Resistors, Part 4: The Cathode Bias Resistor

In this 4th video in the Resistor Video Series, we discuss the purpose and function of the Cathode Bias Resistor. After discussion of the theory of its operation, we will take measurements from an amplifier circuit to determine the accuracy and practical application of the theory.
Here are links to the three information sheets I referred to in the video:
Measurement Data Sheet: app.box.com/s/60rklrlz89t3ahm...
Preliminary Rules: app.box.com/s/g39g2ry3zhmsz37...
Tube Biasing: app.box.com/s/88ir0ai32bvkr9m...
If you enjoy videos that explain in clear, basic language the inner workings of amplifier circuits, and have an interest in vintage tube amps, jukeboxes, and unusual electro-mechanical devices, then please subscribe to my channel to gain immediate access to over 125 similar videos, and to be notified each time a new video is posted.
Thanks for watching.

[@TheChadPad]

I have never seen such loyalty and devotion from a man's pets. They are really supportive and ready to help you any time you need!

[@UncleDoug]

Amen, CP. I worry sometimes that they might actually trample me on their way to doing my bidding.

[@surgingcircuits6955]

@@UncleDoug , all of that energy expenditure would explain Rusty's lean build. Definitely, an example of "man's best friend". No offense, Jack.

[@W1RMD]

Pretty lazy though!

[@brbuckner8]

Uncle Doug, thank you so much for all the time and effort you put into every video! I've read a ton of books on electronics theory, but it's so dry and hard to understand unless you have a teacher showing you how it all relates. (The same goes true if you're studying music theory when you just wants to play the blues). You make every video easy to digest and very enjoyable. Keep up the great work sir!!!

[@UncleDoug]

+brian buckner You're quite welcome, Brian, and thanks for your kind acknowledgement. It's great to hear that the videos are helpful and appreciated :)

[@TheChadPad]

Study music theory until it gives you depression, then you can play the blues

[@MadeAirbreak]

The life long mystery of biasing tubes is no more. Thank you for the explanation.

[@UncleDoug]

You're welcome, Soren. We're glad the video was helpful :)

[@realperson6130]

Doug, I have watched quite a few YT videos to learn electronics and many other stuff. You are the best instructor!

[@UncleDoug]

Thanks so much, RP. High praise indeed :)

[@ianwatkin5452]

Thank you so much. You are a terrific teacher and I appreciate all the hard work you put into your videos. The knowledge you share and learning are invaluable

[@UncleDoug]

You're welcome, Ian. Thanks for your nice comments :)

[@michaelpurchase2586]

Been building small amps and copying circuits but not really understanding the effects of varying values. This series of excellent videos explains it extremely well. Thanks Uncle Doug

[@UncleDoug]

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

[@danhill7134]

I was going to make the exact same comment

[@clownpocket]

Fantastic videos! I took a 6 month electronics course in the early 80's for the FCC General Radiotelephone Operator's license. The theory was much the same. I sat in class thinking about how it relates to guitar amps and effects. I ended up in different fields, but your instruction brings it all back, but with focus on what's important; tube guitar amps. Thanks for making these, I've been plowing through them slowly as time permits, relearning as I go, and planning a possible scratch build. Jack is a real beauty, and Rusty is a real charmer, despite their poor work ethics they really make the videos complete. Thanks again!

[@UncleDoug]

You're welcome, John. Thanks for watching and commenting :)

[@ziggfreud9820]

This whole series is just perfect. You have answered alot of questions I had about these circuits. I hope you keep going. You have a talent for pointing out and explaining certain things at a particular level that seems to get left out just everywhere else.

[@UncleDoug]

+John Miner Thanks so much, John. Much of my approach was developed out of frustration with the inability to find any good, basic electronics explanations or references. I'm glad that you find the videos helpful and easy to understand.

[@andrewgillis8572]

+Uncle Doug Hey Rusty, hey Jack: I asked my librarian for this new book on Schrodinger's cat and Pavlov's dog. - she told me it rings a bell, but she's not sure it's there

[@UncleDoug]

Jack responds that while he has no interest in being sealed in a box with radiation and poison gas......he would be amenable to a spacious box containing sardines and brisket. Rusty concurs.

[@andrewgillis8572]

Uncle Doug gotcha = that joke is told by a live sound engineer

[@UncleDoug]

.....and I must admit that it was definitely better than if it had been told by a dead sound engineer :)

[@MarkusvanAardtBusinessComm]

You are an amazing teacher. Thanks. Been building amps for some time but you articulate the science beautifully

[@UncleDoug]

Wow, thanks so much, Markus.

[@tonyduncan9852]

I had a black cat and a black dog. The cat did all the retrieving The dog went crazy with a balloon. Fond memories. Yes, thanks for re-sparking my obsession with thermionics.

[@UncleDoug]

Thanks for sharing, Tony :)

[@j.w.8663]

Great explanations. Thank you so much! After 4 years on, people will forever continue to refer to this vid.

[@UncleDoug]

Thanks so much, JW. Glad it was helpful!

[@kkenason8728]

Excellent video... May life bring you great things in return !!!

[@UncleDoug]

Thanks so much, KK. Likewise :)

[@joepublic4069]

Excellent. I'm enjoying learning and understanding how the circuits work.

