Black Face Vibro Champ Repair and Bias Discussion

A look at a typical Champ Amp Repair, and one guys solution to the hot biasing issue.

[@UncleDoug]

Thanks for creating this video and providing actual measurements rather than mere supposition. Your solution to the excessive-PD issues with the smaller Fender circuits is quite interesting and I intend to give it a try in my next appropriate repair project. Fact-based videos such as yours are a boon to the tube circuit community. We wish you well.

[@guitarc.p.r.9675]

Thank you Uncle Doug. I found this info in the early 90's while on the quest for proper amp knowlege. I had been working on amps since the late 70s but felt I lacked the info I actually wanted or needed. I took the simplest circuit I could find which was a blown up Champ that I was given for $35. worth of guitar work and began studying it. I had also been seeing lots of champs being blown up 'cause guys were starting to use little amps cranked to record with. I was finding design issues that were overlooked and simply ignored. Something VERY common in this industry....and worse today. So this brought the ability to not only make these amps reliable, but opened the door to an entire different way of seeing things. Then Champzilla was born, but that is another story of its own for another time. Thanks for watching the video, it is an old one, long and boring. I have a hard time staying awake for all of it myself. I may do an updated version at some time. Like I said, I am just trying to present correct info, regardles of how it is accepted. Thanks again, and thanks for the comment. I just had another comment on one of the Marshall videos that claimed it was useless.

[@UncleDoug]

@@guitarc.p.r.9675 You're quite welcome, GCPR. YT can always use more videos that favor measurement data rather than speculation :)

[@subtsound]

Hi, came across this 4yr old video. Learned a lot. I have a 67 BF Vibro champ, recapped it and did the dropping resistor swap with 2watt metal oxide R’s. Really like the results. Really enjoy your videos. Thanks. Towser

[@guitarc.p.r.9675]

Glad it was helpful to you. It was a poor video but full of information. Positive comments are always appreciated. I have taken a break from YT due to the negativity, this may be addressed in the future. I will be back in some degree. Thanks again.

[@subtsound]

Dang. Sorry to hear about the negativity. I find your videos very informative. Hope to see you back up in the future. Best, Towser.

[@fictitiousfreedom]

It was nice talking to you today man! Thanks for your time and information. It is extremely valuable to me and countless others. Ive watched this video twice now, I only almost fell asleep once! Now its off to attempt to breathe some life into my champ, Thanks!

[@guitarc.p.r.9675]

I told you it was long a boring. But there is a lot of info there. I may do a better version sometime.

[@fictitiousfreedom]

@@guitarc.p.r.9675 no no, i was just joking! Its a great video, i learned more from it than basically every other article, forum, or video on the subject that I’ve found. Ive also never seen anyone talk about swapping the two resistors for a darker or brighter tone. I have a 5F2A, which was my first build. I had modified it beyond recognition and have never been happy with it. I replaced all of the component values with the originals and then took your advice. I also put the 12ay7 back that i had originally used and it is now much more stable, predictable, and sounds incredible. The information you possess is extremely valuable to people like myself and truly appreciate you sharing it.

[@guitarc.p.r.9675]

You are welcome.

[@user-bx6qh2ek1i]

Thank you for posting this video. I have learned something new. Currently working on a 72 VC silverface with a hot 6V6. Going to swap those resistors around as you suggested.

[@guitarc.p.r.9675]

Thank you! It is an old poor quality video, but useful. I appreciate the positive comment. I have taken a break from YT due to trolls, and know it alls. I deleted several comments this morning because I just was not going to bother dealing with them. So thanks again.

[@tomrice5796]

Glad I found this. Was gonna put in a 1k bias resistor while servicing a 66. Put in a new 470 and and a 8k and 20k. Also a lot of the resistors in this were way off while changing caps. What a difference,this amp came to life. Thanks

[@guitarc.p.r.9675]

Thank you. I have taken a lot of grief over this from the forum experts and "teks" Unfortunately, trying to correct long term misinformation is a difficult task. I am glad it was helpful to you. Thanks for checking out the video although it is old and a horrible video the information is correct.

