How to Bias a Quad or Quartet of Output Tubes......Theory and Practice

In this technical video, we will discuss the theory and practice of how to bias a quad or quartet of output tubes. Basic biasing theory is covered, then applied to a double-ended pair of output tubes, for review, then finally applied to a matched quad set of output tubes. Calculations are discussed, as well as proper maximum plate dissipation values, and how they apply to cathode-biased and grid-biased circuits. In addition, safety practices are fully explained and encouraged. For a hands-on demonstration of this biasing technique being applied to a Fender AB-763 Twin Reverb, please click on the link below, in the video description.
NOTE: This video depicts a manual method to bias quad amps. If you are fortunate enough to own a Eurotubes Bias Probe, you can simply "march it down the line" of the four output tubes and read the biasing data for each individual tube......eliminating the shortcomings associated with "average" PD for each pair. Also, my suggestion to use the PV to detect disparity in a parallel pair won't work. Since the tubes are parallel, the PV will always be the same for each.
If you enjoy advertising-free videos like this one, please be sure to SUBSCRIBE to our channel and click on the bell to be alerted each time a new video is posted. Also, please consider becoming a Patreon patron of our channel by visiting this link: / uncledougsvintageamps or making a PayPal donation to: [e-mail]. Thanks so much for watching !!!
Link to Fender Twin Reverb video which demonstrates the biasing of quads: • 1968 Fender AB763 Twin...

[@UncleDoug]

I realize that technical videos are not as entertaining or popular as regular videos, but I felt compelled to post an easily searchable how-to video on a frequently-requested topic. As mentioned, the '69 Twin Reverb Extravaganza will be posted soon.....so turn your beer coolers down to 31.9 degrees F and get ready !!!

[@billgreen4592]

For those of us the fix/build amps, these videos are what really makes you channel valuable! Thanks for this content.

[@paulvale2985]

Informative AND entertaining Unc, Keep up the good work 🔊

[@knifeswitch5973]

You do what ever the hell videos you want to do Doug. It’s all brain candy and it’s all good

[@jogreeen]

I love watching Jack riff on refurbished amps, but these tech vids are some of your best, there aren't many people doing this stuff on KZbin.

[@Wildman9]

A AN 764 model Twin Reverb,whoops. These type videos are just fine for those of us who like to do biasing , on our own and doing it right. By the book.📖 🎸👍

[@MrGTO-ze7vb]

Uncle Dougski is one Fantastic Teacher..!!! some teenager in 2056 will need to watch this video after inheriting a Fender Amp and purchasing new output tubes.

[@UncleDoug]

Thanks so much. I sure hope so, Mr. G. :)

[@ElmoSyr]

Thanks again Doug! I remember watching these videos a few years ago and having no idea how to even start touching tube amps. I've since then become a part of a small repair tech team and am making an alright side business with it. A big start and encouragement for me were your videos. So I'd like to thank you once again for making such great videos!

[@UncleDoug]

That's great too hear, Elmo. We're glad the videos were informative and helpful :)

[@willclarke4631]

I’m going through all these technical videos again preparing for my second amp build. Got to say doing a hands on project really helps me understand the nuances of these excellent videos. I’m having so much fun doing these projects and I don’t know if I could do it without Uncle Doug.

[@UncleDoug]

That's great to hear, Will, and the perfect way to master a new skill.....develop a solid foundation through study, then put what you know into practical use.

[@shekador]

As Alanis Morissette once sang, "I've got one hand in my pocket, and the other one is measuring plate voltage"

[@UncleDoug]

Alanis was one of our best students, Shek ;)

[@Rightnow0101]

I thought that but didn't want to post it. :)

[@Rightnow0101]

@@DL-oy2qn Yeah, I really do think....and I've seen the effect of grounding a metal watch band to a chassis in person. It leaves a mark for sure.

[@PandemoniumMeltDown]

@@DL-oy2qn Don't you think? A little toooo ironic... yeah!

[@billguitarvin]

shekador - LOL!!!!! I got a good laugh from your comment. Thanks for the funny comment. Always wondered what the lyrics were in that part of the song. 🙂

[@jeremiahfiek5495]

Thanks man. I've been racking my brain for days... and everyone else on youtube or forums likes to beat around the bush and/or give poor incomplete information. I owe ya one!

[@UncleDoug]

We're glad the info was helpful, Jeremiah.

[@brucebuckeye]

Thank you Uncle Doug!! I have learned so much from you this year, since I started watching. I was an electrical engineer for 34 years in the Air Force (civilian), and I retired on 31 December 2019. I have always strived to be able to explain things easily like you do and keep the calculus out! I like to understand what is really happening and why - that is the way to solve problems! You are a master at teaching! Stay healthy!