[@UncleDoug]

That's good news, Joe :)

[@GaRbAllZ]

Interesting demo.. This reinforces the fact that a positive polarity is the LACK of electrons as referenced to ground(negative) and a positive charge is created by stripping the electrons from the cathode, controlled by the current passing through the cathode bias resistor. Excellent demo!

[@UncleDoug]

Thanks, Dennis. I agree with your analysis. The only cause of a positive charge is the relative absence of electrons. I think of positive charge as being a vacuum for electrons, causing them to flow from an area of relative excess (negative charge) to an area of relative absence (positive charge).....seeking equilibrium.

[@silasfatchett7380]

A lack of electrons implies a surplus of protons, i.e., a positive charge.

[@PatrickInCayman]

Another absolutely brilliantly clear video from an absolute master, the Oracle Uncle Doug.

[@UncleDoug]

Wow, thanks so much, Pappa :)

[@davida1hiwaaynet]

Thanks for making these videos! I really enjoy your explanations. It seems like you're a natural teacher.

[@UncleDoug]

+davida1hiwaaynet You're welcome, David. I'm glad the videos are helpful :)

[@CraigHollabaugh]

Thanks Doug, you hit it on the head with your opening statement about "confusing and unfocused explanations". Excellent clarification. I can go to sleep now.

[@UncleDoug]

+Craig Hollabaugh You're welcome, Craig. From a knowledgeable person such as yourself, a supportive comment such as this is quite meaningful and appreciated.

[@sparkey1958]

another great video, i have read and re-read books on valve [tube] amplifiers,and watched other peoples video's but nobody, and i mean nobody, explains things in such a calm, well thought out and easy to grasp way as you do...simply the best!

[@UncleDoug]

+sparkey1958 Wow, thanks so much, Sparkey. The topic is really quite basic and easy to understand......but you'd never know it from reading most of the so-called "expert" explanations. I finally gave up on them and simply focused on coming to my own understanding of the process. It's a real shame when a basic principle is so much easier to understand than most of the "explanations" of it are.

[@supermist100]

Thank u very much! I finally know the purpose of all those resistors that I had been wondering about. You´ve got the great gift to explain things in an easy to understand way.

[@UncleDoug]

+Haroslav Drastic You're welcome, HD, and thanks for your nice comments. We're glad the videos were helpful :)

[@nicholastrezinicacio2099]

Dear Uncle Doug, thank you very much! I learn a lot from your videos, the best electronic lessons i ever watched.

[@UncleDoug]

You're quite welcome, Nick :) Thanks !!

[@ElmoSyr]

Thank you for the great videos Uncle Doug! I'm slowly starting to understand these things. The video format is a lot easier for me to digest than a text book. And your hilarious in between bits make it doubley fun!

[@UncleDoug]

You're welcome, E.....glad to hear you enjoy our videos.

[@SandersAmps]

Whenever I have a question, I always come back to Uncle Doug. Excellent tutorials. Thank you Uncle Doug!

[@UncleDoug]

It's our pleasure, Shaun.

[@Pollys13a]

As I say, these series of videos, are so helpful, educational, everything, is so very clearly explained, thanx so much:)

[@UncleDoug]

So glad you enjoy our videos, Polly.

[@bennyvandelocht4242]

Uncle Doug, thank you so much for sharing your knowledge for absolutely free. Few people can teach in an efficient, understandable and amusing way like you can. I learned a lot by your resistor and capacitor movies. You can stay to the point and give relevant real world examples and proofs.

[@UncleDoug]

You're welcome, Benny.....glad you enjoy the videos :)

[@christopherjosack9060]

Thanks Uncle Doug! I am trying to recreate the Air Force electronics training I had in 1970 and you have supplied many for the missing pieces. I particularly like the reference schematics and the measured delivery that allows the principles to sink in before scampering off to the next topic.

[@UncleDoug]

You're welcome, Chris. You are are target audience and we're glad to hear that our efforts are appreciated :)

[@m.edwardburton6247]

Fabulous! Thank you. It would seem you've done what so many others have attempted and failed miserably at. That being getting me to a reasonable degree of understanding of tube biasing. Well done!

[@UncleDoug]

That's great to hear, ME. You're welcome.

[@barryballenger7246]

Electrical engineer for 30 years and this is the best explanation of cathode bias that I have seen. Truly feel I fully understand it now.

[@UncleDoug]

Thanks so much, Barry. Please check out our many other videos :)

[@joelmarshall2528]

I think these videos are great. Really clear explanations.

[@UncleDoug]

Thanks, Joel. Glad you like them :)

[@hijmestoffels5171]

After watching this video 3 times all is clear. Thank you very much! The retrieving skills of Jack are remarkable.

[@UncleDoug]

That's good to hear, Hijme :)

[@vw4x4]

Uncle Doug, Thanks so much for these video's. These are truly some of the best info. on the web.

[@UncleDoug]

+Hillman Wents You're welcome, Hillman. We're glad the videos are helpful :)

[@UncleDoug]

+Hillman Wents You're welcome, Hillman. We're glad the videos are helpful :)

[@UncleDoug]

+Hillman Wents You're welcome, Hillman. We're glad the videos are helpful :)

[@UncleDoug]

+Hillman Wents You're welcome, Hillman. We're glad the videos are helpful :)

[@docalex0]

I've been building for 7 years or so . So much more rewarding now that I'm starting to understand how and why thinks work . Many thanks

[@UncleDoug]

You're welcome, Doc.