[@Bloozestringer]

Thanks for this video. I recently recapped my '76 VC. On recommendations from the interwebs I replaced the 40/40/40 can cap with a 20/20/20/20 and put a 1K/10W between the first two caps to drop the B+ from 420V to 380V. Plate voltage is at 367V. Screen supply is at 370V and cathode voltage is at 24.8V using a 680R cathode resistor. I also added a 1K screen resistor based on another recommendation so screen voltage is 364V. It sounds OK, but obviously the bias is way low as I wasn't figuring in the actual screen current. After watching your video I'm thinking I need to pull the series screen resisistor, change the 6V6 cathode resistor back to 470R, and change the 1K resistor feeding the screen between the 2nd-3rd filter cap to somewhere in the 10K-22K range (probably start with 18K). Would you recommend removing the 1K B+ dropping resistor as well, bringing the B+ voltage back up to 420? Thanks for your insight.

[@guitarc.p.r.9675]

First of all thanks for watching the video. It is an old one and I am a repairman not a videographer, long and boring but full of real information. I am considering a redux. There is a huge amount of bad info out there, and most "techs" get it way wrong. I need a clarification; I take it you replaced the 3 x40 with a 4 x 20 (stock value) cap ? You can parallel the first two cap sections ( I like to) or reduce the 1k resistor to 200 to 500 ohms to be used as a Pi filter. The latter will reduce hum more and filter the supply better. Your actual B+ will be under 400v anyway. Remember you subtract the cathode voltage from the plate. Now take the series 1k screen resistor out. You can use a 470 ohm if you want, but don't tell the tone police. Put a new good 470 ohm 2 watt on the cathode. Anything here over 500 ohms is just wrong. Do as I did, replace the screen supply resistor with a 10k 2 watt. You can vary the preamp supply resistor as I mentioned start with the 1k. The screen current here is the culprit not plate current. Hope that helps.

[@Bloozestringer]

@@guitarc.p.r.9675 very helpful. Thanks!

[@JFredick]

Awesome advice and I appreciate your expertise and experience. Would you recommend the 5F1 Tweed 22K(Top) 10K(Bottom) in a SF Vibro Champ or Bronco as well? I love these amps. Is it really common to see a screen voltage higher than the plate without a faulty screen resistor? This seems odd.

[@guitarc.p.r.9675]

Thanks for the comment! Yes you can use the 22k resistor in anything post tweed. What it will do is lower the voltage on the driver and preamp tube. It will "brown-up" the tone a little and make the preamp distort a little more smoothly. It is a matter of taste. You can also add a 470 ohm screen resistor to limit the screen current a little more, but the tone police will tell you it destroys the bloom of the notes. Yes it is common to see the screen voltages high. In fact the tweeds have the B+ connected to the screen first then through the Output Transformer to the plate. Really old school. RULE#1: Screen voltage should be less than the plate voltage. Also, if the plate is drawing excessive current, the plate voltage may be pulled lower than the screen. Thanks again for watching!

[@melnickstudios]

Hi, great video. I have a Vibro Champ from 1978 that uses the AB764 schematic & layout (with the orange secondary psu lead to ground). I was checking the bias and my measurements are: Cathode Resistor: 468Ω, Voltage Drop across that 468Ω resistor: 21.7v which using Ohm's Law gives us 46.4mA. 6V6 Plate Voltage measured at pin 3: 391v, so 391v-21.7v = 369.3v across the tube. Therefore the Power Dissipation (approximate because the current was measured through the resistor, not inline with the PT lead) is 369.3*.0464=17 Watts and seems a bit high. I watched an Uncle Doug video where he replaced the 470Ω cathode bias resistor with a 1000Ω but he didn't do any dynamics testing using an input signal like you do. I'm wondering if I should switch the 1k & 10k resistors like you did and remeasure. But doesn't switching the 1k & 10k resistors also reduce the plate voltage in the vibrato section? Will it affect the vibrato operation or dynamics? Another note - in the schematic, there's a note that says: "Voltages read to ground with V.T.V.M. with vibrato grounded (except v2) - no input signal. Doea this mean that the vibrato rca jack should be shorted when biasing the amp as well?

