Guitar Amp Tubes Part 2: Pentodes and Beam Tetrodes 

The level of intelligence in your presentations is palpable. No "um", no "uhh", no backtracking, no corrections to your statements, no "I think...", no assumptions. Just a hot knife through the butter of education and knowledge and perfect communiction. Thank you so much.

Doug, you are a natural teacher. I find your lessons and presentations to be so informative and illuminating, even if I THOUGHT I had a firm grasp on something, you always find a way to present it such that my understanding is increased. My hat's off to you for another amazing video presentation. In the future, please consider doing a video on the various ways in which one can wire pentodes and beam tetrodes and the consequences as they appear in various amplifier models, i.e. some amps, mostly hi-fi, wire output pentodes and beam tetrodes as triodes, some designs tie grid 2 to the cathode with small capacitors, in some tubes the suppressor grids are internally tied to the cathode, and in others, it is on a pin to be exploited through different designs.

Rusty your videos are an absolute treasure! Of course let us not forget to thank Uncle Doug's minor effort :)

I knew about the beam forming plates, but the alignment of the grids and the virtual suppressor grid were news to me. One learns something every day! Much obliged, thank you!

Uncle Doug, you've done it again! Another excellent video for the new kids on the block, as well as a refresher course for us veterans! I always say this after every one of your videos; "Where were you when I was in electronics school?" My instructor made me learn the theory by using guilt techniques :) Either way, you should be an instructor in an audio electronics technology school; you're an excellent teacher. I truly hope your viewers appreciate what they see here...as a pro audio tech, I truly do! It is information that's valuable to the future of the professional audio electronics technician. Don't stop now, keep 'em coming. God bless!!!

I have learned more in the last 40 minutes watching your videos than in years of reading trying to understand this! Thanks a Whole Lotta!!

Thank you. I just watched about 2 hours worth of your very well done tutorials. I studied TTL circuits and digital electronics at university about 35 years ago. I eventually worked as a network engineer for decades and I am now retired. For much of my circuit theory instruction we were only concerned about cutoff or saturation and NOT the linear response aspect of amplification. This was a great refresher for me back into the analogue world. I rarely leave comments but felt a strong desire to express my gratitude for all your efforts.

I really wish I had an instructor like you when I started learning about electronics in school. These videos are great and I have learned a lot about tube circuits by watching your videos. It has made a lot of the books I have read make sense.

I have a degree in E.E and when I was in college, we only covered transistors and operational amplifiers. I as a boy read a book called "The Boy's Second Book of Radio and Electronics" by Alfred Morgan. I feel that colleges today should still cover tube circuits, because the concepts of biasing a circuit, a cascode circuit, a push-pull circuit, a heterodyne receiver , etc, etc are still applicable today. I feel your videos are so superior and clear, they can be used as course material. I graduated in 1994, 20 years ago and graduated high school in '85, but I feel your videos are so outstanding, they should be used as college material, or material in Physics class in High School. Excellent work Sir and keep up the good work!!!

My most favorite video. Lifelong questions answered. Thank you.

Wow... My brain feels like a bloated capacitor UD. I had to watch these twice AND take notes. I guess that's probably why they're called "Shrinks"; they can reform our brains? Excellent video! I've been staring at a schematic with two instances of the same tube (12AX7) : V1a and V1b. Then I watched your videos and it felt like I've been standing under a coconut tree... Bam! It hit me. Two tubes in one, huh, what will they think of next. Thanks for the education and the entertainment! :)

Wonderful explanation of Beam Tetrodes and Pentodes! Information that isn't easily available to the average enthusiast. I was raised on tubes and graduated tech school learning tubes, but your explanations are far and above anything I have been taught.

This is the best lecture I have seen. You make this difficult subject so easy. Thanks uncle Doug.

