Resistors, Part 5: Plate Resistors 

You should write a book. Your teaching methods are excellent. Thanks Doug.

You are an amazing teacher. You cover relatively complex subject matter with such authority and clarity as to make it easy to understand and digest. I know of no other resource like this for learning the function of tube amps and the whole community owes you a great debt of gratitude. Thank you.

Uncle Doug, I found a link to one of your videos in my recommended RU-vid viewing. I am a retired electrical/electronics engineer, and in the 1960s I used to design commercial high-end audio valve power amplifiers. Watching your videos during the covid-19 isolation has supplied catharsis and nostalgia. You have a wonderful style, provide an excellent service, and exhibit a depth of knowledge. However, I am a pedant, and cannot help myself when I encounter misinformation. I’m sure by now you must be thoroughly tired of me, but here I go again … At 6:17 you state “It should be noted that the value of the internodal resistors is governed by the microfarads of the filter capacitor …”. The 400-volt and 240-volt B+ rails are both design parameters. Therefore, the voltage-drop across the two series internodal resistors is 400 - 240 = 160 volts. All we need to determine the value of these two series resistors is the total plate idle-current of the 5 triodes supplied by this 240-volt rail. We can reverse engineer this by I = E/R = 160/36,000 = 4.4 ... mA. So, the total value of these two series resistors is set in stone by these two design parameters: 1. the internodal voltage difference of 160 volts; 2. the total plate idle-current of the five triode stages being supplied. R = E/I = 160/0.0044 … = 36,000 ohms. That’s it! We have no other choice. The final 240-volt filter/decoupling capacitor in no way “governs” the value of these two internodal resistors. The capacitor value will be determined by the required level of filtering/decoupling (mostly decoupling), and in commercial design, this will often be determined empirically.

excellent, will be replaying this series many times to get it through my thick skull!!!

Until I found this video I was re-reading paragraphs on web sites and not absorbing anything about plate resistors. Thanks again Uncle Doug! I now understand both functions of plate resistors quite well. You even anticipated my question about using large resistor values to maximize gain at the end of the video. Very impressive! Keep up what you are doing. You are sustaining interest in the world of tube amplification! Without videos like this, I would be discouraged from making this a new hobby.

Jack playing the newspaper like Jimi Hendrix is the cutest things ever!!! Great video Uncle!

I love it when I can still get 'aha!' moments at 50! You are an excellent teacher and a value your channel very much. Cheers from Australia :-)

Built my only guitar amp and various radios when I was 15, (Practical Wireless/Radio Constructor 1962). I am now your age Uncle Doug and have played through valves all my gigging life, still do. I was always interested in the workings of but never fully grasped valve (sorry.. tube...I am English), theory ........until now! Not only have you supplied practical explanation but also the maths behind it. Wonderful. I still own a untouched 1973 Fender Twin Reverb that works perfectly well but I now feel more confident to do some component replacement to enhance reliability. The mystique of biasing, plate currents and phase inversion is now slowly unravelling for me at last. Thankyou so much for your detailed videos, the content of which I have never found easy to understand coming from text books. I think I'm on my way to building my second guitar amp now, better late than never! Many thanks again and keep them coming. David. (Bolton, Lancashire, UK)

I have been searching for a logical reason as to how increasing transistor collector resistance in a common emitter solid state amp increases gain (the whole -RC/RE equation). I am so glad I flipped back to Doug's explanation of this in the "tube world". Finally, it makes sense. Thank you.

Whenever other publications leave me befuddled, I return to Uncle Doug to regain clarity. Thank you Uncle Doug!

This is excellent. The best explanation I've seen for plate resistors and nodal resisters. Directly applicable to my tube radio hobby. Thank you, Doug.

Once again, thanks Uncle Doug. I am building my first pedal, one of those 9v, starved cathode, two 12au7 jobs. It looked so easy from the schematic! Not so much. After dealing with bad brand new parts and old sketchy 9 pin salvaged tube sockets and a massive amount of head scratching, I finally got it to work, but the overdrive and distortion was unusable due to way, way too much gain. I remembered that I hadn't watched your plate resistor video yet, and presto! all was revealed. Because the B+ is only 9v, i used a 100K pot temporarily as a variable resistor and was able to dial in the values for the plate resistors for pleasing amounts of gain. Once again, thanks to you and your always superbly done videos, my pedal was spared a quick and violent execution, and the sun is shining in my shop again.