[@UncleDoug]

Thanks so much, Bruce. Will do......you too, I hope :)

[@svartrav8259]

One of the best videos on the internet

[@UncleDoug]

Thanks, Svart :)

[@DonnTarris]

Hi Doug, I've just picked up Epiphone's answer to a twin style, quad output guitar amp. I'm reviewing two of the videos you've done on the Fender Twin models (1968 and 1978) and this video on biasing quad output tubes, couldn't be more valuable. The Epiphone Embassy is very similar to the Gibson GA-95RVT, other than one of the 9 pin tube locations using a 12AU7 (Epi) instead of a 12AX7 (Gibson). The Epiphone I've picked up is from 1966. The amp had been "serviced", but the tech hadn't realized that someone before him had added a 6th 9 pin tube (12AX7) to the chassis in a factory opening - probably used in another model that used the same chassis. Now I'm in the midst of going over the schematics for the Embassy amp, the GA95RVT (which the tech who had checked the amp out figured was closer to the amp) and a fabulous find - a meticulously created drawing of the eyelet board for a GA95RVT that I found on a blog. Like the 1978 Fender Twin you went over, this amp has to be checked end to end to figure out where I'm starting at - a very good part of this educational journey. Getting there, thanks to what I've learned from watching a multitude of the videos you've created! While the more technical videos may not get as many folks viewing, they are greatly appreciated by folks like me!! Cheers from the wet coast of Canada!!

[@UncleDoug]

Wow.....Thanks for sharing this with us, Donn. It sounds like quite an excellent acquisition. Best wishes for continued success with your circuit overhaul.....and for a wonderful holiday season.

[@scottbc31h22]

VIDEO REQUEST Please do a video about the relationship of the tube output impedance and transformer impedance when using a quad of tubes, As a follow up to this video and your series regarding output transformers. Thank you! PS I love your videos! These are the best, easiest to understand, tutorials I have ever seen.

[@UncleDoug]

As you probably know, Scott, identical resistors in parallel have a total resistance equal to half of either of the individual resistances.....i.e. two 100 Ohm resistors in parallel have an effective combined resistance of 50 Ohms. The same is true with quad tube sets. The Load Resistance is one-half that of a double-ended pair.

[@scottbc31h22]

@@UncleDoug Thank You.

[@bertrodriguez3087]

Technical videos are the reason I fell in love with your channel. I don't know if I love you or hate you (LOL), but having worked on just about everything in my lifetime (I'm sure, just like you), your "How Tubes Work Videos" set me on a path. Again, I don't know if that was a good thing, because I've had to fix a ton of amps for my musician friends (and I have many) over that last few years. And I have fixed a bunch of circuit board amps (which I know you don't like to do), Peavey 6534+, 5150, Marshall 800, 900, 2000, etc. My background is in computers so I was used to circuit boards. This was all thanks to your very clear and precise explanations on the "WHY and HOW" it works like that (for tubes and amps). To me, at least, that's top of the list. You must have been a hell of a teacher. We need more teachers like you in this day and age, and I don't mean to embarrass you, that's just a fact. God bless you and thanks for all you do.

[@UncleDoug]

Wow, thanks so much for your very nice comments, Bert. Hearing from viewers like you makes it all worthwhile :) Likewise on the blessings.

[@erra9490]

This is gold! Thank you. If I had this useful application of math in high school, I might have retained it back then. Thank you Uncle Doug ( and all your supporters) for sharing these videos and tutorials.

[@UncleDoug]

You're quite welcome, ER. Glad you enjoyed the video.

[@jedishaw6771]

I personally really like your technical videos! Thank you.

[@UncleDoug]

I'm glad to hear that, Jedi :)

[@jasongcicero3488]

That was the best explanation/presentation of the process of biasing that I have ever seen. You are a really good teacher!!!!

[@UncleDoug]

Wow, thank you, Jason.

[@anthonyderosa5181]

Love your videos Uncle Doug! With all due respect I would like to point out one thing. In your example of the quad output tubes (14:28) you discuss plate voltages on the two plates which are connected in parallel. The voltage measurements will be the same, they will not differ on the two tubes. What will be different, unless the two tubes are perfectly matched is the plate current of each tube. The rules for a parallel circuit are as follows: The voltage on each branch of a parallel circuit is the same, the branch currents add up to equal the total current. Thank you for making these great videos.

[@UncleDoug]

Good point, Anthony. Thanks for your helpful input. I have added a statement to the video description clarifying this issue. Regardless of the relative current flow through the paired, parallel tubes, the voltage measured at the plate (anode) will be the same.