Yes let the Necromancing continue. Just a shout out for the mighty 6550, used in Leslies far and wide. Now the interesting part, they were also used in Ampeg SVT bass amps, along with 12au7s. I get the feeling this combination 12au7, and 6550 worked very well for getting a really punchy bass. In fact, a properly maintained B3/C3 combo with a a similarly maintained 122 leslie, at only 40 watts sounds a whole lot louder and bigger than 40 watts. Back in the day, our bass player had an SVT with dual speaker cabinets, and well, he was never drowned out. May The Leslie Be With You BlueMule

This must be one of the best videos on this topic out there, thanks

Uncle Doug, your channel here is just a kind of treasure! Your teaching shows god given talent to explain things and also very inspiring! All the best wishes and many many thanks to you and Rusty!

If I would have been the type of student I am now, at my advanced age, I would have enjoyed learning from my electronics instructor then as much as I enjoy learning from you sir! Thanks for posting these valuable lessons!

Just brilliant. You're grasp of and ability to communicate this technology is profound. You and Rusty have put together a channel that is an inspiration to those of us likewise inclined.

What a wonderful discussion of the 6L6 family of tubes. I knew nothing about the history of this family of tubes other than there were many subsittutes available. But after watching this, it is easy to see how different they are from one another, and why it can mean a difference in the performance of some equipment. Thanks again.

It just blows me away that people figure stuff like this out. I mean seriously, WOW!

I love these videos where you talk and point at paper. A+++

Very simple explanation and a history lesson on tube innovation! My understanding has gone to the next level. Great job

I'm barely able to follow this as I gain a better understanding of tube electronics, but I know genius when I see it, and the beam tetrode design is brilliant! Thanks for a very easy to understand explanation with these videos.

I cannot like your videos enough, Such a great teacher!

I keep on going further down the rabbit hole. Thanks Uncle Doug.

It is always a true pleasure watching these tutorials again. From time to time, I go back and start from the beginning. A combination of your patience, voice, pictures and the pace itself, makes these tutorials the best ones on the web! It's a shame that Rusty is not so interested in electronics, though! :-D A mighty thanks to Uncle Doug for all the lectures so far!!!

Your lessons are easy understand and you present it well. Moreover, it's through. Thank you Uncle Dough.

Who would have thought that the "K" in an KT 77 tube stands for "kinkless", what an enlargement of my general knowledge. It is these side comments who make your videos so informative, the rest is good and sturdy camera work of Rusty. He excelled again give him a great hug.

Excellent lesson. I learned more information than I thought I already knew about vacuum tubes!

Finally, a fantastic description of the venerable KT66! thank you uncle Doug, Rusty, and Jack! very physical description of how tubes are a physical part of amplification.

The "Getter" is interesting. I've never even thought about its function. Thanks Uncle Doug.