[@guitarc.p.r.9675]

Hi, first thanks for watching! OK the vibrato thing. You definitely want the vibrato turned off to check the bias. You do not want any signal into the 6V6 at all while you are taking measurements, and you certainly do not want it pulsing. Now on to the biasing thing. The 1k resistor for the bias is just wrong. I was misinformed about 25 yrs ago about the same thing, and was educated by a higher power. Like I said in the video it bounces too much and drops out of class A. Replace the resistors as I mentioned. ALL three, and use Metal Film flame proof resistors. Yes, changing the resistors does drop the vibrato voltage but does not have any adverse affect on it from working properly. I suggest watching the video again, yes I know it is boring, but there is a lot of important information there. Now that being said you should look for something close to 40 ma plate current. Yes, I know the common theory is to bias at full plate dissipation. But you want it biased at the point that when signal is applied it averages at plate dissipation as the signal swings positive and negative. This would relate to the center of the linear portion of the tubes operational curve. Then it does not drop out of class A, becoming thin and buzzy. I bet when you change the resistors the current goes down. I would be curious what your screen current is right now. Hope this helps.

[@melnickstudios]

@@guitarc.p.r.9675 Hi and thanks for the reply, so I measured all the voltages listed on the schematic as well as the resistor values necessary to calculate current flowing through the amp. It turns out that there is a plate dissipation of 14.4w based on the following calc: Vp - Vc = 383v - 21.3v = 361.7v Ic + Is = Vc/Rc = 21.3v/468Ω = 45.51mA Is = 8v/1007Ω - 25v/10600Ω = 7.944mA - 2.359mA = 5.585mA Ic = 45.51mA - 5.585mA = 39.925 mA Ppd = 361.7v * .039925A = 14.44w Where Vp=Plate Voltage, Vc=Cathode Voltage, Ic=Current measured across Cathode Resistor, Is=Suppressor Current, 8v/1007Ω is the current across the 1K resistor, 25v/10600Ω is the current across the 10K resistor, with the tremolo off (rca switch connector grounded) there is zero current flowing into the tremolo section. What are your thoughts, should I still reverse those resistors in my situation? I guess that will lower my suppressor current but couldn't I add a resistor from the 1K leading to the suppressor? Why switch them instead of adding a resistor to the suppressor? And also, would switching the resistors lower my plate dissipation enough? I felt the 6V6GT tube (it's the type with the black coating inside but not on the top) and it was really hot, much hotter than the rectifier tube. If you made it this far, thanks for listening! And your advice would be much appreciated!

[@melnickstudios]

Oops - I think I'm calling the 6V6 Screen Grid the "Suppressor" by mistake...

[@guitarc.p.r.9675]

Measuring screen current across the 1k is misleading because it is also supplying voltage to 4 12AX7s down the line that are drawing about 1 ma ea. AND this is only screen current at idle. The only way is to measure current inline with the screen. Also keep in mind that as you calculate the cathode current across the 470 you are also reading screen current which is probably around 5 ma on top of the plate current. Yes reverse the resistors, in fact as I said before Replace them along with the Carbon composition cathode resistor. It may change value as it gets warm. I have also seen the original 1ks cook to various degrees. I also want to mention the 10k resistor is the norm in many amps including previous models of Fenders, and suggested by tube manuals. Yes you can add a resistor to the screen, but don't let the forum guys know 'cause they will bust you for "destroying the Bloom of the notes" (btw, my modified Champzilla has a 2.2k screen resistor, but it is running a 6L6 in fixed bias mode, but we do not want to get into that here!) Now for the 5Y3....basically a dual diode rectifier, unless you have a big problem it should run much cooler than than the power pentode 6V6. Speaking of which, you might want to check voltages again with another 6V6 if that one is old it may be running hot on its own. As I said in the video, swapping positions with the resistances solves a lot of problems at once. The 1k did not show up until the Blackface era, and I believe it was carried over through all circuits at that point, unless a choke was used. This works fine in push-pull circuits, but single ended is a whole 'nother game. More resistance or even better, a choke here limits the current to the screen.

