallis You're welcome :)

[@MichaelLloyd]

Hey Doug, I just used this procedure to check and change the bias on my old RCA AM/FM/Phono amplifier. It was using about 10.52 / 11.9 W for a pair of 6V6's in push pull. Average was 11.24W so bumping max. The cathode bias resistor was 270Ω. The readings never really stabilized at that output and the audio wasn't that great even at a low setting (3 of 10). I have a few 6V6's and I selected a pair with the same Gm. I installed a 330Ω cathode bias resistor and power came down to 9.81 / 10.88W. Average was 1035W or about 86%. The readings were stable. Plate voltage came up 10V (must've been stressing the transformer a little?). And the plate current was a lot closer. I'm going to leave it there and see what it sounds like. I might've left it the way it was if it was a guitar amp. I don't need distortion in this setup :o) Thanks a bunch for the video. Mike

[@UncleDoug]

All your measurements and modifications make good sense, Michael. Plate current and plate voltage are inversely related, so one always goes up when the other goes down. I hope it sounds great. Best of luck.

[@MichaelLloyd]

I finished working on the amp this afternoon. It had a few other issues. The worst was a loud pop followed by a crackling sound that decreased in volume when I turned it off. That's not good. I replaced 5 bad resistors and touched up some ground connections then reinstalled the amp. I am very happy with the way that it sounded and how the volume control worked. And the pop was gone. You can't beat that with a stick. Now if I can just get the FM section to work and align the AM section I'll have a nice, oversized, home heating and audio cabinet :) Thanks for your great videos

[@UncleDoug]

You're welcome, Michael. Best of luck with your alignment and repairs.

[@taddy666]

Thank you Doug! You're the best!

[@UncleDoug]

You're welcome, TA. Thanks !!

[@messmer777]

I was going to try changing tube types in my Orange Rockerverb from EL34 to KT88, as the amp is advertised as able to handle several tube types. I opened the amp up and was faced with the back end of a PCB with all the components and traces on the other side. I stared at it for a moment, then put the amp back together, suddenly feeling satisfied with the EL34s.

[@UncleDoug]

I understand completely, Sam. Rusty and I have opened up a few chassis, taken a look around, and decided to move on to other projects.

[@messmer777]

I'll gladly pay my favorite tech to deal with this!

[@jorge195501]

I’m impressed , thank you.

[@UncleDoug]

You're welcome, Jorge.

[@alexabdulkader7423]

Great Video!

[@UncleDoug]

Thanks, Alex :)

[@FluxCondenser]

Phenomenal info! Thanks, Uncle Doug.

[@cosmonaught7690]

Uncle Doug, this video has helped me immensely. Question: The center tap in my output transformer is not quite on center. There is a 30 Ohm difference measuring to the anode pin on either side of center (150 Ohms, 120 Ohms). Can I (should I) add a 30 Ohm resistor to one side to balance this, or does this cause a problem with the music (AC) signal? THANKS!

[@UncleDoug]

That is perfectly normal, Cosmo. They all have some variation between the two windings. Leave it alone and enjoy it :)

[@paulkielt9301]

What I've done is to add a second bias supply with a separate potentiometer for each tube. Now I can even buy unmatched tubes and bias them properly, across a 1 Ohm resistor placed between the cathode and ground of each valve.

[@UncleDoug]

Excellent idea, Paul. I'm impressed.

[@ingrofrano]

Hello Uncle Doug, thanx in advance for all the unvaluable info. Today I've completed the Plate dissipation test for my recently ended 6V6GT Stereo Push-Pull tube amp. The 6V6GT's Plate dissipation results was 12.24w, 11.99w, 12.26w and 12.05w. What do you think about working so close to the dissipation limit? The cathode bias resistor is 250 ohms with a 47uF@50v capacitor in paralell. How can I reduce the Plate dissipation? Thank you very much for your time and help.

[@UncleDoug]

Gabriel Rofrano You're welcome, Gabriel. I tend to bias just below the maximum.....around 10 or 11W. It sounds about the same and the tubes last longer. You can reduce the PC and PD by increasing the value of the bias resistor, i.e. to approximately 275-290 ohms. Good luck.