[@rossthompson1635]

I thought that but then I wondered - yes that would be true for grid biased tubes (ground to the common point between the plates), but wouldn't the voltage still differ for cathode biased tubes - i.e. you are measuring voltage across the tube which will be proportional to the current through each?

[@Dr_Reason]

@@rossthompson1635 I think they cannot vary in voltage since they are connected to the same voltage source. Their amp flow can vary if the tubes vary.

[@bobvecchi7981]

@@rossthompson1635 no, the voltage will be the same, but the current will be proportional as a result of the tube’s internal resistance being different, hence the term mismatch”.

[@ichbinnutzlos5561]

Change one opposite pair of tubes to check wether plate current changes without changing bias.

[@gavindunn8770]

Nice Video, for anyone looking for this info, this is definitive. Wish i Had your Vids 10 years ago! Thanks Doug

[@UncleDoug]

Thanks, Gavin. I wanted to release a dedicated, searchable video on the subject, so people could access it when needed.

[@smeh3929]

Дуже дякую вам за відео і чудову інформацію ! 👍👍👍🙂

[@UncleDoug]

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

[@Lee-NN7X]

I really enjoy the tech segments.....good lessons for my little brain...

[@UncleDoug]

Glad you enjoy them, Lee. Thanks !!!

[@lifegettingintheway2710]

There is an error at 14:39 when describing the plate voltages of tubes with dissimilar plate current. Because the tubes are hardwired in parallel they will always have identical plate voltages regardless of the plate current each carries. It is identical to two resistors in parallel but having dissimilar resistances - the voltage measured across them will not change but the current they carry will be different. With tubes this simply means the tubes have dissimilar internal resistance. In cathode biased circuits the cathode to ground voltage will be very different between parallel tubes. The cause of unbalanced plate current could indicate a manufacturing error or external component/soldering problems (screen or cathode resistors, for example). Thank you for all you do to educate all of us. These technical discussions are fascinating to this old tube amp engineer.

[@UncleDoug]

Thanks for your knowledgeable input, LG.

[@hestheMaster]

Amplifiers are very dangerous to work on unless you know what you are doing. Think safety ,safety ,safety. Plug your amp into an isolation transformer while working on one if you need to power it up to test voltages and currents. If after you do power an amp up do not assume the electrolytic capacitors are safe to work on because they can still hold a lethal voltage as well. They must be discharged through a resistor . They can hold a high voltage for hours or even days. Thanks for the biasing output tube video professor Doug! Always something new to learn!

[@UncleDoug]

Thank you for your wise advice, HM :)

[@DrHumbertoable]

Thirsty for theory , thanks alot for a great video. All thumbs Up!

[@UncleDoug]

Glad you enjoyed it, Dr. H.

[@Silent-Lucidity]

Another excellent tutorial! Thank you UD for all this great content!

[@UncleDoug]

You're welcome, S-L :)

[@alexkatsaros6180]

I’ve been ready Uncle Doug!

[@UncleDoug]

Good to hear, Alex :)

[@rolandmengedoth2191]

Execellent explaination of the subject without very deep maths. Just ohms law, thats it !

[@UncleDoug]

Glad you enjoyed it, Roland :)

[@richreid7098]

A great teacher at work is a real blessing!

[@UncleDoug]

Thanks so much, Rich.

[@zaraak323i]

Great information, as usual. I always appreciate those willing to share their knowledge. Thanks!

[@UncleDoug]

Our pleasure, Z. Thanks for watching :)

[@jayahn9399]

I have been enjoying your video so much as of late. Thank you so much, please keep it up, and keep us informed on the subject matters.

[@UncleDoug]

Thanks, J. Will do :)

[@tsloth1390]

Great video, Doug!

[@UncleDoug]

Thanks, Tommy. Glad you enjoyed it.

[@vhm14u2c]

Wonderful information as always, and thanks uncle Doug!

[@UncleDoug]

My pleasure, Vince :)

[@charleslambiase5670]

Learned a lot from this conversation here of yours today!!!!!

[@UncleDoug]

That's great news, Charles.

[@Bigjoedo66]

Another great educational video Uncle Doug!!

[@UncleDoug]

Glad you enjoyed it, Big Joe.

[@suomi35]

I love your technical videos...I always learn something from you! Cheers!

[@UncleDoug]

Glad to hear it, Suomi :)

[@andrewhorne7443]

Thanks Uncle Doug, that is perfect information for my Vox AC30. You're the best!

[@UncleDoug]

Glad to help, Andrew. Thanks !!