Fantastic lecture...the theory of the KT's opened my senses to which I've adhered to an element of historical witness being while searching for a great tone I also needed somewhat of a payday for the work applied. I studied the magnificent McIntosh MC-275 which after viewing it's outlines I noticed that regardless of how much use it would never ever reach full potential and operating at full throttle it's audio response is 2nd to none even to this day. It's secret was great engineering and built way above overkill circulatory components. Now the baby I chose and swear by to this day very much resembles the very best of the Vintage tone(s) EL-34 but as you explained the kink and lost emission(s) due to the negated suppression of working out the kinks ..lol...on the screens and eventually burnt out tubes leading to the overworked eventually burnt grid resistors time and time again,..at this point the the conventional choice were 1k ohms but the HotRodded ''Super'' series were 470 ohms,..I experienced losing really expensive vintage speakers. From '60s stock Fane to pre Rola Celestion White Covers, Altec Lansing among several Power Tubes including a set of Mullard '70s EL-34s!!!!. All on my favorite '71 Hiwatt 50 watt SA-2x12'' Combo which Jimi Hendrix and Huey Thomasson a very big fan of Hendrix from the the Outlaws, specifically used these for their best leads. It's primary 15.3 ohm(UK) 16ohm(USA) impedance really jerked my chain so I instead changed the primary over to the 4ohm load and using two diodes(IN4007) rectifying each side of the sine wave @ 1 per, changing the resistors from 220k to 80k-100k @ 2 in parallel this change allowed me the freedom to use the incredible ''BIG'' Daddy to the EL-34 220k 8ohm to as you said perfectly the KT-88 @ 80k 4 0hm. At first I used the GE-6550 @ 100k 4-2 ohm but the break up was a lil too sterile plus nowadays this tube's worth is astronomical as I have a set of 6, more or less matched just couldn't consider using them except for something special they are fantastic sounding lil tone machines which is what led me to research for an alternative the performance of the McIntosh MC-275 for it's overkill build and components...see those very concepts you so elegantly elaborated on are somewhat all worked out,.. so in theory use the best as a clone relatively ,...btw the problem was easy to finally repair after almost 20 years it has to do with the 110v line it was converted to when it was first introduced here in the USA,..I figure at least one of 5 repairman knew it but chose to just use my service for repeat business. I switched it over to the 120volt line and things have been sweet,.. maybe something you've spoken of that should be noted I believe on your AO Reverb 18watt amp conversion,,good call . I did finance the overhaul with the best of practical needs and then for the best audio choice I tried several different but decidedly chose Gold Lion KT-88s which is well over 800 volts peak and the amp never reaches that threshold and it sounds very much like an older '60s Park amp a lil superior in tone to the PLEXI of the Marshall but the Plexi of the Park with the wattage now around 60-80 according to Ohms Law and a great high headroom with a Semi-Active tonestack the Bass no joking has a Lowend Octave Harmonics that sounds like an octave doubler after 6 with a 4x12'' cab adjacent to the 2x12'' propriety . It did have a very powerful toned clarity alot like the 6L6 STR's of the Mesa Boogie's. I didn't know how to explain this in theory,.. before now...Uncle Doug I'm getting alot of juice from what you do explain and I do get,..maybe not all but like today I can at least relate alot to other Vintage Power Tubes and choice preamp tube applications with a better understanding of what to suppress and what to allow for,...Thanks and please give a big loving petting to your Tech Buddy ''Rusty The Wonder Dog'',..Later Friend!

This is great! Best explanation of tubes I have ever watched. I am a beginner in electronics and have always been curious about how tubes work and here application. Thank you!

Very nice, especially how you showed us the development that took place through time and the background story of the beam tetrode and Mullard.

Sir Uncle Doug - your knowledge is truly amazing and invaluable!!! many thanks for sharing it in these remarkable videos... Inon.

Doug, you have a gift of teaching. Great videos.

What a great video. I have an old Philips book with tube engineers discussing tube production. Crazy stories. If someone used even a bit of teflon in the factory, tubes simply did not work anymore. Production had to stop and the whole place cleaned. Some handcrémes were forbidden for the assembly ladies because of the same effect. If a new one came out it could stop everything until they found out what it was. Also I read that a tube does not have a very strong vacuum because they actually stop working if they do. Tubes are so interesting

Excelent! Helped me to refresh everything after a couple of decades not caring about tubes. Thanks

I wanted to thankyou very much for your channel and time. You have given me anwsers to questions i have had for many Years.. I am truly grateful sir.

This channel is much better than TV, I love it! Thanks Uncle Doug!

More than 1,900 likes, with just 13 dislikes (they are obviously from those misguided souls known as 'fans of solid-state amplifiers'). This ratio must be one of the best on RU-vid, and it is nothing less than what Uncle Doug deserves. Well done Sir, and thank you very much.

After spending my whole working life involved with circuitry using solid state devices, your descriptions of these thermionic devices has successfully filled in some gaps in my knowledge. I did find it very interesting however, to see that there are similarities between the thermionic triode device and the depletion mode insulated gate field effect transistor. Both are are driven by a voltage input and control a relatively large current (i.e have a mA/volt mu value) The inter electrode capacitance is also a problem with some IGFET’s which also limits their application area. But, as a keen guitarist and owner of a fender pro-reverb, your internet posts are now very relevant and are useful for maintenance and perhaps for fault finding purposes. So thank you for your time and effort. Also, I recently came across your other you tube posts relating to your “hot rod” restoration, wow. You certainly are a man of many talents. I restored an MGBGT some years ago. That was time consuming but it was not in the same league as your re-build project. So keep up the good work, best regards from a snowy England still in lockdown.