[@Ivesh1000]

Hi Uncle Doug, super series, hope you can explain something to me in your wonderful understandable way, I see different calculators and values for cathode based push-pull amplifiers and they refer to class A and class A/B with the same set-up and diagram, is that really the case in a class A/B for a cathode based push-pull amplifier ? Is it only the bias current that is lowered ? or are circuit changes required ? I do not seem to find a comprehensive answer. Hope you can shed some light on this subject, thank you and stay safe.

[@UncleDoug]

I personally find the separation of amp circuits into specific types to be an unnecessary distraction, Ives. I have serviced hundreds of amplifiers and the amp type is never a consideration.....you simply work with what you've got and set it up according to basic rules, as depicted in most of my videos.

[@sledge776]

I thank you for putting these videos together and tremendously appreciate the time you’ve taken to put these videos out. Tremendous appreciation I also give for sharing your knowledge. You make me see the “good” in humans, damn you for that! 😉🤣😂

[@UncleDoug]

You're welcome, Greg. We've been damned for less :)

[@jimmcdowell6000]

Great video. Very informative..

[@UncleDoug]

+jim mcdowell Thanks, Jim. Glad you liked it :)

[@1977sadhana]

Hi Doug, I have a 1995 Bassman Reissue, and it made a burning smell, and a loud buzzing. Since then, while testing with this method, I found that ohmage pin 8 rectifier to pin 3 plate of 6L6's, in both cases, each side of the output side of the output transformer read at MEgohm readings. IS this due to a fried output transformer, or some weirdness of the Solid state Rectifier that's in place? (thanks in advance if you get to see these late comments)

[@UncleDoug]

I'm confused by your descripton of the symptoms. If you mean that the output wires from the secondary winding of the OPT have virtually no continuity ("megohm readings"), then yes the OPT is fried. Both the input (primary) and output (secondary) windings should have measureable resistance far below the megohm level.

[@jeffcaldwell2544]

Fantastic info, Uncle! I was paying close attention to the AA764 piece. I have an AA964--would the bias recommendation be the same since, like the 764, the 964's cathodes are connected and grounded and it's grid-biased with the grids connected to the trem intensity pot? Of course, this video was 10 years ago--maybe you've found another way to cool the bias on these beasts? Thanks for listening.

[@UncleDoug]

Yes, the bias procedure would be the same, Jeff. You can alter the value of the 27K resistor (to ground) to change the range of bias adjustment.

[@The-Master-of-None]

Hope uncle Doug will give us some tutorials about the difference between the cathode vs fixed bias and a modification to switch between cathode and fixed bias. :) Happy New Year Uncle. Your channel is my favorite channel in KZbin land. :)

[@UncleDoug]

Thanks, MT. I'll keep your suggestion in mind.

[@ezra360]

Great stuff, thank you!

[@UncleDoug]

Glad you enjoyed it, Ezra :)

[@alvarofalcon7136]

Hi Uncle Doug, I finally decided to try your method for biasing fender amps. I just had the power transformer from my princeton reverb amp rewired (rebuilt) for the third time in several years....I checked and found the capacitor in the bias circuit loose, thinking that could be the cause of the failure, I fixed that and installed the transformer. The amp works well and sounds good but I feel its running hot, so I will check the Plate dissipation etc as you suggest and see if its too high. If thats the case Ill probably be writing you again.....Thanx

[@UncleDoug]

Excessively hot tubes = too much plate dissipation, Alvaro. Check it an see before more problems arise.

[@alvarofalcon7136]

Thanx Uncle Doug ! so far I’ve checked the resistance of both halves of the OT 162 ohms outer 166 ohms inner, voltage drop 1.3 vdc and 1.52 vdc, now I will measure the plate voltage do the math and see what I come up with. I’ll probably be in touch with more questions, this biasing thing is hard work......Rgds

[@UncleDoug]

@@alvarofalcon7136 I get values of 8mA and 9mA for your 6V6's, Alvaro. If your measurements are correct, the bias is ice cold.

[@alvarofalcon7136]

Hi Uncle Doug, it turns out that since I made the measurements with the amp pugged into a 60w light bulb (I was only getting around 99 vac from the 117 vac wall voltage.) after plugging directly into the wall and changing some tubes I got these new measurements. Outer tube: R pin 3 pin 8=162ohm VD= 6.4 PC=.0385 PV=399vdc PD=15w Inner tube. R. =162ohm. VD= 5.91 PC=.0356 PV =399vdc PD = 14.2w Now they seem too high for 6V6 s Rgds and thanx for taking the time to explain all of this information!