Uncle Doug is the best! Mr. Kraft was my high school algebra teacher. He came to us by way of the US Navy. He played in the Navy band also. As I recall we weren't totally respectable to him(bunch of eight graders), BUT I did learn algebra from him and he as you was a great teacher! When I got to college, I had a lady algebra teacher that taught high school algebra in the daytime. I could not help but make an "A" in her class. Her explanations made everything sound simple and she was one of the best teachers I ever had. I am working on a 1967 Fender Bassman at this time and Mr. Doug you are a great help. For all attempting to learn something about electronics, I commend you, salute you, and subscribe to your channel for your dedication and time !!! P.S- I still wish for a video on the signal chain(not thru the schematic view) but thru a chassis view on an eyelet board. A lot of your info is starting to soak in!

Takes me back to Navy Electronics schools in the 80s. Love these videos!

Thank you for tube lecture. Georgette looks like high maintenance and trouble. Rusty is a smart cookie.

Calling from Lisbon, Portugal... finally I understood, also by watching the all series on resistors, how the Plate Resistors work... thank you so much, Doug and Rusty, for such excellent explanations! :)

You mentioned that you were a teacher. It all makes sense now. Teachers in the 60s - 70s were arrogant, apathetic and incompetent. However, there were those that were so good at it, that they developed a following of students who truly loved them. I suspect you were one of those! Top notch stuff. Thanks. Side note, I spent many years in the 70s - 80s in Corsicana TX. Loved it!

Very good explanation Uncle Doug! This sure clears up why there are all these Resisters in a Guitar Amp at each Tube. Now I see why you measure Bias Voltage after your repairs. Thank You & sure appreciate all your videos.

I can not thank you enough for these wonderful informative videos.

Thank you so much for this......awesome explanation of tube cath-grid-plate function! It was outstanding! Add in the Jack hendrix and the Rusty antix and this whole series was off the charts!! Thx so much Doug. A loss in such fine teaching was seen when you retired.

That all made sense to me. Simple arithmetic, simple logic. Starting to think there's no magic after all. Thanks for that...

I knew that increasing the plate resistor value would increase gain. Now after watching this, the reason is much more clear . Thanks Doug !

Wish you had been my algebra teacher ! My roommate in the late 70`s took 4 years of television repair classes at a local two year JR college , Took it twice and really knew it pretty well by then , back when they taught tube TV repair together with transistor TV repair . I had a 69 Twin Reverb added 4 gain stages to the non reverb channel . My friend recommended putting these 250 K , 5 watt Allen Bradley potentiometers as the plate resistors . You can imagine what fun it was to experiment with adjustable plates ! Thank You so much for the clear presentations ! It is like gold for us old tube builders and helps us with the algephobia !!!!!

I like rewatching videos that were a mystery just a few weeks or months ago. Thank you, Uncle Doug.

Excellent. Very direct and clear (love being able to comment and watch at the same time on my phone).

Thank you Uncle Doug! That was so very helpful in understanding vacuum tube circuits!

Well, making a little progress. I just finished part 5 of the Resistor series of videos. Very interesting. One take away is the interdependence of DC and AC in proper tube function in the amp circuit. My purpose is that I have a Sound City 50R amp and it's not well. I know there are some dirty pots and possible 1 or more at least need cleaning or replacement. So, once i get the special fasteners off the pot stems I can remove the chassis and start making measurements. the fastener nuts are not hex but have split slots on either side so I need a spanner fork shaped wrench of sorts. I'm having one made since the ones I've found online are not wide enough. You are a Very good teacher. Thanks!

Before you, it was all a black voodoo, or at the best, super isolated islands of knowledge without any chance to be connected. This a-ha! moment I can only compare to the one 40 yrs ago when I discovered how pentatonic scale works and started soloing for the first time. Hat's off to you Sir, and thank you very much. Highly addictive channel...