[@roadshowautosports]

Amazing all the detail and take our hands though you do! You are such a kind individual on your teaching! Thank you uncle Doug

[@UncleDoug]

You are very welcome, Edson. Thanks for your nice comments.

[@kirkgreenman1386]

Awesome! Going to watch that again.

[@UncleDoug]

Things generally make more sense the second time around, Kirk :)

[@robertjones6324]

Excellent lesson! Glad you gave us the safety precautions too.

[@UncleDoug]

Glad you liked it, Robert. Be safe :)

[@GScott50]

Thank you so much for posting this Uncle Doug, I am about to check the bias on my Princeton Reverb.

[@UncleDoug]

Glad it was helpful, GS. Good luck with your project.

[@oldguy5381]

Thanks Doug, always good information from your channel

[@UncleDoug]

You're very welcome, OG.

[@bennynelson7220]

Another excellent video , I very much appreciate your time and effort in putting forth these technical videos and enjoy them very much.

[@UncleDoug]

Thanks so much, Benny. Glad you enjoy them.

[@richardstanley8460]

Your technical videos is what led me to subscribe to your channel in the first place.

[@UncleDoug]

That's great to hear, Richard. Thanks !!!

[@magnusekhall]

I love the more technical videos!

[@UncleDoug]

Thanks, Magnus. They aren't as popular as my regular videos, but it's important to have this information on YT as reference material for viewers needing help.

[@magnusekhall]

@@UncleDoug To me the technical videos are at least as interesting as the regular ones. I have learned so much from watching your technical videos. I don't think I've seen better videos explaining how electronics with tubes works.

[@stephenrioux6821]

Thanks, Uncle Doug. I just started studying these circuits in the last week. This is a big help.

[@UncleDoug]

I'm glad to hear it, Stephen. Best of luck with your self-education.

[@alasdairmunro1953]

I applaud you for you detailed safety brief when working with energised equipment. 👍🏻👍🏻👍🏻😀

[@UncleDoug]

Thanks, Alasdair. I hope it leaves a lasting impression :)

[@irusman62]

I love the technical videos the best, but its great when you mix it up.

[@UncleDoug]

Thanks, IM.

[@johnwebb2562]

Thanks Doug, for another Jem info video. Best regards, John.

[@UncleDoug]

You're very welcome, John :)

[@rjprivate]

Very useful and as always interesting to learn. Thanks uncle Doug

[@UncleDoug]

You're very welcome, RJ.

[@wegert1]

Thanks UD. You too. Great vid as always.

[@UncleDoug]

Our pleasure, RSF :)

[@Danyael850]

If I had such a teacher in my electronics class in high school, I would surely remember anything :-)

[@UncleDoug]

Thanks, DR :)

[@46bovine]

Thanks Uncle Doug. A great video.

[@UncleDoug]

Glad you enjoyed it, Bob.

[@mattrogers1946]

Thank you for another informative video.

[@UncleDoug]

Our pleasure, Matt.

[@jameswatkins7263]

Once again, excellent information. I'm working on a another Peavey Classic 50 2x12. Having issues with reverb, so I'm hitting up your Reberb videos again. You are a virtual book of info.. love it..

[@UncleDoug]

You're quite welcome, James. Best of luck with your repair.

[@framusburns-hagstromiii808]

Excellent explanation! Short, sweet and to the point. Thank you for stressing the ' keep one hand in your pocket' safety rule...it saves lives. 😀😀👍👍👌👌☕☕🤗🤗

[@UncleDoug]

Glad it was helpful, FB. Thanks !!!

[@michaelbutler2312]

another great video. thank you!

[@UncleDoug]

My pleasure, Mike.

[@MrCayoCoco]

I'm watchin' and learnin' !!

[@UncleDoug]

Good news, Mr. CC :)

[@chivato666]

save lives with this video.,. Thanks uncle

[@UncleDoug]

That would be wonderful. Thanks, Orlando.

[@gerryjamesedwards1227]

Hi Doug, I just wanted to extend my heartfelt thanks! I've just completed (all bar the faceplate) a conversion of a horrible, noisy Hayden PCB-based amp into a Brownface Princeton clone. Only the chassis and cabinet made it all the way thru the process, which are fine, sturdy items. The end results sound great, it's very quiet -noise-wise- and the bias-vary tremolo is just as nice as I hoped. I couldn't have even attempted such an undertaking without your valuable tutelage. Thanks again!

[@UncleDoug]

That's great news, Gerry. It's really gratifying to hear that our videos have been put to good, practical use. Thanks for making our day :)

[@RHR-221b]

@@UncleDoug 💚

[@kkrekula]

Thanks for the insights, humor and nice choice of songs for strumming on the guitar ;)

[@UncleDoug]

The pleasure is ours, Ken :)

[@Rightnow0101]

Interesting theory and technical content as always!