[@UncleDoug]

@@alvarofalcon7136 Jeez....from one extreme to the other. Good luck with this one, Alvaro.

[@jamespaterson7597]

very interesting uncle doug thank you

[@UncleDoug]

Glad you enjoyed it, James.

[@scottjamable]

As usual I am learning so much from your patient and descriptive approach to teaching. I Just bought the same 67' PR but has a silver face AA764 and I looked inside and a bias pot had already been added and I just did all the calculations for PD and my tubes are running at 11.46 and 9.15 watts. Is this OK? it sounds very good very clean headroom with volume up to 5 or 6. Weber F150T 10" 50 watt speaker. I wasn't clear from your table on tube dissipation if we should be going with 70% of 6V6 12 watts (8.4 watts) or stay closer to 100%? Thx.

[@UncleDoug]

Since the PR output tubes are fixed bias (grounded cathodes) you have to limit the PD to 70% of max, which would be around 8.4W. This allows for an increase in PC and PD when the amp is driven hard. Your output tubes are not well matched and are biased too hot.

[@Fighterjetpinups]

I ll try that, I read on fenderguru the wire from middle lug leads to 220k resistors connected to grids and can be grounded when changing to cathode bias. I could disconnect and tape the wire instead.

[@UncleDoug]

You should disconnect the end of the wire that connects to the center lug of the bias pot and ground that end of the wire (which will then ground the junction of the two 220K resistors). Leave the bias pot (and the other two lugs) alone.

[@pasqualinocecere3432]

Beautiful, very simple, very clear explanation. Too bad I miss a few words (I'm Italian), but thanks for the videos. like, like,like,like........like.

[@UncleDoug]

Thank you very much, Pasqualino :)

[@jimotranto546]

Doug, great videos and instructional method, wish I had teachers like you in high school. I hope you can help with my issue, I'm working on a Princeton AA964 and did some upgrades on the passives that needed it. I'm getting some strange readings on the voltage drop, 1.96 and 2.03, resistance 146 and 149 which gives me plate current of .0134 and .0136 respectively. Plate voltages are 431 and 430 which gives me plate dissapation values of 5.77 and 5.84 respectively. Now the amp sounds OK, but those readings seem very much on the cold side, taking in consideration you comments regarding bias adjustment on these Fenders, is there some I need to do to or should do bring those values up? Thank you for your hard work.

[@UncleDoug]

If your measurements are correct, the biasing is way too low. Your PC should be around 30 mA. This video, or any number of more recent videos, should explain how to do this.

[@LEANDROID14]

Uncle Doug excellent video! I want to ask you about the value and power rating of the resistor you use to discharge the filter caps. Thanks!

[@UncleDoug]

LEANDROID14 Thanks, Le. It's a 100-ohm 10-watt resistor. They are fairly common. You may even be able to get one at Radio Shack, if they are still in business.

[@kevinwilkins9926]

The transformer on my Bassman build I am doing which was an old PA, doesn't have a bias wire. The schematic shows that the Bassman original transformer does. I assume I can than just cathode bias this amp. You mentioned 100 ohm resistors for the 6V6, but my amp has 2 6L6GC's. What wattage resistor do you recommend me using? Thanks Uncle Doug

[@UncleDoug]

Cathode biasing works fine and it's less "fussy" than grid biasing. I would start out with a 1000 Ohm resistor (or pot, so you can adjust it to find the right resistance then replace it with a fixed resistor), probably around 10W. After running the amp for a while, check to see if the resistor is too hot to touch. If so, increase the wattage.

[@kevinwilkins9926]

@@UncleDoug thank you sir. Appreciate it.

[@jmeds_jammin]

You are the man.

[@UncleDoug]

Thanks, Justin.

[@michaelwoods9005]

On fender amps with a standby switch seperating the center tap from the rectifier tube, do you still measure to the rectifier tube and simply close the standby switch? For the measurement?

[@UncleDoug]

Yes, or you can measure the resistance from the OPT side of the standby switch.

[@Ben-yy7io]

Hey Uncle Doug, I've got a question. When I measure the pd with the output transformer method on a se 6aq5 amp im working on, the plate dissipation is a spooky 20W, while when I meaure with the cathode resistor method, its a much more reasonable 10.75W. I'm not getting any red plating and it sounds about how I want it to. My suspician is that my cheap multimeters autoranging feature will not give an accurate readout of the vd across B+ side of OT and 6aq5 plate. Do you have any other ideas why this may be? Thank you so much for sharing your wealth of information.