[@melnickstudios]

@@guitarc.p.r.9675 Since the tremolo is off and draws zero current, and I subtracted the current across the 10K resistor from the current across the 1K resistor, isn't that equal to just the screen current? Here's my math: Is = I(1K) - I(10K) - I(Tremolo 470K) = 8v/1007Ω - 25v/10600Ω - 0Ω = 7.944mA - 2.359mA - 0mA = 5.585mA. If I run a 9K resistor up to the screen between the 1K & 10K isn't that exactly the same as switching the resistors around?

[@gonzinigonz]

Just fixing up my 81 Vibro Champ now and come up with the same problem with the bias and following advice have looked at upping the cathode resistor value to 820R & 1K. Interestingly on mine the 1K had been replaced with a 2k7, not sure whether i had done that or as it came when i brought it S/H around 1987. Its been pretty much in storage for the last 30 odd years...! Anyway after changing the cathode resistor away from 470R looking at the output its not very satisfactory and you loose at lot of voltage swing so clips very early, not sure what it sounds like though...! Also the H.T is really high which doesn't help matters. I think I'll look at your trick at swopping round the resistors and see what happens. With the H.T I've found with my Champ which is an export model with line voltage selector that setting it to the 260V position with our 240V line mains (as measured) the H.T comes up to about 420V. If i set the line voltage selector to 240V H.T jumps up to around 444V..... To get the 240V setting to come in spec line mains needs to be around 220V having checked with a variac and looking at the resulting H.T.

[@guitarc.p.r.9675]

420 volts is about right. However consider your cathode to be at ground when measuring plate voltage. Which will give you around 390v. The whole thing about changing the cathode resistor is simply ignorant. The theory of changing it to 1k is just wrong. It irritates me to no end to see this constantly considered as a solution. I even saw one "tek" Referring to biasing at the 75% dissipation point which is equally wrong. Class a single ended should average around full plate dissipation during operation. My video is long and boring but it had to be to cover the info. Thanks for watching.

[@michaelknight4041]

I can think of quite a few classic amp designs that have this problem to some degree, screen voltage equivalent or in many cases exceeding the plate. Crank it up, and of course we're going to, and the screen starts doing the job of the plate...well at least for a little while!

[@guitarc.p.r.9675]

Yep, see it quite a bit. Just in the last couple weeks I dealt with pretty much the same issue in a Marshall Mercury, Ampeg B-25 and a few others. Sorry you had to wade through that video it is an old one of quite poor quality.

[@michaelknight4041]

@@guitarc.p.r.9675 not at all Charlie! Ive taken some things to you over the years and every time I've been in your shop I always thought to myself, Man I'd just like to hang out in here all day watching and listening to this dude and learning about this stuff! Then I would think about what a pain in the ass i probably would have been! Hey Charlie whats this? Whats that thing for ? Hey Charlie how's this work? 😆 I tell you tho in the pre internet days when i was trying to learn about this stuff but there weren't many resources, to go into your shop and talk to you was like visiting a wizard or something.

[@guitarc.p.r.9675]

Thanks Mike!

[@alexnuzzi4780]

Hi I’ve got a 65 Vibro champ and I need to replace the speaker. Can’t seem to get a straight answer on whether or not I should go with a 3.2 or 4ohm speaker

[@guitarc.p.r.9675]

A 3.2 ohm speaker is a 4 ohm. The D.C. resistance is 3.2 ohms. The nominal impedance is 4. Hummmm.... I wonder if I have any of my 8's in 4 ohms in stock?.....

[@zjokka]

Thanks so much for sharing this. I face a similar problem with a tweed reproduction -- would it help to switch the 10k and 22k around, or should I just recreate the Blackface power rail to stabilize my amp? The answer is of course: just try it. This still is the most relevant video on Champ biasing on the yt.

[@guitarc.p.r.9675]

I would need to se a schematic of the tweed repro. The original late tweed circuit is actually more correct than the BF. Personally I believe 22k is too large for the screen resistor. Thanks for the comment, I appreciate it.