[@UncleDoug]

Thanks, CR. I hope it's helpful.

[@Rightnow0101]

@@UncleDoug Very helpful. Soon I plan to begin going through a signal tracer and vtvm by Eico (kits my dad assembled) and a millivoltmeter by Leader. I've also been restoring my 1928 Brunswick 5N0 (RCA Rodiola 60). I'm a machinist by trade but electronics have become a new hobby. I also turn wrenches from time to time hince the caveman. repair name.

[@nelsonmedley4349]

Congratulations... Real good explanation. Lots off sucess for you my friend, big hug from Holland.

[@UncleDoug]

Thank you very much, Nelson. Likewise on the hug :)

[@cassvirgillo3395]

Hello Uncle Doug & Family, What a great tutorial, you are a really good teacher. Always great to see an Uncle Doug video. After nine days of smoke, starting on Labor Day, the air is clear and have had some good rainfall. Catching up as have been working some crazy long day's at work, 10 to12+ hours a day. Glad you are doing well. Checking the Hot Rod channel as am able. Lately, bed has sounded pretty good. Came through the fires OK, closest was 10 to 15 miles away, nothing in my immediate area. Like Arnold might say, "If you want to live, keep one hand in your pocket". Be good, all the best from Oregon, Cass.

[@UncleDoug]

Thanks for the update, Cass. It's great to hear that you came through the conflagration unscathed, especially your left hand. Catch up on your sleep and stay healthy.

[@TonyAllmanMusic]

This Video is very informative and right on time. i look forward to part two in order to observe your actual measurements and calculations on this twin. i am currently working on my twin Reverb AA769. This sure does help. Thanks Uncle Dough.

[@UncleDoug]

Glad it was helpful, Anthony. Best of luck with your project.

[@olipito]

Superb video! Of course we want the practical demo!! Thanks!

[@UncleDoug]

Coming soon, Oli.

[@chadcohoon8867]

Thank you very much for the video .. Top shelf info

[@UncleDoug]

Glad it was helpful, Chad. Thanks !!

[@deanmesserer3554]

Just realised that for both scenarios you could measure plate to cathode as a grid biased amp would have cathode connected to ground anyway. That saves me trying to remember. Thanks for another great video UD and all the best from Australia!

[@UncleDoug]

You're right, Dean. You can't go wrong with plate-to-cathode.

[@gerrabath]

Doug you read my mind!

[@UncleDoug]

That's great news, G. I hope the video is helpful.

[@gerrabath]

@@UncleDoug Doug my friend your videos are never anything but helpful. You have taught me so much over the years. Thank you.

[@claudiuclaudiu8331]

Thank you for a great video !

[@UncleDoug]

My pleasure, CC. Best of luck :)

[@steverollinson6906]

I do wish you posted a Eurotubes probe owners version too! I think most of your fans have bought them and would love a tutorial!

[@UncleDoug]

It's so easy with the Eurotubes probes that you really don't need a video, Steve. Just march down the line and get measurements from each tube.

[@steverollinson6906]

@@UncleDoug Thanks!

[@djfrank59]

Hi Doug! once again, another valuable and informative video. Entertainment is one thing, but the young and upcoming techs NEED to watch the most important videos such as this one. I like entertaining videos, but I also like the meat and potatoes when it comes to the theory. Your explanation is spot on. Without the theory, the how and why remains unknown, and its like shooting in the dark without a flashlight which isn't good; the bullets can fly anywhere. There are so many different ways to measure and set bias, but a VTVM and pencil and paper tells the accurate truth which I feel is important. I think a balance of both, entertainment and theory would be the best direction to go in. You will have it all covered on both ends that way. I enjoy the non-entertaining as well; that's why your past videos were a great teaching aid when I taught at the vocational school. Keep this great flow and balance going. you're onto something good :)

[@UncleDoug]

Wow, thanks so much for your very nice comments, Frank. Coming from you, they mean a lot.

[@Eddy63]

Great informative vid ...

[@UncleDoug]

Thanks, Eddy.

[@Justplast]

A1 as as usual.

[@UncleDoug]

Wow, thanks!

[@GL64]

Clear and concise and informative as always. Thanks! I've been steadily adding 1% 1 ohm resistors to the cathodes of my grid bias amps to get quick and easy bias readings at each tube. It's a little work initially, but the end result is easier than fumbling around with hot tubes and bias probes. The manual math method is about as true as it gets though.

[@UncleDoug]

Thanks, GL. That's one way to determine the PC matching within each pair of tubes......instead of just getting the average.