[@74cannelle]

I’m always amazed by how clear and structured your videos are. I bet you put in a lot of work. Thanks for that 🙏

[@UncleDoug]

Thanks, 74C. Yes, each video takes about 2 weeks of recording and editing.

[@74cannelle]

@@UncleDoug Two weeks !! Mamma mia ! So you deserve more mercis

[@UncleDoug]

@@74cannelle Thanks :)

[@Rad10head707]

Hi Uncle Doug! I just wanted to say thank you. It's not often you find people on youtube that are not only very knowledgeable about the topic they are explaining but also very good at explaining it, while also exhibiting a great deal of selfless passion in what it is that they do. Your videos have helped me advance in this hobby greatly. I am looking forward to the next resistor video. This series has been very informative. Thank YOU!!

[@UncleDoug]

+Rad10head707 You're quite welcome, RH, and thanks for the very kind assessment of our videos and methods. A big part of the enjoyment we derive from this hobby is hearing from nice people like you :)

[@DeadKoby]

Another fine video that proves you can understand tube amps without a calculus degree. I've always done my builds based on a proven design... where I can match the voltages to their expected values.... now I'm getting the Why, and because of it....

[@UncleDoug]

+DeadKoby Thanks, DK. I agree with the use of proven circuits. Why try to re-invent the wheel? RCA engineers designed about every possible amplifier circuit over 50 years ago, and I always find it best to rely on their (infinitely superior) wisdom, rather than semi-educated guesswork, when building my own :)

[@BAMPSANDPEDALS]

Very interesting video Uncle Doug. No easy stuff, i must say 😕. I would like to take the opportunity to thank you for all the fine videos and the quick response to all my, sometimes silly, questions. Thank you ! Benny

[@UncleDoug]

+BAMPSANDPEDALS Thanks, Benny. I think the only silly question is the one not asked. Never hesitate to ask for clarification from me, or others, if you feel you need it.

[@treborheminway1196]

Rusty gives us a visual example of the term referenced to ground. Good dog Rusty. That is Dog-gone easy to understand.

[@UncleDoug]

He was definitely well-grounded almost all the time, Trebor, until food was served.

[@jenniferjohnson708]

I have taught math for 15 going on 16 years now and I greatly admire your technique and your teaching style. You have helped me defog a number of different issues concerning amplification and electronics. You have the mark of a great teacher and your consummate skill in explaining these concepts is greatly appreciated. I have been watching your videos just to pick up some techniques and style points to assist me in my lesson planning as well as learn about guitar amps. Thanks for everything and please keep the videos coming. PS. My twin boys love the bits about Rusty and Jack. The amp stuff is over their heads but watching Jack surf the duster is just pure gold as far as they are concerned:)

[@UncleDoug]

+Jennifer Johnson You're welcome, Jennifer. I too was a math and science teacher for 17 years and am quite pleased that you find my videos helpful. I found that to be an effective instructor, you had to have empathy with your students and put yourself in their position.....how would you want the topic explained? A boring recitation of mindless, nebulous details, beginning in the middle and jumping all around the subject? Or a coherent, even-paced, incremental progression of facts, explained in basic language....with a smidgen of humor and fun thrown in, like chocolate chips in a cookie. It appears that the latter not only works, but gains the trust, appreciation, and (most importantly) cooperation of the audience. Thanks so much for your kind appraisal of my channel, and for your input. Last, but not least, Jack send his very special regards to your twin boys. Have a wonderful holiday season.

[@AdmiralQuality]

Another excellent installment. I dare say I'm starting to Grok this stuff!

[@UncleDoug]

+Admiral Quality That's great news, AQ. After a while, it really does begin to make sense as the knowledge "blanks" get filled in.

[@BryanSavage]

Informative and entertaining as always. Thank you.

[@UncleDoug]

+Bryan Savage You're welcome, Bryan. Glad you liked it.

[@MrUbiquitousTech]

Uncle Doug, more great info! No one has done a better job at explaining tube biasing! It now makes clear sense to me and also helps clear up why a circuit can be at ground vs above ground. Jack did well too! :)

[@UncleDoug]

Thanks, Shy. I'm glad the video was helpful.

[@karatzdi]

Hi there Uncle Doug. Am delighted with the explanations and the ese of which anyone can follow. I studied Electronics but have not had the pleasure to work with tubes. Graduated from a US university in 1989, and since that date opportunities had me working in the IT Industry. I have been on the tech side to troubleshoot expensive Transistored HI fi equipment. I decided recently to engage my self in Tubes where now I am positive to the fact that I want to build one HiFi amp for home. Your name came up on a facebook group, where I decided to follow on how to get some technicalities and help me built one. Thank you for all shared knowledge.

[@UncleDoug]

You're welcome, Dimitris. Best of luck with your HiFi project.

[@michaelpukmel9175]

Thanks, Uncle Doug, awesome video as usual.