[@UncleDoug]

Since I didn't watch you make the measurements, it's impossible for me to guess what went wrong, CJ......but apparently the cathode resistor method yielded a more realistic result.

[@Ben-yy7io]

@@UncleDoug Thank you for responding. Determined it was my dodgy multimeter. Doh.

[@evj63]

Bless you Uncle Doug for sharing your knowledge in a most concise and easy to understand way. I just noticed right at the end of this vid where you're talking about the biasing arrangement in the Princeton amp. I've just put a 10k pot into the bias supply of a Princeton in an attempt to make the grid bias adjustable but the plate current is jumping about all over the place when I try and measure it. Is the 10k pot a waste of time? Would it just be better to adjust the bias supply resistor if the tubes are running too hot/cold? Thanks again for your videos, they really are great. Cheers, Jeff.

[@UncleDoug]

Thanks, Jeff. I'm glad the videos are clear and helpful. The bias voltage in the Princeton circuit is controlled by the 27K resistor to ground (in the schematic, just to the right of the tremolo intensity pot). Increase the resistance to increase the bias voltage, and reduce it to reduce the voltage. There is a correct way to install an adjustable bias control in the Princeton circuit. It's described in detail in another of my videos: kzbin.info/www/bejne/eprVfmycqrqbgNE Good luck :)

[@60retired]

Uncle Doug great video as always but I have a question for you. What about biasing a Cathode Bias Kalamazoo Reverb 12 it has no Rectifier tube, its solid state diodes, were exactly would you connect the multimeter ground. I have a good idea but I’d hate to be wrong a damage this pristine condition amp, it was the first amp I bought back in 1966, thank you.

[@UncleDoug]

If you are trying to measure voltage drop across the cathode-bias resistor, simply attach the probes on either side of the resistor.

[@60retired]

What I meant to say with that ground connection question, if using the second method to bias the two tubes using the output transformer method, where in the circuit on the schematic would be the best place to connect the multimeter ground test lead on a Kalamazoo Reverb 12 with solid state rectification. There is no rectifier tube pin 8 to connect to like in your video. Thank you😊

[@UncleDoug]

I have no schematic for this model. In general, with SS rectification, connect one lead to the B+ wire from the diode rectifier (which connects to the center tap of the OPT primary), and the other to either of the plates of the output tubes. @@60retired

[@60retired]

You are absolutely correct the center tap is connected to the two diodes that go to the B+. I traced this out again last night and thats the place I figured would be best but wanted you opinion on it. I’ve only worked on four Vintage Fender amps with fixed bias, never on SS but it looks like no big deal. I’m considering possibly doing a mod so that both tubes have their own cathode capacitor and resistor rather than both tubes sharing. Theres a perfect spot in the chassis to do this. Thank you so much and for all the great videos they have been very informative and greatly appreciated. Nobody online does a better job at explaining and demonstrating this info than you. Thank you once again, stay safe and healthy.

[@UncleDoug]

There is no benefit to splitting the cathode bias resistor and bypass caps, 60R, other than allowing you to measure the individual plate current for each tube.....which can be done via the OPT half-winding method. Good luck with your project should you choose to proceed.@@60retired

[@robferguson3419]

Hi Doug. Just wondering why the voltage off the rectifier goes through the output transformer. Is it a convenience thing rather than hooking up directly? And would their be no issues with all the dc from the rectifier flowing while the ac audio is trying to share the same space (i know transformers wont let dc pass)? Lastly, i'm struggling to understand how lowering the cathode voltage helps tube life, especially if you already have more than the max rated plate voltage (say 390v on an el84 and then 16v on cathode trying to go lower with bigger resistor). I suppose it all has to do with relational voltages inside the tube?