[@ertreri]

super good video. many thanks

[@UncleDoug]

You're welcome, E. Glad you enjoyed it.

[@emergencynurse02]

Great Video...! Thank you ..... Indecently Defibrillators DO NOT Restart a heart....They will not deliver a shock unless they sense a rhythm....Defibrillators are intended only to correct a life threatening rhythm....Like Vtach or Atrial Fibrillation that causes loss of consciousness and impending death. If the heart is in asystole (no detectable rhythm) then you need to continue CPR until the problem is found and corrected.

[@UncleDoug]

So those little paddles they apply to start a stopped heart after surgery or a transplant are just TV make believe, GH ?? Those lying phonies !!

[@rogerhowell9474]

Great and informative video as always Mr. Doug. Thank you, Roger in San Angelo.

[@UncleDoug]

Our pleasure, Roger. Always good to hear from a fellow Texan.

[@andreberger7847]

Hi, I love your video’s. Very educational and inspiring. I love to build tweed amps. They sounds great en and they’re easy to modify. Ive learnt a lot from you. Many thanks. Greetings from Holland. Drebus

[@UncleDoug]

Thanks so much, Andre. We're glad to hear that our videos are helpful. Continued good luck with your projects.

[@michaeldowning796]

Thanks UD!

[@GeorgeChristofi]

A great explanation video of how to measure plate current Doug. Looking forward to the video showing how you actually set the bias on both cathode bias and grid bias. I'm guessing with the cathode bias you add to the plate resistance and with grid bias you change the grid voltage.

[@UncleDoug]

Thanks, George. In cathode bias the cathode bias resistor value is adjusted to alter the negative charge on the grid, relative to the cathode.

[@terencekaye9948]

Hi Doug,, Super Great Tech Video,,,, Thank you for posting it,,,

[@UncleDoug]

You're very welcome, Terence. Thanks !!!

[@nickdevriese]

And this is why I stick with single ended class A amplifiers. My brain can’t comprehend anything more than simplicity. But, I appreciate your videos. God bless!

[@UncleDoug]

Thanks, Nick. Likewise on the blessing.

[@nickdevriese]

Uncle Doug On a side note, which brand of current production tubes you recommend? Obviously NOS will always prevail. But, money is tight at the moment. Just an honest question.

[@billphillipstube]

Great video. Got a little confused at first because you mentioned 6L6s in the intro, but your paperwork used a 6V6 example. However, I understood perfectly what you were explaining. Cheers!

[@UncleDoug]

I'm glad it made sense, Bill. Thanks !!!

[@MrGTO-ze7vb]

I just purchased a 1990? Fender Twin Amp. It was just missing 2 output tubes, all the pots needed Deoxit cleaning and the 2 speakers were out of phase. I purchased a matched quad set of tubes and installed them. Luckily on the back panel there is a bias adjustment pot, test points and output balance adjustment

[@UncleDoug]

I hope it turned out to be a great amp, Mr. G.

[@MrGTO-ze7vb]

@@UncleDoug So far ... Very good.!! Someone replaced lots of the caps!! It does put out some great tones..! Thank you Uncle Doug..!!

[@darthbubba866]

Always good to revisit the basics every now and then, especially the "keep one hand in your pocket" rule for measuring voltages greater than your shoe size. I wish that I worked on hollowstate devices more often (and actually charged for it.) Then I could afford a Eurotubes Bias Probe.

[@UncleDoug]

We're glad you joined us, DB. Best of luck to you :)

[@richreid7098]

Excellent!

[@UncleDoug]

Glad you liked it, Rich.

[@edwardhannigan6324]

Great video Doug, so much information and especially the safety instruction..I'm still learning...Look forward to more videos.I'm like a kid in a sweet ( candy) shop..Geat stuff Doug, thank you once again for sharing your knowledge..Ed..U.K.

[@UncleDoug]

You're quite welcome, Ed. Best of luck with your continuing education.

[@Yupppi]

I'm a big fan of these videos, I've learned incredibly useful things from these tech talks. Would you touch the topic of cold/hot biasing and how it changes the behavior and sound?

[@UncleDoug]

That's good to hear, Yup. I have been considering such a side-by-side comparison for quite a while. To be honest, much that I have read about the effects of hot/cold biasing does not seem to be accurate. Such a comparison might be very interesting.....so stay tuned.

[@jayster.k.wiseguy]

am so honored, sir~....jayster~

[@UncleDoug]

I thought your painting really looked great in the video, Jay. Just wait until Twin Reverb video in a week or so. Both the paintings and you get recognition.

