Resistors, Part 4: The Cathode Bias Resistor 

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!

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

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!!!

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

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

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!

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

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

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.

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.

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

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.

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

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!!

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

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

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

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

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!

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

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.

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!!

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

Informative and entertaining as always. Thank you.

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!

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

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!

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

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

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

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

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!

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.

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:)

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.

I think these videos are great. Really clear explanations.

Thanks, Uncle Doug, awesome video as usual.

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.

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

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

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

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

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! :)

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

Thanks so much Doug, really helpful.

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.

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....

Very nice job on this series Uncle Doug.

Brilliantly explained...

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

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!

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.

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

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 !!

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

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

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

Thanks for understandable content on important topic! 😄

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

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!!!

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.

Love these films. Rusty is hard act to follow!

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.

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!

Very well explained.

amazing video. so well done

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.

Thank you very Much Doug 🙏

Very good explained. Thank you

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

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!

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.

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!

Can't wait for the plate resistor video.

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.

thanks for sharing great vids.

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.

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

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.

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.

Thank you Uncle Doug.

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

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.

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!

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 🙏

Uncle Doug, Please consider a comprehensive series on capacitors as well, discussing ESR and leakage and why/where these values are important. throw in red hot tubes comments if you want. Craig

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!

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.

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 :)

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.

Great - Thanks 100000

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

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?

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

In cathode biasing, the whole tube circuit is below ground voltage, making ground positive, and allow ground to be used as a grid voltage. This is the essential part to understand. So +25v cathode and 0v grid, is the same as 0v cathode and -25v grid, or say -50v cathode and -75v grid. You just need to have the grid more negative. If the cathode is +25v to ground, the anode must be even more positive.

Hey Mr. Doug, your videos are a delight for the knowledge thirsty! I got a question. What was the point of having a switch between two different resistor values for the cathod biasing? Is there a difference soundwise or maybe fidelitywise of the circuit?

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

You mentioned that the ideal plate dissipation is 11.65W (achieved with the higher value cathode bias resistor). If that is the case, why do you have the switch option for the lower value resistor? I'm thinking you probably answer that in the build video but I figured to ask here too. I'll try to watch that video as well. Thanks for sharing your wealth of knowledge, this is incredible!

When measuring the voltage drop across the cathode bias resistor and the voltage from cathode to ground (pin 8 to chassis) it appears to me to be measuring essentially the same thing from the same test points albeit different locations along the same line....what am I missing.

Ha! A retrieving cat! Another good video