Thank you so much for your videos. I'll keep watching them over and over again....and maybe some day in the future I will understand tube-amplification. Keep up your excellent work !

Great stuff once again. Really appreciate your straight-forward approach and practical examples..helps cut through the fog..Many thanks for a great series of videos . I'm going through them all and greatly enjoying them!

Now I get it! Been wondering for some time why a larger plate resistor value gives more gain. It seemed that it would just reduce DC to the plate, thus lowering output. Didn't know about the AC signal component and the increased voltage that a higher resistor value sends to the next stage of amplification. Thank you for a very clear explanation!

I am really grateful for these videos. I can't stop watching them! I have learned so much that I've always wanted to know. Thank you Uncle Doug for posting these! You are a fabulous teacher.

"with his teeth! Must be Jimi Hendrix...." oh man....I laughed at that one!

Thanks for making this series. This was very helpful. I've downloaded them all for future reference.

An outstanding series. Many thanks! 👍

The best series ever! Thank you so much

Great series on resistors and tube amps! Looking forward to the capacitor series!

Such a good series. Thanks Uncle Doug!

Awesome video - one of your best I think !! It totally clarified for me the balance between DC Plate voltage and interstage AC gain driven by the plate resistors !! Loved it. Very much looking forward to the Capacitor series !! Have a great Holidays Unc Doug !!!

I just finished your resistor series, and I realize that I never did learn this properly way back when I was learning my theory (about 50 years ago!) To be fair to them, it was more aimed towards repair, and I was a pretty effective repairman for the 10 years I did that (before big box throwaway culture sent me back to college and a career as a science teacher). My amp building now is a hobby, and of course, my friends still drag their sickly amps my way, but after watching your wonderful lessons, I feel like a curtain has been lifted - and I'm an old dog! You have wonderful teaching technique, complete with rest stops with the animal family. I am kicking myself, though, for my procrastination - I could have started these two years ago when I started dabbling again, but I was hooked on your repair videos. "One of these days I'll watch the theory..." I'll have the rest finished by the end of the week!!! Again, thanks so much for your contributions to the rediscovery of vacuum tube technology - it's heartening to see young people stepping up to the mystery and romance of working with voltages that can catch things on fire, send you across the room, and at the same time reward them with delicious tone for their guitars. Keep up the good work!

Thank you! This 100% clarified my understanding of plate resistors with relation to AC. Really appreciate it.

You provide an invaluable service to the community, thank you!

Thanks Unc, this one turned on a number of bulbs for me! Hope you and yours are well, Cheers!

fantastic video. finally i fully understand what happens from the plate to the grid of the next tube. well done. bravo.

I had watched this series a year ago and going back to it again, I understand it much better. Thanks for the hard work put in by all 3 of you.

Thanks 😊 Doug for your most fantastic description and priceless knowledge and help in these matters of plate resistor functions. Hope you are all well in these Covid times and look forward to your new videos. Very best regards. John. WIGAN. UK.

you are correct, you did save the best for last. Thanks for the informative yet fun look at resistors!

Thanks for all your work making these videos Uncle Doug. Your one of the best I've found where I learn the easiest and you make it fun. Say hi to Rusty and Jack for me. Thank you for all you do and passing on very valuable knowledge.

Just had that wonderful feeling I get when I discover a video of yours I’ve missed!

Excellent... You made it easy and clear on how the stages work..It's implied that the tubes operate and are limited by physical parameters...I think, maybe showing and explaning how these designs work with information from a data sheet would be helpful to see entire picture. I wish I had you as instructor when I was learing this stuff in college. Your methods are easy to follow and understand...Thanks!!!

Thanks the great series, ive been watching these over and over again. This time I wrote it all down. Thanks for all your hard work I enjoy your channel everyday

Happy new year, Uncle Doug! I have been thoroughly enjoying this series (and the scratch built amp)! I find your teaching style to be exceptionally easy to understand, informative, and entertaining. You' re an asset to the Vacuum Tube community. Please keep the excellent content coming!

Thank you so much, you make it so real, the passion for the whole thing. I was so confused before I found your channel

Excellent video.Really informative.Many thanks Doug.