[@remi3741]

very usefull video doug thanks

[@UncleDoug]

Glad it was helpful, Remi.

[@danielsaturnino5715]

Still subscribed! Thanks for sharing!

[@UncleDoug]

Good to hear, Dan. You're quite welcome.

[@cbennz514]

Another wonderful video, thank you!! By far you have the most informative youtube tube amp channel. Any chance you could explain an ultralinear amp to power low efficiency speakers? Thanks again!

[@UncleDoug]

Thanks so much, CB. I will keep your suggestion in mind.

[@jorgemellooliveira9611]

Obrigado mestre

[@UncleDoug]

Likewise, Jorge :)

[@kenporonin1]

Great explanation..... thanks for sharing your knowledge to a new tube guy.....!

[@UncleDoug]

Thanks, Ken. Glad you enjoyed it :)

[@kenporonin1]

@@UncleDoug Thank you uncle Doug. I recently purchased a Dual Mono Block Marshall EL-34 100/100 which I plan to use as an audio amp/receiver (not as it's intended use as a guitar amp). I don't think the tubes need repacing as everything sounds pretty good as is. However, I was interested was interested in tube biasing when the need comes.

[@UncleDoug]

@@kenporonin1 It's best to check the biasing before use, Ken. I have found it hazardous to trust previous owners.....or even builders.....to adhere to proper biasing levels.

[@kenporonin1]

@@UncleDoug Thank you..... I will check them myself.

[@UncleDoug]

@@kenporonin1 You may be surprised by what you find. Good luck :)

[@AALONSO1962]

Thanks In Advance...!!!

[@UncleDoug]

Our pleasure, Antonio.

[@jmonk77]

First of all, who wouldn't find this interesting? In fact, just last night part of that question came up. I was looking over the specs of my recently purchased Deluxe Reverb, saw the output tube specs of(2) 6v6s. The amp is 22 watts and fits everything I need in home or occasional performance use, especially these days where we mic all the instruments for a performance. But then, I remember in my band during high school years back in the 70s, the guitar players had Twin Reverbs. A 65 Twin reissue has 85 watts and in the70s I think it was closer to 100 watts. Why did we have those high wattage amps? We didn't have the$$$ for 20+ channel boards and we had to be louder than the drummer. So we sacrificed sound quality for sound quantity. Anyway, I was looking up the tube complement for a 65 Twin Reissue and it is (4) 6L6. Hmm UD has never talked about 4 output tubes... But wait, now he has provided instructions on how to bias the tubes. The only thing left is how does the push pull work with four output tubes? Well I guess that's for another video.

[@UncleDoug]

Thanks for your comments, JD. The pairs operate exactly like single tubes do in DE amps. One pair "pushes" while the other pair "pulls".....but since each pair is twice as powerful as a single tube, the output from the amp is much greater.

[@Claymansd]

How could anyone thumbs down this???

[@UncleDoug]

I have asked myself this question, Scot, and have come to the conclusion that somewhere in a basement in the mid-West, there must be a lunatic asylum with computer access ;)

[@SIXSTRING63]

Just an observation, As plate current changes with volume and frequency so does the speaker impedance which will change the reflective impedance that the tube will get. An 8 ohm speaker can go from 2-30+ ohms when pushed. Guys like Dave Friedman say to run a 30 ohm dummy load on an 8 ohm output for running without a cab if you don't have a dedicated reactive dummy load box for recording. My point is, I run my tubes at 65% and have for years and never seem to wear tubes any worse than running colder. I usually try to look for notch distortion on the scope and split the difference. Lee Jackson biases all his amps that way, takes the signal to clipping and backs off to a nice shaped sine wave. If there is crossover distortion he just heats the bias a bit till it is gone and leaves it there. Said in his 40 years of building custom high gain amps and modding amps to the stars that has been his method that gets the best overall tone. Lately I’ve measuring my plate current using the shunt method which is probably the most dangerous using your multimeter in DC Ma mode from plate pin of tube socket to center tap of output transformer while amp is at idle. I have the bias sockets with the one ohm resistor that uses Mv to measure Ma using ohms law. I usually get a higher read off the sockets than I do using a quality Fluke 189 in Ma mode which would make you bias it colder from that higher reading. Using this method I want a meter with good front end protection and an accurate current shunt. Your method of measuring the resistance from center tap to plate is great advice to people new to this game of lethal voltage. I will have compare readings from the resistance reading to the shunt reading. They should be very close. I don’t think I’d try the shunt method with a free Harbor Freight meter and cheap leads. Might have a smoking melted plastic blob. 😲 Just my Flukes, Tektronix or Brymen meters for that test. Here’s a cool article with charts showing the wide swing of impedance a speaker will go through with frequency changes. audiojudgement.com/speaker-impedance-curve-explained/

[@UncleDoug]

Thanks for all the helpful info, 6S.