[@zjokka]

@@guitarc.p.r.9675 I'll rephrase my question: how would you tackle this problem of drifting bias in a 5F1 Tweed Champ with the 'more correct' 10k and 22k already installed. I tried installing a screen resistor on the power tube, which really improved the sound of the amp, but didn't fix the drifting bias... EDIT: Now that I watched the video again, there is a screen resistor on this amp -- at least there is a resistor mounted on the 6V6 tube socket. I'm confused you didn't mention it and I didn't see it until now....

[@guitarc.p.r.9675]

Sorry for the late reply, somehow I missed your comment. OK, short answer. Are we drifting hot, or cold? As far as drifting bias goes in a champ there are just a few possibilities. (1) Old, worn, damaged output tube. (2) Drifting cathode resistor, and or bad or leaking cathode bypass capacitor, and (3) a leaky grid capacitor. There are also the possibilities of output transformer issues, or a non-audible parasitic oscillation. But I would bet on the top 3. Oh, also there was no screen resistor in this amp. The tone police frown on them, plus it did not need it. Thanks for watching.

[@YEARS_OF_TEARS_BLUES]

I have a pre CBS 65 Fender black face vibro champ then I'm wanting to trade all original unmolested sounds good does get a little hot but every single thing on it is original except for half of the cord an old lady used to play Amanda Lynn through it and on a gig back in 1973 I think she told me they was down in Tennessee leaving a gig and she slammed the cord in the door which cut the cord so they just wanted a piece of a cord off of something on to it and she played a couple times after that and it's been sitting ever since I just got it on the second owner but every single piece of its original except for the end of the cord power cord I'm looking for something a little bit bigger I'm a blues player someone stole my Fender Blues the veil I wish I had to amp back had the four blue 10 in Alico speakers in it

[@trbr1799]

Hello! First, thanks for this video. I have a quick question. I've watched this video a couple of times, and want to clarify. You left the bias resistor at 470 ohms, correct? Then, you swapped the 1k with the 10k, and that increased stability with a signal, and it also reduced signal going to the 6v6. Correct? Thanks

[@guitarc.p.r.9675]

No. Swapping the resistor was to limit screen current. Same circuit as in earlier and other amps. It has nothing to do with reducing the signal to the 6V6. Hope this clears it up. Thanks for watching!

[@trbr1799]

@@guitarc.p.r.9675 thanks for your response! I didn't word my comment well. I didn't mean reduce signal but reduce voltage under signal, when turning up the amp. I see you've stabilized screen current. 👍 thanks for the video!

[@oldmanofcotati]

Oh boy.

[@dembydish]

Hello, would you recommend 1 watt (as schematic) or 2 watt resistors for the change over (1K/10K). Thank you.

[@guitarc.p.r.9675]

I always use 2 watt resistors in the power supplies. Thanks for checking out the video.

[@sallhame]

Hello and thank you for uploading this. Today I got an old Super Champ delivered by mail. Serviced last september. Turned it on, and it played well. Then, while playing very quiet, it blew the T315mA fuse. I changed fuse and the amp played well again, just to blow that fuse once more. Then I called a technician who told me to check the bias. Could my problem be fixed by swapping to two resistors you are mentioning here? Or, is it likely that the tubes have been damaged while being transported? (by mail) Martin, Norway

[@guitarc.p.r.9675]

Thanks for watching and commenting. Unfortunately the Super Champ is an entirely different amplifier, in a push pull configuration instead of single ended as an original champ. This does not work in your amp. It is hard to guess what may have happened, way too many variables.

[@guitarc.p.r.9675]

Also I want to add..... If the amp uses a 3 amp fuse I assume it is a Super Champ X2. This amp is entirely different than the older ALL tube Super Champ (1 a fuse) The output stage is the only thing similar. The X2 has a tube phase inverter and 6V6 outut tubes as well, BUT has an entirely solid state and digital front end with 3 voltage regulators and a bunch of stuff to go wrong. Look for some one to service it.