Truly excellent video series you have here. Thank you very much for taking the time to make these!

Thanks thanks Doug I've watched almost all of your videos now about how tube amps work I never could have learned this stuff any other way thank you

The beam tetrode design is just utter brilliance. Coming up with a virtual screen by focusing the electron paths was genius.

Amazing detailed and clear tutorial about these tubes. Excellent!

Awesome videos. Thank you for producing them.

Dear Uncle Doug, this video as well as part i is just excellenant in the way to teach tube theory, You are truly a master in tube theory, I have watch both part 1 and part two and have learned more about tube theory then any other source I have read.. Great job.. I thank you you for taking the time to explain about this complex subject.....

these are the answers that I, as a guitar player ( and I suspect all guitar players), appreciate. it takes the "magic dust" and gimmick out of popular held beliefs about amps and gear. I love to tinker. watching these vids just makes me want to find excuses to get out the soldering iron. I really wish someone would bring in an old marshall amp for you to work on. or an old ac30. anyway. as my old man used to say: " wish in one hand, piss in the other. see which one fills up".

Thank you for sharing your sharing your knowledge in such a clear manner. Super video !

Many thanks. My second run through these great tutorials of yours.

That one hit the spot. Great info. I love it when some of the mystery gets taken out of tube tone.

BRILLIANT! Especially the side view sketch. That, for me, was a real eye-opener. I had no idea that the SG was virtual in the 6L6's. Excellent explanation! Every time I watch your vids I learn something. Another commenter said that you were a natural teacher. I completely agree. incredible teaching skills mixed with a smidge of humor and, of course, the kitties and pups.. Many, Many thanks for posting these!

This is the fourth video of yours I've seen, and it's particularly great! My favorite was the virtual cathode / virtual suppressor grid, bypassing the patent. Incredibly insightful and practical, thank you!

I appreciate a lot your teachings Mr ! Your way of explaining things is very inspiring.

Your videos are clear, concise, easy to follow ……… and so educational . Thank you!

I never had tube theory in school, as it was the early 80s, and everything was about digital circuits. This was a really great presentation for me. I learned a lot, and now the mystery of vacuum tubes is solved!

A big thank you for the course dear uncle Doug. i really appreciate your channel. Greetings from France.

Great explanation. I know you put a lot of work into these videos and it is appreciated.

Thank you Uncle Doug !!..............I'm learning step by step and you are a great teacher !!

Wow!!! what a fantastic video, I'm so glad to found your videos which are so informative. very crisp explanations and to the point. please keep doing what you do.

Always beneficial and fun, Uncle. Thanks again for all you’ve done and continue to do. Great to see ol’ Rusty as well. What a champ. 🤙

Happy 2015 Uncle Doug and Rusty! I hope this post finds you both well and happy. I wish you all the good you can have in the new year. As one of your loyal followers I will be looking forward to your next video. Your videos are like a great movie you can watch over and over again. I do and will watch all of them again and again. Thankz!

OMG, this is such an amazing tutorial! Thanks!!!

Thanks brother.... Watching your videos gets me thinking more and more about what's going on inside my amps... and it's helped me pinpoint issues with better ease. I'll stay tuned for whatever is next.

These videos are fantastic, Uncle Doug. Super clear and concise. I'm just getting into amp building and these videos break the subject matter down very well. Thanks!

Doug, This video is the most informative tube video I've seen since the 40s Army video. Thanks for including the discussion surrounding beam power and the pentode patent, love the historical references to it all. I especially liked the views under the glass explaining the upper mica connections and tube identification. Very nice work. Craig

Very interesting series. I learned a lot. Thank you!