First off Uncle Doug I want to thank you for this series. It has been great and I have learned so much more to help me along my amplifier building purposes, so as always just a pleasure to watch. My greetings to Jack and Rusty and a Merry Christmas and a Happy New Year to you too.. Thanks

I tell ya. This opened up a lot as far as understanding goes. Thanks a bunch

Holy smokes Uncle Doug. There were so many light bulbs going off in my brain during this episode that I was able to watch it in a dark room ! Now I understand WHY the failing plate resistor caused my vintage hi fi amp to lose so much of it's volume. Once again, thank you so much for producing the most informative and easily understood series of educational videos on the web. Priceless !

Thanks so much Doug. I'm literally studying all this information. I do appreciate the time and effort you put to make this videos. Greetings from Spain

Really excellent, great clarity and such valuable comprehensive information for us who dabble in the science of valve amps :-)

Excellent video and tutorial!

Great series on resistors Uncle Doug! You are an excellent teacher and you make complicated concepts much easier to understand. Thanks for all your inspiring work!

Great videos, thanks for your great way of presenting this material.

Hello, Another great series of tutorials. Thank you very much. Waiting with impatience for the capacitors video series. Best regards.

This is a real training masterpiece in the nearly lost art of detailed tube amp understanding. Thank you , I'm going to go watch the 1st 4 now. I hope you are well and happy.

Excellent video series once again Uncle Doug! You are truly gifted at teaching about electronics. I have learned a heck of a lot from your videos and as a amp building hobbyist I am extremely grateful for your time and efforts that you provide the RU-vid community about amplifier components and circuits. Thanks again, and cant wait to see what you have up next for us!

Another excellent video and very enlightening in helping to unlock the mysteries of how tube amps actually work. Well done!

hello uncle, thanks to your impeccable and very clear explanations, and the help of your good employees I learned many things about tube amps, I even learned that for Rusty a plate resistor is shaped like a bone .. :-) many thanks

Excellent! The fog is beginning to lift as most EE books on tube theory are long on theory but short on practical application. Also, I know it's been about 5 years and your assistants may have retired or even passed on by now, but I have found that one almost sure-fire incentive for increasing work output is to use an ordinary red laser-pointer. My mini-poodle Jack (looks a lot like Georgette) goes nuts but his baby step-sister, Clara, couldn't care less. On the other hand, my cats who have since passed on, were fearless chasers of the red light.

Your video is pure gold! A BIG thank you from the Far East

Really nice job on this one Doug. Looking forward to the capacitor series!

You have amazing teaching skills! Thanks for explaining "stuff and junk" in a way that is clear and easy to follow!

thx Uncle Doug! after 1 year (study and reparing tube amps) the fog starts to clear up. after watching your video again i think i understood 100 procent. thx!

I’m so grateful for these videos. I can’t tell you how many hours I have spent wading through texts and the internet for explanations that made sense to me. I wish I could articulate this better but there is something about your explanations which gets to the heart of the matter plainly and directly, building each thing on the next, explaining the significance and connection of each and every piece, and I’m left understanding more. It’s a feeling I would sometimes get with some classic ARRL manuals from around the 60s and before. I feel like texts and explanations written today are so hard for me to follow. I know there must be a lot of reasons for that and maybe I’m a part of that equation. Along with your other videos, I have a much clearer picture about how all of these amps really work and I’m having a ton of fun learning this stuff. I’m excited to build some circuits and learn more. Thank you.

Dear Uncle Doug, interesting and informative series about resistors in tube amplifiers, trust it takes ages to prepare the content. To you, Rusty, Jack and all the ones you love a merry Christmas and a happy and healthy new year. We all wait for more!

Uncle d, I have been completely in awe of how clearly and easy to digest your explanations are. You have a real gift for teaching. I know all this stuff but I feel like I was learning these things from the beginning the way I was meant to learn them. I know this is outside of your wheelhouse of videos but I hope one day you can break down vari-mu compressors and how the do their thing. Things like voltage controlled tube current and how they affect gain, time constants, etc. I think you would take this to a whole new level of understanding for many people. An altec 436c or UA 176 would be good, simple vari-mu tube compressors to break down. Thanks again for the knowledge.