[@sallhame]

@@guitarc.p.r.9675 Thanks for your answer. It is an old Super Champ from the Rivera periode. I have been told that the blown fuse should control the output tubes, and that this again in controlled by a bias resistor. I'll try to find a capable technician then. Thanks for your consern :-)

[@guitarc.p.r.9675]

@@sallhame Just a few more tips. The main fuse in that amp should be a 1 1/2 amp Slo-Blo. This fuse is on the AC line coming into the amp and controls the entire amp. If the tubes are old or damaged it may draw more current as it runs until it reaches the limits of the fuse. Super Champs run a bit on the warm side anyway, so that does not help. If the tubes are good, which they may not be at this point if they have red-plated, another culprit can be the bias supply capacitor. Good luck, I hope this has helped.

[@sallhame]

@@guitarc.p.r.9675 Thanks again for your answer. My Super Champ has two fuses. One marked T1A, and the one that blows are marked T315mA. I am told this fuse controls the current to the outputtubes. The tubes might have been damaged under transport, so I will try with new tubes and have the bias checked/adjusted at the same time. Thaks again :)

[@pabloquiroz4022]

Hello. Thank you for your video. I just did this swap on a '76 vibrochamp but the dissiation is still a bit high at about 17 watts. Would I keep increasing the resistance of the 10k ohm resistor until i get an acceptable dissipation?

[@guitarc.p.r.9675]

No, do not raise the 10k higher. First check the 470 ohm resistor and 25uf cap on the cathode of the 6V6. You could also have a hot tube. I am curious as to how you are measuring the plate dissipation.

[@pabloquiroz4022]

@guitarc.p.r.9675 I measured the voltage drop across the 470 ohm, which was 21V and then divided that by the resistance value, which was around 465 ohms to get 45 mA of current. I then multiplied this by the plate to cathode voltage, 380 V, to get 17 watts of dissipation. The bypass cap should be fine. I just replaced all the electrolytics, since it came to me with the originals.

[@guitarc.p.r.9675]

You are calculating screen current plus plate current. Measure only Plate current.

[@michelkowalczuk6921]

Hello, For measuring accurately the plate current without opening its loop, what about soldering a 1 ohm 1% resistor in series with the plate and using a (milli)voltmeter? I guess the DC resistance of the primary output transformer is largely higher than this 1 ohm so it would be negligable. Good idea? Thx for the numerous vids you provide. I would say " keep goin' "! Mike.

[@guitarc.p.r.9675]

Yes, you certainly can add 1 ohm resistors in line with the plate, and the screen. This also still allows you to operate the amp at high levels and track operation like inserting the meter. Just measure in mv across the resistors. And Be Careful these are 2 of the 3 highest voltages in the amp.

[@natep77]

Sir, I’m looking at a friends silverface Vibrochamp. Measuring ma from 6v6 pin 3 to center tap I’m getting 74 ma! With plate voltage of 354. Voltage at center tap 367. That bias current seemed way too high so I searched around and found your video. I swapped the 1k and 10k resistors as suggested (lacking a 22k on hand). This change resulted in plate voltage 364v, center tap 376v and plate current 62MA. This is an improvement but can you suggest a next step to get the bias closer to the suggested 30ma?

[@guitarc.p.r.9675]

First the plate voltage for a silverface is too low. This is most likely due to the high plate current. Make sure you have no signal when shunting the output transformer to measure current. As for suggestions, first thing check the cathode resistor and bypass capacitor. These fail regularly. Do not raise the cathode resistor over 500 ohms... period. Also the 6V6 may just be failing and pulling too much current, try another one.....after changing the cathode resistor and cap. Tubes vary widely in plate current, you may need to find one it likes. If you can get the plate current down to around 35-40 ma you are good. Thanks for checking out the video, it is an older poor one but the information still conveys.

[@natep77]

Thanks for the response. I’m going to get back into this another day but I will take your suggestions. Interesting idea to use the tweed circuit values.

[@guitarc.p.r.9675]

As I said CBS had a LOT of good ideas...... the practice of raising the plate voltage to 420 volts and uping the wattage to 6 watts than the previous 4 was one of them. The Champ is a very basic minimal circuit, and also lacks screen current resistors and and grid resistors on the 6V6. So this does not add to the stability.