[@SkitaEkul]

I guess uncle Doug is busy :-) Here comes a crash corse that might clear up a few things. The point of the power tubes is to drive the transformer. That i why the B+ voltage is run true the transformer. The transformer works as a gearbox so you can get the get the high voltage signal that appears at the plates to mach the speakers that likes to se low voltage and high current. Transformer will let DC pas from center tap (CT) to the plates but not to the secondary so it isolates the high voltage from the outside world (speakers) . Not a convenience but a necessity. The cathodes emits electrons that ends up on the plate but you can control how manny electrons end up there by inserting a grid with a signal in between. To help that grid to control the amount of electrons flowing from cathode to plate you place the grid at a negative potential compared to the cathode. The more negative it is the fewer electrons can pas it and end up on the plate. The point of bias is to place this negative potential (Bias) so that A. we reduce the flow of electrons so we don't destroy the tube with to much flow. B. The signal that we want to amplify needs to be floating on a negative bias just right so that it really controls the flow of electrons. Ex If you have 300V on the plates of an EL84 bias is usually around -7 volts. If you increase the plates to 390 volts the plates would attract more electrons but the plate can't take that kind of beating so you have to lower the bias to reduce the amount of electrons that can get past the grid and end up on the plates. You do this by increasing the resistor connected to the cathode. Hope this helps.

[@UncleDoug]

Sorry for the delay, Rob, but YT never alerted me to your inquiry. First, I appreciate the excellent answer provided by Luke, and I would add the following: The output transformer (OPT), since it is an inductor, acts like a (for free) filter choke to smooth out any ripple in the B+ (high voltage) while it is distributing the high voltage to each of the output tube plates. The DC in the OPT primary can happily coexist with the AC signal that is traveling across the OPT to the speaker. Tube life is largely determined by their plate dissipation value. PD is a product of plate current and plate voltage, so anything you can do to lower either value will result in lower PD and longer tube life. Increasing the cathode bias resistor value will result in lower plate current and lower PD.

[@silasfatchett5693]

In addition, since the current is flowing in opposite directions in each of the half-primaries, the hum induced in one half is cancelled by the hum of opposite phase in the other half.

[@shirladams5204]

Hi, do I understand that you can get the correct maximum plate dissipation values for each tube from the tube manuals??

[@UncleDoug]

Yes, it is one of many operational parameters that are provided for each tube in the manual.

[@misterc1957]

Hello Uncle Doug. Thank you for all your informative videos! I tried this procedure with my bulb limiter and found that my voltages were far lower of what they would be without the limiter. Example my plate voltage without the limiter is 420 VDC, with the limiter it was around 270 VDC. This results in lower values when doing the calculations. Is there something wrong with my bulb limiter? The amp (1974 Fender Bassman 100) draws under 1 amp when running. Thank you.

[@jjo5917]

I have found the same when using a 40 watt light bulb in the limiter... maybe a 100 watt bulb wouldn't limit voltage? I only use the limiter for finding issues (shorts of some kind) and for getting rough "relative" voltages (i.e., is the voltage drop about the same across this resistor as the schematic shows? Even that is only a rough ball park because the current and voltage are both limited so voltage drops will differ from schematic voltages.

[@UncleDoug]

It all depends on the bulb wattage you are using, Mr. C. If the bulb was glowing at all, it is of too low wattage. The bulb should only get slightly warm to the touch......never hot or glowing. Too low wattage = too much resistance.

[@jjo5917]

Doug uses a 250 watt bulb, thereby probably very little resistance and voltage drop. He discusses the use of high watt bulbs here: kzbin.info/www/bejne/rYOpg6qFo7GmkKs

[@misterc1957]

Sorry for the late response. The bulb is an outdoor flood lamp and had no wattage specified. Last night I calculated that it is about a 75 watt bulb. I will look for a 250 watt bulb. Thank you to all.

[@UncleDoug]

Yes, the higher wattage bulb will greatly reduce current restriction to the circuit.

[@mlculwell]

Uncle Doug, I have one side of my 6v6 push pull that will not bias. I bias one side and the other side goes out from bias no matter what I do. would you have any idea what causes that?

[@UncleDoug]

Yes, ML, your output tubes are not balanced, so when one is "in" the other is "out". If the difference is less than 10%, simply live with it. If it's greater than 10%, order a matched pair of output tubes.

[@allanpennington]

Hi Doug, why is it that you never see the DC resistance of an output transformer listed in their spec? If for example your cathode biased amp had a 130 Ohm bias resistor, would you expect similar resistance on each side of the OT when you measure it? Should they at least be in the same ball park? The other thing I have seen on some schematics is 'Imp Anode to Anode 8000 Ohms'. How does or rather I should say does this relate in any way to the DC resistance? Please forgive my silly question if the answer should be obvious.