[@UncleDoug]

You're welcome, Michael :)

[@SeamusMcNeil]

Excellent mate - Thank you for knocking the dust of my valve theory ;) Nicely done mate Cheers Jim

[@UncleDoug]

+Jim Oates Thanks so much, Jim.....glad you liked the video. I have many more, similar videos posted. Please watch them if you get a chance :)

[@SeamusMcNeil]

lol. Watching Part 3 (Resistors) now as it happens. ;)

[@UncleDoug]

Rusty and I hope you enjoy it :)

[@donaldfilbert4832]

Great video Unc Doug! It caused me to realize a confusion I have had for a long time (and didn’t know I had). And it’s caused by the names of the bias methods. The “Fixed” Bias method actually has a way to “adjust” the voltage on the grids (via the bias pot) ; So - maybe not so - “fixed”. Where-as the Cathode bias method provides a truly “fixed” bias - based on the size of the Cathode bias resistor. The names of things can really cause confusion if not aptly applied !! ;) ;) Thanks for all the great videos !!

[@UncleDoug]

+Donald Filbert You're quite right, Donald. The term "fixed bias" is indeed quite misleading, but that's the generally accepted name for the process......which was probably established before someone thought up the idea of making the grid bias voltage variable.

[@donaldfilbert4832]

+Uncle Doug Ya - that's probably it. Your last comment clarifies it !!

[@PeterWalkerHP16c]

Grid Cathode differential was very well explained. I remember when the penny finally dropped for me and it was a revelation. I arrived at the same place you did but the way I did it was to treat the plate - cathode as a resistor in between the 'real' plate and cathode resistors and that way realised the cathode could be more positive than the grid or more importantly how the grid could be 0, -2, -4, -6v etc. with respect to the cathode. It's a very simple thing but if you don't use it every day for biasing valves and transistors it 'slips away'. Some of these videos probably require watching several times in conjunction with playing with LTSpice et al until they are second nature. Anyway thanks for the free video course.

[@UncleDoug]

You're quite welcome, Peter. Thanks for your detailed and informative response.

[@jeffcampeau9491]

A couple of hairy resistors indeed!! I must say it felt incredibly satisfying to see Jack go fetch prance back with his toy… 🤣😃 Thank you for the wonderful videos sénior uncle Doug!!

[@UncleDoug]

Glad you enjoyed it, Jeff :)

[@marklange8894]

Once again very informative and well done video. Just one thing; you do realize that Max is not your pet cat, you are HIS pet human. And he has worked very hard to teach you to throw and pick up that piece of paper. That is why sometimes he does not bring it all of the way back. He is reenforcing his superiority over you. Well done Max!

[@UncleDoug]

+Mark Lange Thanks, Mark. You're right about Jack.....he is a very good teacher (of humans) :)

[@jenniferjohnson708]

I agree with Craig a series of videos concerning capacitors would be very enlightening.

[@UncleDoug]

+Jennifer Johnson This seems like a good suggestion, Jennifer. I will start planning some videos relating to capacitors.

[@thomthumbe]

Self bias is an excellent/inexpensive method to bias a tube, but it comes at an operational cost. That cost is a slight loss of dynamic, useable voltage range in the tube. Some of that range can be recovered by adding a cathode bypass capacitor, but that too has a cost. In the end, fixed bias is the best if you have the available component room and money to build the extra parts. This is a great tutorial!

[@UncleDoug]

Thanks so much, Thom. Glad you enjoyed it :)

[@michaelnorback4717]

Love your Videos. Started building my own amp now based on the mesa boogie dual rectifier.

[@UncleDoug]

Thanks, Michael. You're starting with a very complex circuit. Best of luck. Let us know how it turns out.

[@kosycat1]

great stuff man thank you,it clicked when i see the relativity between each part of the tube,it's just just about balance

[@UncleDoug]

+Jonathan Kosyjana You're welcome, Jon. We're glad the video was helpful :)

[@cassvirgillo3395]

Hey Uncle Doug, Rusty & Jack, I hope you and yours are all doing well. I can't tell you enough how excellent your video tutorials, explanations, and, electronics, workmanship, performance are. Yep, I'm a Rusty & Jack fan, half the reason I watch, ha. I really appreciate the time & effort you spend making them, and, sharing your wealth of knowledge, thankyou. I would like to wish you & your family a Happy Thanksgiving & Holidays. Take care, be good, C.

[@UncleDoug]

+Cass Virgillo Thanks so much for the very nice comment, Cass. Rusty, Jack,and I sincerely appreciate it, and send you our best holiday wishes.

[@HammerToneAmps]

Very nice job on this series Uncle Doug.

[@UncleDoug]

+HammerToneAmps Thanks, HT :)

[@garysewell5465]

Brilliantly explained...

[@UncleDoug]

Thanks so much, Gary.

[@curtisprice9806]

Thanks for understandable content on important topic! 😄

[@UncleDoug]

You're welcome, Curtis.

[@peterbustin8604]

Love these films. Rusty is hard act to follow!

[@UncleDoug]

Thanks, Peter. He is, indeed :)

[@ProfTrout]

Thanks so much Doug, really helpful.

[@UncleDoug]

You're welcome, Prof

[@onefoolishidiot]

I so appreciate your videos Thank you Doug rusty and jack :)

[@UncleDoug]

+onefoolishidiot You're welcome, OFI. We think you should change your screen name to onesmartfella :))

[@BrokenPortriats]

Ok you did it, Reminded me when I go that D in tube biasing in college. Now someone explains it so I get it. I wish I had a time machine. I am ready to retake that test.