[@RS85851]

Hello @Guitar C.P.R I built a 5F1 champ (mojotone tweed kit) and installed it in a silver faced style cabinet and chassis that I also built. I want to upgrade it to a vibro champ without the tone circuits. What are your thought? Can I email you the schematic that I'm thinking of using? I have a few questions about where the vibro circuit meets the cathode of the 12ax7 preamp cathode. Thanks

[@guitarc.p.r.9675]

Sure, you can email me. The contact info is in the about section. I already have questions and suggestions. Plus, I have a design issue with all preamps without a tone circuit. We can discuss that as well. In the mean time I will check transformer voltages and impedances. I hope you found this video useful. It is an old one and poor quality but the info is correct.

[@RS85851]

Yes I did enjoy the video. Also subscribed. I may be missing something or just dumb haha, but I don't see an email address in the about section.

[@guitarc.p.r.9675]

It is there..... open the about, scroll down to details below the description. It is to the right of business inquiries, click on view email.

[@RS85851]

@@guitarc.p.r.9675 I sent you an email. Thank you

[@CC-te5zf]

First, thank you. I'm a novice. I built a 5F1 from a kit. It has the 10K in the first position, between positive leads of 16 uF and 8 uF filter caps, then 22K in second position between positive leads of the two 8 uF caps. With this 470 ohm cathode bis resistor at 470 ohms, I get the following readings: Plate to cathode, pins 3 and 8, 6V6: 323 VDC Voltage drop across the resistor: 19.53 VDC Plate voltage: 342 VDC Screen voltage: 303 VDC Cathode voltage: 19.5 VDC CBR Value: 478 OHMS According to what I've learned up until watching your video was that plate current is 40.8mA and plate dissipation is 13.97W at idle. What would you suggest to improve this; or is it good as-is? Thanks so much! I will watch this video a couple if times - learned allot on the first watching for sure!

[@guitarc.p.r.9675]

First of all, thanks for checking out the video. That one is long and boring but there is a lot of info , and I could not convey it any other way. There is a lot of bad and wrong info out there, and it really ticks me off to see the "experts" regurgitating it constantly. So according to your plate voltage numbers, I would say the 6V6 needs to idle at about 38 ma. I am curious as to how you are measuring the plate current or just doing the math. Make sure you are not measuring plate+screen current. That will make the dissapation look too high. Once we get past the 6V6 bias thing, I do have a few circuit suggestions for you.

[@CC-te5zf]

@@guitarc.p.r.9675 Just math: I used the following: voltage drop across bias resistor divided by value of the resistor. Then multiply that number times the plate voltage. Thank you!

[@guitarc.p.r.9675]

Forget the math numbers can lie. Learn to do actual measurements. I would say it is probably runnung very close to correct. But I would like to see the actual plate current.

[@CC-te5zf]

@@guitarc.p.r.9675 OK - I can certainly check that. What is the best way to actually hook up the meter, etc. Which pins and what setting on the meter? I recently acquired a nice set of clip on leads that I've been waiting to try out. Thank you!

[@guitarc.p.r.9675]

To measure plate current put the meter in between the output transformer lead and pin 3 on the power tube. Set the meter for DC MA. I use the 200 ma setting for idle measurements. You can do the same measurements on the screen by connecting between the screen supply wire and pin 4 of the 6V6. USE CARE these are the highest voltages in the amp!

[@mikeault]

The upper blue resistor color codes are hard to see with the yellow cloth wire blocking and I'm not understanding the color code for a 22k should be red red orange correct? thank you for a great and helpful video.

[@guitarc.p.r.9675]

The top blue resistor is 1k, The lower resistor is 10k and yes a 22k would be red red orange. Thanks for checking out the video.

[@Tony-st6yd]

What if you uses both 10ks?

[@guitarc.p.r.9675]

It will just drop the pre-amp voltages a little. It will darken the tone slightly and compress quicker in the pre-amp. Thanks for watching!