Great series. Looking forward to the next "ones"...Thank you so very much! Happy holiday's to you and your family!

What a well done video series, thank you! The technical information is very interesting and your pet clips are entertaining.

Yes, the best for last, in a way, lol. Best in bringing my understanding to another level but, without the other videos first, I might not have made the leap here. Also, starting parse out the roles AC and DC play together. It is a beautiful dance and and electricity becomes less of a mystery. I cannot tire of thanking you.

Hey Uncle Doug! As an old Electronics Tech your videos here REALLY take me back to electronics school. I love anything tube related and totally enjoy ALL your videos and appreciate your professional delivery. Not to mention the entertainment value that Rusty and Jack bring to the class! Can't wait for more!!!

I just wanna say that I’ve learned so much from watching your videos. I’ve watched several of them multiple times as I try to learn the concepts. I love your teaching style. Thanks for making these videos!

I think I see a future John Entwistle in Jack; he plays a mean bass. And Rusty is simply a genius. Brilliant as usual, Uncle Doug. Learning so much... Thank you!

Excellent series, very helpful! Thank you!

Just watched your capacitor video's and the resistor video's even though i took in the resistor video i think i might have to watch it a couple more times to keep it in the mind as i watched the 5 parts altogether. The way you explain these videos just bring home what the valves go through when these values drift as components age and how important it is to check these biasing resistors and coupling caps, thankyou uncle Doug for another great lesson and video.

Another great video, I will be waiting in anticipation for the next video. thanks again, and happy holidays.

Every time Jack meows in the background I start looking around for my cats!

Thank you so much… looking forward to more...

Great explanation! Totally get it now. Thanks!!

Uncle doug your methods of teaching are stellar, I am trying to go throughout your entire catalog of video lessons but as you know most adults can only focus for about an hour at best, that being said I still find myself hungry for more .the little breaks with rusty and jack are quite the comic relief needed to release the mental tension created in the learning process, very good technique sir.

Such a great film! Hope Rusty gets his new collar! Thanks uncle Doug, I would have loved to have you as my teacher back in the day. The way you explain things is awesome and captivating. I could listen to you all day long! God bless.

fantastic series! The practical examples in your videos help bring to life the theory that I learned in my youth! THANK YOU for putting these wonderful videos together. I'm currently working my way through every one of your lessons.

Its kind of funny, I actually have degrees in math and statistics but electronics have made zero sense to me until I started watching your videos. I guess everything just needs to be related to guitars:D thank you for all the work you do!

Hi Uncle Doug. Greetings from Denmark. Just wanted to thank you for making these videos. I'm a "young" elelctrician/musician just getting in to the whole tube amp thing. I'm intending to scratchbuild a high gain amp, and your videos are an invaluable resource when it comes to learning what i need. Keep up the exellent work!

Just to say thank you for your excellent tutorials. I still have a relatively basic understanding, I've had audio tube amps for years ( work wonders with big Tannoys ) and am tempted to build some simple designs to begin. I'm ok with the maths ( degree ), but your videos have given me the understanding of the functions of components to set up the juggling act ! I hope you and your family are well - strange days indeed !

Thanks, best teaching ever!

Thanks for sharing !! You are an excellent teacher !!

That was brilliant, thankyou. I think you mentioned you were a teacher - if so, your students were lucky. It looks like an interesting tradeoff trying to choose the correct resistor for each plate.

Outstanding Uncle Doug, Rusty & Jack the jamming cat! Thank you so much for taking your time to help educate those of us who need educating! lol Now I can actually properly calculate if this little GA-5 I am trying to resurrect has been hacked improperly and check that my repairs are producing exacting results to get it back to original specs. I think you need a little cat amp now and a little keyboard the mimics a strat so he can jam too! :) Thanks again and please continue educating! It IS greatly appreciated!!!!

Thank you once again Uncle Doug. I now have a much better understanding for the values used in providing the proper gain setting for a tube. Before this, I was understanding that plate resistors was what was used in a china cabinet to keep the dishes rom falling (:>}

I learned more before work in the train ride in today than the army will do all day!

Thanks for the great information. Lots of learning here...thank you.