[@UncleDoug]

+BrokenPortriats If you do happen to get a time machine, BP, I'd like to borrow it so I could visit a local music store in the early to mid-60's to buy a bunch of amps and guitars :) Seriously, glad the video was helpful.

[@robertway5756]

I'm beginning to feel more confident about pulling out a meter to check some things on my slightly noisy '63 Supro Corsica. TY! 👍

[@UncleDoug]

That's good to hear, Robert, but please be very careful.

[@saturn5tony]

yay Jack, he succeeded, not once but almost three times. Great topic on the 3rd part of the tube Doug. Thank you so much for sharing. So funny for me to see almost 45 years of transistors and Ic's take a back seat to the almighty valve! I never got warm near a 2N2222a but I did burn my finger once when I gave it to much current by accident..haha!

[@UncleDoug]

+saturn5tony You're welcome, Tony. I'm glad you are enjoying the series. It is strange how vacuum tubes fell out of favor for so long and now are back in vogue. I keep hoping the same thing will happen with all the old clothes I've been keeping in the back closet :)

[@saturn5tony]

+Uncle Doug - Naa, I think my 1969 Hippie suede leather fringed Jacket will not be coming back soon, but I wish I still had my Gretsch duo jet electric guitar... Those were the days Uncle Doug, those were the days! hehe!

[@UncleDoug]

No kidding, Tony. I remember slobbering over displays of brand new early 60's Fender guitars and amps in the windows of the music shop at the local shopping mall. They came back, so maybe there's hope for my madras bell bottoms :)

[@salonrojo3233]

Yes, i suspect that too, and i had my doubt about the voltages, i thought the amplifier was the problem. Your comment was very usefull, and now i feel more secure to reclaim to the salesman, cos' It was new (the genalex original tube) i suppose. Thank you so much.

[@UncleDoug]

You're welcome, SR. Good luck.

[@acceler9]

I am loving this series! Thank you so much for the time you've invested into them! I'd be happy if there were another 100 videos just on resistors. That being said, may I make a request? Could you do a similar video on DHT tubes (300b for example)? Thanks again!!!

[@UncleDoug]

+acceler9 You're quite welcome, Accel. I'm glad you're enjoying the videos :) I'll keep your suggestion in mind, but please understand that my area of interest is in vintage tube amps, primarily for guitar use, and not HiFi......but I must admit that direct heated triodes are quite interesting :)

[@SidewinderINC]

Throughout this video I kept thinking my cat wanted to be let in the front door or the window because I could hear Jack :D He seems just as talkative as mine! With every video I'm learning more about my Engl Screamer E330. There's a lot more going on because it's got 4x 12AX7 valves in it, two channels with a high/low gain switch that covers both, but clearly the principles are all the same! Fortunately a schematic was available so I'm following along directly.

[@UncleDoug]

That's great to hear, Matt. We're glad the videos are helpful.

[@tonyloh6140]

Hi uncle Doug, I finally found the ‘culprits’ a pair of coupling caps b4 the grid of the 845... Checked their value looks ok 0.47uF but they leaking DC... Replaced them n all is good now...neg bias volt quite stable after an hour Many thanks!

[@UncleDoug]

That's great, Tony. Congratulations on a successful repair.

[@masteringhouse]

Very well explained.

[@UncleDoug]

That's good to hear, Tom. Thanks !!

[@buschfsu]

amazing video. so well done

[@UncleDoug]

Thanks so much :)

[@davindra67]

Thank you very Much Doug 🙏

[@UncleDoug]

You're welcome, D :)

[@khalidwht6534]

Very good explained. Thank you

[@UncleDoug]

You're welcome, Khalid :)

[@brianbuckner2768]

Can't wait for the plate resistor video.

[@UncleDoug]

+Brian Buckner Video production has been slowed by the holidays, but it should be posted in the near future......so stay tuned :)

[@thomass7877]

Thx for teaching us master Dough... My brain is smoking.. but i got it finaly... "just" need to re-watch few times :)

[@UncleDoug]

A smoking brain is a healthy brain :)

[@robertkeller5536]

Holy shit, Jack did it. Its not just comedy relief.

[@UncleDoug]

Nope.....he's a KZbin star :)

[@AndyWragg]

I was watching this on the tram into town (for my physio session) earlier this morning, listening with earbuds in, and had to smile when I could hear Jack mewing in the background. Great explanation once again my friend.

[@UncleDoug]

+Andy Wragg Jack is a shameless ham, Andy. I hope the session went well. Thanks for your kind words :)

[@UncleDoug]

+Andy Wragg The amp sounds quite nice, Andy. If the rectifier is arcing, are your filter caps of excessively high capacitance ?

[@AndyWragg]

The caps are as specified in the design a pair of 50+50uf 500V. I've ordered a new rectifier from a different brand - TAD this time. Should be here tomorrow, so I will soon see if it's the circuit or just a bad rectifier tube. I've not got over how good it sounds for a first time build. Really happy with the sound, WHEN it's not blowing fuses.