[@SIXSTRING63]

Uncle Doug Just some tidbits of info. You are one of the best techs on here and I never miss a video and have watched your whole catalog of videos! Especially in the long boring winter months of the northeast 🙂. I watch tech videos much more than any TV and have used your methods and advice in many amp rebuilds and repairs. Your concern for peoples safety is priceless. When guys like Bruce Egnater mention you as the guy to watch if they’re getting into amp repair that is quite a compliment. I read a lot of books, watch your videos, Lee Jackson, Dave Friedman, D-Lab, Joe Smith(probably the smartest EE that puts videos on here), W2aew...............thought I’d share some thoughts and ideas I have wondered about concerning constant impedance fluctuations and what it does to tubes and transformers. Subtle changes in speaker wiring can have such a profound change in amp tone, feel and performance, parallel wire two 8 ohm speakers and then wire same two in series. Quite a difference in frequency response and tonal quality. I have the tone chasing addiction as do most gigging guitarist. I have always been an amp overdrive guy so I learned how do my own amp repair and mods. Lately after playing many vintage Fender amps and how great the react with good overdrive pedals. Easier to change out pedals than mod a front end of a high gain amp. Love Fender clean tones, nothing like it. I guess that’s why they are great pedal platforms. Getting too old to be an Eric Johnson tone chaser taking out 4 amps like plexi’s, twin’s, old echoplex’s and fuzzface pedals. Keep up the great videos Doug! There is hope for this country if you pass that knowledge on to just 1 or 2 out of every hundred who watched and learned. I try to do the same with my modest knowledge of amps and my 45 years of guitar playing, 40 of them have been gigging years. I do lessons at the same store I do repair work for.

[@UncleDoug]

@@SIXSTRING63 Wow,, thanks so much for all your interesting and informative comments, 6S. It's always great to hear from you.

[@bullthrush]

Just a few extra steps for the quad. Surviving amp repair made easy. :))

[@UncleDoug]

Yes, it's just a couple extra steps, but it seems to cause a lot of consternation among quad owners :)

[@nicewilbur1]

Hey Doug, love the channel - my friend turned me onto it over the weekend.. Hoping I can find a tech in AZ to help build a custom low watt amp from a 1950/51 Magnavox entertainment console amp.. Not sure where to start. thanks for all the videos!! These are great.

[@UncleDoug]

Welcome aboard, David. Maybe one of our viewers will step forward to help you out. Best of luck.

[@nicewilbur1]

@@UncleDoug it's all I can hope for.. :) I have the two 12" 5-prong plug speakers, too..

[@ImmitH]

Thank you for the video. Turns out things wasn't as complicated as I thought. I was wondering what happens to load resistance in quad and parallel se circuits and how to impedance match them.

[@ImmitH]

I realized that you already answered this in a comment down below. I tend to miss what is right under my nose. Have a great day

[@UncleDoug]

Glad it helped, Ikoll

[@alanbrower9969]

Hi Doug, I love your videos! I have a question that's a little off topic for this video: I saw on the data sheet in your video the max plate dissipation of 12w for the 6v6. I also saw the max plate voltage at 315v. I see dual ended 6v6 amps typically above 400v plate voltage (kudos to you for using a real world voltage in your example calculations). I know that's what sounds good and I've used that voltage in some of "my" designs. How is it that we all get away with applying a voltage that is that far above the listed max plate voltage? Your videos rock, the only request I could make is to ask for more Rusty and Jack!

[@UncleDoug]

Thanks, Alan. Apparently the 315VDC max rating is absurdly conservative. Fender routinely drove their 6V6's in Champs, Princetons, Deluxe Reverbs, etc. to 425V and above, with no apparent harm. As long as the plate current is sufficiently low to provide a sub-12W (cathode-biased) or 8.4W (grid biased) plate dissipation, all is well.

[@michaelhager4270]

Hi Uncle Doug, another great video. Thank you. You mentioned the bias probe. If you plugged a bias probe into one of the 4 tubes, would both the tubes on that sides current be read? Hopefully that makes sense? Once again thank you from down under Australia. Your videos are such an incredible resource for the DIY community.

[@UncleDoug]

Thanks so much, Michael. Using a bias probe removes all the uncertainty......each tube's PC, PV, and calculated PD are individually tabulated, so you know the operating parameters of all four tubes, not the average of each pair.

[@michaelhager4270]

@@UncleDoug Thanks Uncle Doug