How to re-purpose build a custom homebrew 6L6 tube guitar amp Junk box special part 1 

Nice Terry! Repurposing vintage freebee junk is awesome and fun. Imagine if we had to buy these parts new.

Hello Terry, Your videos are a treasure house of great information. I love your old school approach. You have military training and know how to do the job correct, the first time. Plus you have a great style and a way to communicate your skills. Good stuff from the old wino. Hey, everyone needs to find a way to relax. Dancing with 500 volts on a daily basis keeps your spine in line. It would be great to show us how to voice the circuit to behave. Dial in a great tone. Like voicing an amp for harmonica. You are the master. Keep up the great work.

You could actually use to DROP your B+ a tad. You don't need 500+ volts. One easy way would be to wire the choke in right off the cathode of the 5U4 = choke input filter, instead of between the filter caps = "pi" filter (as it now is, just tacked together for testing) Probably not dramatic, but could knock off 20-40 volts. Yes, and as someone downthread suggested, balancing resistors across the series filter caps are called for. 220K/330K are fine. They also provide a mild bleeder function.

Terry - a suggestion for a future video. Build a spring reverb from an old piece of test equipment. Accutronics makes the 8 ohm reverb tank you need for the original Fender circuit. You can also add some extra gain to overdrive the amp without reverb. The outboard Fender unit is very lush and springy compared to the blackface amps. Very Dick Dale sounding.

I suggest you connect 1 resistor in parallel to each capacitor to equilibrate them in DC voltage. A good value could be about be 330K. This is a common used technique when connecting capacitors in this way.

The Tweed Fender Bassman and High Power Twin used 600 volt caps in the power supply. Unloaded, neither one went over 500vdc on 110vac line voltage. There had to be discussion at the time about standardizing US line voltage to 117vac at the time, otherwise Leo wouldn’t have wasted the money. The only 600v 20uf caps you can buy today are made by Sprague, and they are NOT cheap.

Hi Terry...I have stripped a Stromberg Carlson Model 32 P.A. head and now have Two Tung Sol 6550 power tubes, a 5Y3 rectifier and a handful of Mullard 12AT7 tubes, the original metal clad 6SQ7 tubes,a6K6 and an aluminum chassis drilled and ready for my first Class AB? push pull build. I built the fun 5F1 champ with an adjustable negative feedback circuit courtesy of Uncle Doug clips but this will be my first build with a phase inverter circuit and I want a good circuit that I can complete and be proud of. This P.A. head was designed for 6L6 tubes and the power transformer checks out ok with 720 volts AC available with no load .The power transformer has all the needed taps for tube rectifier, high voltage and center taps where expected The output transformer has taps for 4, 8 and 16 ohm speakers so my transformer needs are covered. I was considering a 5E3 Tweed circuit but that may or may not be the best circuit I could build to take advantage of the power of these mighty 6550's. Your advice please on a circuit that is simple enough for me to complete considering what I have told you. I play Country, Blues, Swing music and am a far better guitar player than amp builder and dabble in jazz a little bit so I like overdrive and speaker break-up but am by no means a Hard Rock player so I'm basically looking for good clean tone with maybe a little hair right at the upper limit of my volume knob. I expect these two 6550's will easily make 50 watts and I have two Jensens just waiting to be put back in service from an old Silvertone set up I bought years ago. They sound fantastic on the back end of a Fender Bassman AB165 that belongs to a friend of mine .Your clips ,and others are very inspiring and I thank you for sharing your knowledge with the world...and me ! So...which schematic should I tackle ? I will be retired at the end of the year and this will become a regular hobby I suspect. Many Thanks ...Daniel

Part 1 was awesome....but was missing just one thing which I hope finds its way into future parts. Wine!

Glad you ran a load on the power supply Those rectumfriers can take the voltage way high over the caps voltage rating and go kaboom without the output tube/s in place Other issue that certainly only very few techs realize is the screen grid voltages exceeding the maximum recommended by over 10% due to the rise in mains voltages in the US will certainly kill your tubes much quicker Everybody should open their data books and check this I don't know why it gets dismissed by most In actual fact a separate regulated supply would be the ultimate for screens

Awesome video series! Now I want to go back to my swap meet days!!

At this point in my life, I have become REALLY interested in how to service and repair Tube Amps. I have a been a musician all my life. I am no dummy with a Soldering Iron. However, I have a long way to go when it comes to repairing this stuff. I am REALLY looking forward to doing this stuff. I appreciate your videos and have learned quite a bit from you already. Thank you!

love to see you tinkering, Terry. Inspiring and educational, but also fun!

Hi Terry the original configuration was choke input, that would have reduced the voltage by 30% and improved regulation of the supply. Very common to have this arrangement on this old military test gear.

Excellent video. Looking forward to part 2.

I would be worried about electrolytic caps, you need 600 VDC rated or some 450 VDC in series. Happy New Year Terry and to your better half too. I got excited and made a comment before the end, I knew what you were going to do. Many more to come this year too, Thanks Terry!

Yer vidyas are always a delight, Thanks Terry!

This was excellent Terry! Always nice to see an old device given a new lease on life! Especially as a guitar amp! I also loved how you showcased the new variac! Love that thing! Looking forward to the next chapter!

Love how that old Polytechnic was wired. Would be a cool tutorial showing a clean point to point like that

Terry, Bless you for sharing your wisdom with us. You are the Man when it comes to guitar amps an educational videos. Thank you so much for educating the Next Generation of Electronic Repairers. There are so many people that cannot afford the price of schooling. That is where the school of youtube and you came in. All your amps sound amazing. Thank you again for all your kindness!!!