[@UncleDoug]

The amp does indeed sound great. What rectifier tube does it use? The tube handbook states maximum values for the first capacitor in the power supply....50uf might be too much.

[@AndyWragg]

The Tube Depot kit which accompanies the design comes with a JJ GZ34, which is the same one as I have been using.

[@machinimafan27]

Thank you Uncle Doug.

[@UncleDoug]

You're welcome, MF.

[@christophersmith2165]

Great series of videos. I have come back to them having not seen them in a while. They are always good, and you have helped me change my opinion about cats. Jack seems like such a nice guy. He has great personality. Totally confused why the voltage measurement over the cathode bias resistor is different to the cathode to ground measurement though. I thought they would be the same. But I'll keep thinking about it. It will come to me eventually. Thanks a lot.

[@UncleDoug]

Thanks so much for your kind words, Chris. Jack is pleased that you appreciate his performance in the video. Now I'm confused.....the voltage drop across the cathode bias resistor is the same as the voltage between the cathode and ground. They are both measured in the same way, since the bias resistor is grounded at one end.

[@christophersmith2165]

@@UncleDoug Thanks for the reply. ... but that is what I thought too. It is that one is minus 25V and the other is positive 25V. I am sure it is my misunderstanding but I have been baffled about this. I will study it all again and see where I have gone wrong.

[@UncleDoug]

@@christophersmith2165 The -25V is because it is a voltage drop, Chris. "Drops" are negative. The +25V is simply the potential between the cathode and ground, and it's positive because the voltage in the circuit is positive (B+).

[@christophersmith2165]

@@UncleDoug Thanks a lot. The thing with electronics is that it is so often counterintuitive.

[@UncleDoug]

@@christophersmith2165 The complexity is what makes it both challenging and interesting :)

[@stefanvaneylen9488]

fantastic explanation again. thx so much. i hope in the future you will make a video about the operating classes of output tubes in single ended and push-pull amps. class A,B and AB. i really like to hear your approach :-)...greetings from Belgium.

[@UncleDoug]

You're welcome, Stefan, and thanks for the suggestion. I will keep it in mind for a future video.

[@groovedodger]

thanks for sharing great vids.

[@UncleDoug]

+groovedodger You're welcome, GD. Thanks for watching :)

[@Pollys13a]

Thanks for this, very interesting, love the pets :)

[@UncleDoug]

Our pleasure, Polly.

[@aland2253]

I have to tell you Doug, sometimes I come back to these older videos JUST to see ol’ Rusty.

[@UncleDoug]

I have a hard time watching them, Alan. I miss the old pup.

[@julesl6910]

One key fact I believe was omitted that will help beginners understand the theory, which is that electrons are negatively charged and flow towards the positive side of the circuit - meaning that for example the cathode is at a lower voltage than the plate / anode which allows for the flow of electrons in the tube. The greater the voltage difference between the cathode and the plate, the stronger the electric field directing electrons from the cathode to the plate. This electric field is what enables the control of electron flow within the tube, which is the basis for the tube's amplification and switching properties. A good way to think of this is that the positive terminal of a battery is not actually where the electrons flow (it is positively charged from ions), instead the electrons flow from the negative terminal toward the positive. Also worth mentioning is that the grid carries a DC bias voltage as well as the AC current of the audio signal. It has been described as the water level of a pool (DC voltage) with the waves on top being the AC current. Next time you're listening to Hells Bells, remember that the song is being amplified through the AC/DC current in the grid. As always, thank you for your wonderful work Uncle Doug, you and your team of hard working four legged friends are greatly appreciated.

[@UncleDoug]

Thanks for your helpful input, Jules. As viewers well know, I am a strong proponent of electron flow theory (from - to +).

[@brennanweiss8969]

Your videos are amazing and have helped me exponentially! With your help I have successfully created my first build of a 5f1 and inspired me to move further. May I request that a video be made for a pedal operated channel switch and or boost on a simple circuit? Thanks for everything, keep the videos coming!

[@UncleDoug]

+Brennan Weiss You're welcome, Brennan. It's great to hear that the videos are helpful. I'll keep your suggestions in mind for a future video. Thanks for watching :)

[@jarredhager6271]

I've learned more in one of uncle Doug's video than I did the entire past semester

[@UncleDoug]

Thanks for your very nice comment, Jarred :)

[@victorvma8170]

Dear Uncle Doug & Rusty!! Thanks you very much for uploading all your videos, they are so useful and very fun!! Have you ever thought about making one related to reverb circuit? Best!

[@UncleDoug]

+Victor Martínez Armijo You're welcome, Victor. We're glad you enjoy our videos. I have posted a video on my scratch-built Fender 6G15 clone and reverb circuits in general....but it's on another channel: kzbin.info/www/bejne/pX2ZaqugfL5mg7s Check it out :)

[@alext9067]

We like Rusty. Point of order: As soon as you put the grid leak resistor in, i started thinking that the voltage drop on the grid leak resistor was much higher than the bias resistor voltage drop. One meg is almost an open circuit. I'm confused, and that's fine, but others might also be confused. Great videos. PS. Resistors part 1 explains it very nicely.

[@UncleDoug]

Rusty appreciates your nice comment, Alex. Voltage drop is related to current flow through the resistor. In the case of grid leak resistors, there is virtually no current and thus very little voltage drop.