[@UncleDoug]

allanpennington Greetings, Allan. Your questions are anything but silly. I have never noticed any equality between the bias resistor value and the primary winding resistance. There may be some correlation, but I'm not accustomed to seeing them be equal. Impedance is purely an AC phenomenon and, again, it probably relates to the DC resistance of a coil, transformer, etc.....i.e. a voice coil with 8 ohms of impedance generally has a DC resistance of 6 ohms.....but impedance is also affected by frequency and other variables that interfere with the establishment of an infallible formula to correlate it with DC resistance.

[@allanpennington]

+Uncle Doug Thanks Doug you are a great instructor. I am a little confused though. When you demonstrated biasing the single ended amp you obtained the resistance by measuring the resistor itself. This in turn gets divided into the voltage drop to obtain the plate current. So even though you use a slightly different method to obtain the resistance (via the OT) I can't understand why the OT resistance on each side should not be close to the value of the resistor. This assumes the resistor is near or equal to its stated value. I recall you did compare the end result of the Part 2 biasing procedure with the other method and obtained very similar results; 14.5 vs 14.68. This is why I concluded that the resistance in half of the OT must be close to or the same as the bias resistor value; since all the other derived values relate to the value of the resistance. The only thing which I don't know, is the value of the voltage drop using the first method, however it must again be similar to obtain the same result. The only way they can be different to this method is if both resistance and voltage drop are different but in this case their ratio, thereby defining the plate current is the same or similar to the results from either method since the plate voltage should be the same regardless of where you measure it from. Sorry for such a long winded explanation of how I thought this through but I just want to try and understand the methodology of how we arrive at the answers. I should have paid more attention in school but I did do Pure and Applied Maths to first year college level. Sadly that was eons ago and brain atrophy happens when you don't continually practice.

[@UncleDoug]

Allan, there are two popular methods to determine plate current (PC). 1.) You can measure the voltage drop across a cathode bias resistor and divide it by the value of the resistor: i.e. Resistor = 100 ohms and Vdrop = 3V then divide 3 by 100 = .030A or 30mA This is the method generally used for single-ended amps. 2.) For double-ended amps, measure the resistance of the OPT primary (CT to tube plate) and the voltage drop across it. i.e. Winding R = 150 ohms and Vdrop = 4.5V then divide 4.5 by 150 = .030A or 30mA. In both cases, the resistance is different and the voltage drop is different, but the PC is the same. I think your confusion is caused by thinking that the voltage drop for the cathode resistor is the same as for the OPT winding.....it is not.

[@allanpennington]

+Uncle Doug Thanks Doug I understand now. The methods achieve the same result therefore the ratio between R and V drop is the same but V drop and R for each method will differ in individual values. Cant wait for the next vid on the Elvis Amp.

[@UncleDoug]

You're exactly right now, Allan. Glad we could clear that up. Part 2 will probably be posted today (Monday), so stay tuned.....lots of pet silliness :)

[@vidkids7878]

@Uncle Doug I had a real head scratching moment when trying to measure the resistance between pin 8 and pin 3 on my Fender 5E3 (that I built). Until I stopped being an dunce and remembered that pin 8 on the 5E3 goes through the standby switch (on amps that have one). So if anyone is trying this and isn't able to get that measurement (with the amp off, which it is for this particular measurement) flip the standby "on" and it will open this circuit and you'll get a measurement:)

[@UncleDoug]

+Vidkids You're not the first.....or the last.....to make that mistake, VK. If you check through the comments below, I think you might find a few others who learned this "the hard way". Personally, I think solving mistakes is a great way to learn. Thanks for your input :)

[@Satchmoeddie]

VALCO National Dobro. The Dopeyra brothers kind of founded National Dobro and the halfway financed & founded Valco. Supro was the first non-"National" brand name that ever Valco used. By the time Baldwin piano bought Valco, Valco was making amps for Oahu Publishing, Supro, Dwight, McKinney, Airline (Montgomery Wards catalog order), Silvertone (Sears Roebuck Catalog order), PennCrest (J.C. Penny's), Spiegel Catalog order Company, Gretsch, Harmony, Kay, Weber, and lots of other obscure names, and there were even some just Valco models with names like "Rock Star" model. The Gretsch Chet Atkins model is a Valco and it it has a tone oscillator tuner built into to it. There is NO SCHEMATIC for the tuner part.

[@UncleDoug]

Thanks for the informative input, Seth.