Uncle Doug, Another great video. Well done and clearly explained. Nice that you put your dogs in the videos too. Thanks again for taking the time to make these. Yours is a very positive contribution to the understanding of vacuum tubes and particularly as used in guitar amps.

Just happen to stumble onto your channel. Great information and thank you for sharing! I have a lot of videos to watch it looks like

I'm rebuilding a vintage Rock-Ola Juke box and I've learned so much from all your videos! Just wanted to say thank for making these. You explanations are very clear and easy to follow.

What a great tutorial, you are the master of giving a simple explanations to very complicated concepts. Thank you Uncle Doug!

This video presents a LOT of new information!

Brilliant video, once again. Thank you Uncle Doug (and Rusty). You are providing a vault of knowledge for people, like myself, who are truly interested in learning about valve amps. It is greatly appreciated. Best wishes and thanks from the UK. Rob. :)

Thank you for demystifying vacuum tubes for me. These videos were exactly what i needed. Gathering knowledge, to hopefully start building amps soon. I highly appreciate your content.

I studied all this in high school and college back in the 60's. You did a very nice job. I particularly appreciated your discussion of the differences between pentodes and tetrodes.

Superb description of how the beam tetrode works. Very impressive, not an easy thing to explain. Enjoyed this very much.

Thank you for explaining this to us so understandably.

As always, Rusty does all the work, While Uncle Doug relaxes! Great informative videos to feed the mind. I have to aquire some old tube manuals. Keep up the great work! Thanx Uncle Doug and Rusty too!

This one was spot on Doug! Thank you so much! It was great to learn the origins and history of the beam tetrode.

Dear Uncle Doug, a happy new year to you, Rusty and the rest of your production team. Lots of health and stamina to produce more interesting videos. Happy New Year

Uncle Doug. You are a master of your craft. Greetings from South Africa.

Great videos, man. Thanks a lot for making them. Feel like I'm learning a lot.

My man Doug has taught me so much.

A very nice explanation on the tubes! Thanks.

Uncle Doug, you never cease to amaze me! The patented suppressor grids factoid, the sounds of those old Vox Beatle amps! I remember how kinda "muddy" I thought they sounded! LOL Tell Rusty his camera work is right on the money..as per usual!

Gleaned a lot from these instructional videos . Thank you 🙏🏻

Greetings from France ! That was an incredible lesson Unc', managed to understand tubes basics in 50 minutes thanks to you. What was black magic to me one hour ago is now crystal clear! will never thank you enough for your videos and the knowledge you share in such a clear and entertaining way👍 As a bonus I feel my English gets better just by listening at you 😉

Hi Doug and Rusty just to say thanks for all the videos this year and all the best for Christmas and New year. Colin

I always give you a thumbs up before I watch your video.. cause I know the content is always awesome...

Great explanation Doug, thanks

Such enlightenment was only found on mountain tops before your videos, thank you. Rusty looked eager to get at it since no young pups were around copy his technique. :))

Thank you very much for your time to do this :)

Thank you so much for these videos Doug. I'm slowly working my way through them and loving every minute of it. I'm a self taught electronics hobbyist, and I've discovered that I love the older circuit technology so much more than modern transistor based components. The analogue stuff is so much more intuitive. I feel like I can really get a sense for what is happening on a physical level. It's too bad it's become such a niche area. But thankfully the internet and electric guitar amps have sustained a small market and community of enthusiasts.

Thank you for this brilliant vid.I have now learnt so much.

Once again... Fantastic. The extra layer of information on tone and harmonics is like the cherry on top. Not only do we get the technical info, we also get the juicy secrets on why certain amps have a different personality than others. Thanks UD&R!

Wow.. Thank You very very much.. I really respect you for making these videos.

Thank you for these last two videos in particular. I was actually going to request exactly this as, while I've understood triodes for quite some time, pentodes and beam tetrodes were still a complete mystery to me. I feel like I get it, at least a little, now.

Great video. Very well explained.

Thank you for teaching us about this subject.