[@ImTheReal]

Super Uncle Doug! I was thinking about something has missing on your last videos. Where are your guitar test's? (to finish the video) I want to hear you playing guitar 😺 Thanks again for sharing a piece of you time with us 🙏

[@UncleDoug]

+Humberto Martins Ferreira Junior You're welcome, Humberto. On technical videos like this I don't generally do any musical interludes, since I feel that you all have suffered enough :)) I'm flattered that you would ask, however, so please check out the Supro TV Series (Episode 8) and the final Kay 720 video for some serious string thrashing :)

[@ciprianwinerElectronicManiac]

Now I get it, I was thinking in the conventional flow from positive from negative and that's why I weren't able to understand where that voltage drop was coming from. Cheers :)

[@UncleDoug]

+ciprianwiner Yes, occasionally current flow is represented as traveling from + to -, but this is not correct. Current consists of electrons, which are negative, and it flows from areas of high electron concentration (very negative) to areas of lower electron concentration (less negative, i.e. relatively positive)......from negative to positive.

[@silasfatchett5693]

'Occasionally' is not quite correct. If you read books and articles by tube amp gurus such as Randall Aiken, the Valve Wizard or Rod Elliot, you'll find that, whereas the operation of the actual tube itself is described in terms of electron flow, tube amp CIRCUITS are almost invariably described in terms of conventional current. If you are hoping to build on Uncle Doug's excellent explanations of the basics of tube amps, developing the ability to think in terms of which ever scheme is most appropriate in any given situation is a useful, if not an essential, skill.

[@silvermica]

18:19 - The most amazing part of this video.

[@UncleDoug]

Glad you enjoyed the retrieval demonstration, SM :)

[@silvermica]

@@UncleDoug - But, have you have tried this with a tube amplifier? kzbin.info/www/bejne/bJiynXuEnNZ_oKs

[@UncleDoug]

No, but I had a football game when I was a kid, that did the same thing with little players.

[@carvingbone]

Awesome video as usual, very well done! Do you have or could you do a video on the term and use of 'loadlines' and how to use that in amp desing? Cheers

[@UncleDoug]

Thanks, CB. I will definitely keep your suggestion in mind for a future video.

[@tonywainhouse163]

Another fantastic video, thank you Uncle Doug. Within the context of this lesson on Cathode Bias Resistors, I am still curious as to why most are Capacitor Bypassed and some - like the WILLIAMSON circuit - are not. What purpose does the Cathode Resistor Bypass Capacitor serve? And what drives the design decision to use one - or not? Very much looking forward to you crystal clear unpacking of these closely related questions. Appreciated.

[@UncleDoug]

I have posted a video explaining how the bypass cap works and what its overall effect is on the circuit, Tony. Please check my video list.

[@bobbarcus8310]

I love the videos you do and I watch them over and over again but it’s very difficult to figure this stuff out but I greatly appreciate it maybe someday you show us how to get some more gain out of our Marshall plexi

[@UncleDoug]

Thanks, Bob. I'm not sure why you would need more gain from that monster, but I'll keep your suggestion in mind.

[@bobbarcus8310]

Uncle Doug think of that tone of the rhythm track to running with the devil by Van Halen also when I’m playing my amp lags behind my fingers if that makes any sense I would like to get quicker response from my am but I greatly appreciate all of your time You put into these wonderful videos

[@UncleDoug]

@@bobbarcus8310 Delay like that is generally attributable to a relatively slow rectifier tube/power supply, Bob. You might check it out and convert to diode rectification, if you don't already have it.

[@bobbarcus8310]

Uncle Doug thanks it is my 100 watt plexi That’s why I can’t use a wireless that really lags You Would think a 66-year-old man with slow down a little bit LOL thanks a lot for all of your time and knowledge you shared I just wish I understood it a little bit better but I do play pretty good

[@UncleDoug]

@@bobbarcus8310 It sounds like you do, Bob. Keep up the good work :)

[@fongmaho]

Very good video, Explanations are easy to understand. Thank you to have printed them, that helps a lot for people like me that don't speak English. I have a question: "Tube Biasing Purpose: To regulate the current flow through the tube when no signal is present" As the grid becomes more negative relating to the cathode when the current flow, doesn't the cathode resistor also tend to affect the current flow when there is a signal, like a negative feed back? How does that sound compared with a grid bias with a small battery inserted in the input - grid side of the circuit instead of a cathode resistor? I once made a one stage amplifier with a 9 volts battery as a grid bias and it sounded very well, even with only a regualr 220 volts to 12 volts power transformer used as audio transformer. I obtened the high voltage source directly from the 220 volts AC outlet, rectified and filtered. There is something I don't understand with your measures: you first measure the voltage drop of the cathode bias resistor, and then measure the same voltage between cathode and ground. As the resitor ends are connected to cathode and ground, aren't these 2 measurements the same, but just taken a bit more far away? At 11:55 "the first measurment I intend to take is the voltage drop across the 750 ohm cathode bias resistor". Sorry, but the colors on the resitor doesn't match 750 ohm. Isn't that rather a 680 ohm resistor?

[@UncleDoug]

Greetings, Fong, and thanks for your nice comments. 1.) The value of the cathode resistor doesn't change, only the amount of negative charge on the grid changes in response to the alternating music signal being applied to it. 2.) To measure bias with a cathode biased circuit, first you measure the exact resistance of the bias resistor, in Ohms, then you measure the voltage drop across the resistor, in Volts. Two different measurements with two different units. 3.) You can't trust the marks on the resistor. You must actually measure it with an accurate ohmmeter.

[@W1RMD]

I love that Jack and Rusty help out. Well, Jack tried....a little.

[@UncleDoug]

Behind the scenes they really pitched in to help......I swear ;)

[@treborheminway1196]

For the best definition of resistance, please refer to Jack's reaction to Uncle Doug's commands not involving food. However, when Food IS involved, Jack becomes biased and is positive relative to ground.

[@UncleDoug]

Excellent observation, Trebor, except it involves Rusty, not Jack :)

[@treborheminway1196]

@@UncleDoug Sorry about that Rusty. My bad.

[@fuckinA79]

Hi Uncle Doug, First off big thanks and appreciation for putting together your last few series, loads of practical knowledge explained in well... a practical way. if that makes sense. I have a early 60's Bantam Supra came stock with no power transformer, single ended-35Z5 rectifier tube-50L6GT output tube and 12AX7 pre. Have not seen a lot of references to this model and was wondering if you have any insight to impart. Thanks

[@UncleDoug]

+N Grey The transformerless circuits with filaments in series get a bad rap because they can be potential shock hazards. You should install a three-wire power cord that securely grounds the chassis and has a polarized plug to ensure that the hot 120VAC lead goes through the power switch and fuse and then to the tube filaments. I posted a video on a 1950 Supro amp with a series-filament circuit that you will find interesting. Also, there are all sorts of sites and forums that discuss this type of circuit. Don't listen to those who badmouth the design without having the knowledge required to fully understand it.

[@raymondheath7668]

Ha! A retrieving cat! Another good video

[@UncleDoug]

Thanks, Raymond :)

[@st2nh]

Great - Thanks 100000

[@UncleDoug]

Our pleasure, Nader :)

[@geekslip]

Thanks uncle!!

[@UncleDoug]

+Chuck D You're welcome, Chuck :)

[@OptomodMix]

Thanks for walking this through again Uncle Doug. I have a blown cap on a 5W tube practice amp and was wondering might have casued a 22u/25V polarised cap to blow from the cathode of a 6V6, also in parallel is a 1W/470r resitor and a 104/63V cap? Personally I think, but do not know exactly how to check, that these amps run very hot and have a fixed bias. From what I am picking up the 1W/470r is common.

[@UncleDoug]

Apparently the voltage across the cap exceeded 25V, Opto. I always replace them with 50V caps. The presence of a bias resistor would argue against fixed bias.

[@OptomodMix]

@@UncleDoug Thanks, still learning a lot, this is the second amp of the species, the other one burns out tubes, after a while they start making a lot of noise with input from a guitar. It's a champ copy amp. I thought the bias was fixed by the rating of the resistor as the there was not pot to adjust it. 50V caps as a goto is good advice, thank you.

[@UncleDoug]

@@OptomodMix Fixed bias amps have the cathode of the output tube grounded.....no bias resistor. The bias is usually too hot on Champs. Increase the value of the bias resistor to reduce the heat/bias.

[@OptomodMix]

@@UncleDoug Thanks for taking the time out to answer these questions Doug. You have confirmed my next move of increasing the value of that bias resistor. I'll hunt around to see what might give my tubes a longer life :)

[@UncleDoug]

@@OptomodMix Something in the 800 to 1000 Ohm range, most likely. Good luck :)

[@ImTheReal]

Nice :) tnx again master

[@UncleDoug]

+Humberto Martins Ferreira Junior You're welcome, Humberto. I just posted Part 5 about plate resistors, so please check it out when you get a chance.

[@ImTheReal]

+Uncle Doug your videos heal my soul, I'm very grateful for this!

[@UncleDoug]

That's good to hear, Humberto. I'm glad you enjoy them :)

[@jteichma]

Thanks again uncle Doug for your great contributions to my knowledge! I hope this isn't a redundent question as I plan to watch all of your videos. Is there really a self- biasing amp and what does that mean? Is it a fixed cathode bias cap? I have a Crate V18 as a sand box to start getting immersed in tube amps and mods. Still has the original tubes from 20 years ago and support said i can just plug in new power tubes (el34s) because the amp is cathode biased, therefore no adjustment necessary. What do you think?

[@UncleDoug]

You're welcome, John. Yes, a cathode-biased circuit is self-correcting, within certain limits, and generally does not require re-biasing after the replacement of output tubes. It never hurts, however, to double check the bias of the tubes, simply because the value of the cathode bias resistor may change over time. I have posted a video showing how to do this. Good luck.

[@sethbrandes2979]

Struggling to find a positive (pun intended?) reason for fixed bias configurations. It sounds like cathode biasing is superior - what situations would it be advantagious to design a fixed bias instead of a cathode bias method?

[@davidfuller581]

Seth, in the unlikely case that you haven't figured it out - fixed bias is more efficient than cathode bias (you can get more power out of the tubes). This means the output section stays cleaner and can be louder before distortion. The resistor power dissipation values you need also get to be somewhat... crazy.