Great video with excellent practical knowledge! Thank you! I like the Variac power supply. I've built one myself that also has four switches with 5, 2x60 watt and one 1000 watt bulb in series with the auto-transformer. I can also switch to bypass all of the bulbs and go direct once "the coast is clear". An isolation transformer is also a great (necessary) improvement I want to add in the future!

Nice project Terry, interesting series that I´m going to follow. I would put a resistor voltage divider over the filtercaps, or there might be a problem in the future. I have seen this happen even in low voltage equipment where the values change over time and one capacitor blow up from overvoltage. This is no issue when the caps are new, good brand and from the same batch but down the road…. Have a Good 2020

with that voltage , you can build a nice guitar amp with some 807 output tubes !!

Absolutely fantastic demonstration on the practical design of power amplification using tubes. This will be a great series on reusing old equipment to make an amplifier! 🍷🍷

Looking foward for part 2 :) thanks for sharing

This is grand start to the year! Interesting because you cover the broadest array of tube electronic discussion and fact-finding. Totally enjoyable.

Great project! I look forward to part 2... Thanks

Good video Terry.

Once again great work Terry!

This is good stuff. I like the new series format. I was glued to the screen. Repurposed boutique amps. Such gems. I am rubbing my hands together like an evil villain. The additional output of the older transformer because of the higher wall volts is intriguing. It's like getting a free boost just for making the conversion. Like cash back! If you find the right candidate for a refurbish. That amp is going to sound real nice Terry.

Great work D- lab can't wait to see part 2 thank you.

Great video Terry

Here's a tip for your viewers. Say you didn't have a 25w resistor but only a 10w. Just for the purpose of this test you could put the resistor in a jar of water, then the 10w resistor could easily take 25w or more.

Looking forward to part 2. I've been planning on doing some hotrodded single ended 6L6 powered amps here with all the parts I have laying around. You've given me some good motivation to get started.

This is why I originally subscribed to d lab. Can’t wait to learn about how you’ll rig the gain and tone circuit and route for low noise. Thanks for this series.

Thanks Terry. I'm most definitely enjoying this re-purposing theme. Looking forward to the next part(s) of this project. Wishing you and yours a happy, healthy and prosperous New Year!

Looking forward to the new series,thank you for the clear and concise explanation, most instructive and enjoyable.

5Y3 wants a 20uf reservoir cap max but a 5Y3 would drop the pV a bit compared to a 5U4. A 5R4 or 5V4 would maybe be a better choice

Looking forward to the series. And, would love to purchase a little practice amp that you make.👍

great step by step procedure with explanation.

Wow this is perfect for me cos ive got a bunch of old junk i wanna make into cool stuff.

Nice bones! I've been collecting a lot of radio and console hi-fi chassis but that power transformer really puts out the voltage. I can't wait for part 2. I think I'll draw up a load line just to see where you're going to end up with this thing. Thanks for the videos, Terry. HAPPY NEW YEAR!

Hi Terry, I have paralleled and seriesed transformers as well, as long as you get the phasing correct they work fine. If going for high voltage you do have to make sure the transformer insulation will take it.

The last time I saw an instrument with that kind of power filtering it was a Precision Apparatus E-300-C signal generator, and the line filter was in its own copper coated steel can.

Love the home brew amp

Really HEAVY DUTY power transformer.....& a really sweet choke. BUT I’d go class AB with that kinda B+ voltage tho.....SE class A 6L6GC is like 375vdc if memory serves. 500vdc class A ???....that’ll be way super clean...too clean for guitar...might eat 6L6’s too

I always find you brave for use those alligator leads on 500V or so.

Outstanding! Terry, this is a great change of pace from the normal Break/Fix that you do such a great job on. I look forward to the rest of this.

I wish I had even half your knowledge. I am really enjoying this. Thank you.

Thanks Terry, This is great watching, looking forward to part 2

High rectified voltage was my problem with the transformer I bought for my 6AQ5 amp project. Good stuff, waiting for next vid!

Great Job. A great lesson Since you mentioned that the line current is higher today I would have opted for a 10K more than 25 Watt just to be safe.

Excellent Terry !! Love the new series idea !! Creating a new application with old components requires a deeper understanding of electronic principles and allows the illustration of new ideas !! Looking forward to 2020 !!! ;)

They don't make 'me like they used to! Except at D-Lab! Terry, I have to say you heard us tube ampaholics and this new series is just what we need! Ham radio is so important, there maybe 1 Ham guy for every 1000 ge-tar players out there, so I am thrilled for this new series. It's probably too late, I think, with this power supply a 30-50 watt class AB output would be great! You could make the input switchable for guitar or bass. It will end up a head not a combo, so you could use it for either. With that kind of power supply voltage you could also think about a 6550 output tube instead of the 6L6, maybe get 20 watts class A output. Just an observation. Love the D-Lab channel! Happy 2020! Thanxz

Fantastic Terry /keep ‘em coming.

I would of loved to see a 807 output I believe they handed more voltage but you’re the master and I’m the student

Happy New Year Terry! The "pissed off transformer" statement gave me a good chuckle. The repurposing of old electronics offers more opportunities for me to learn than say, troubleshooting an AA5. Can't wait to see the next installment! The lack of vino in the video was surprising.... 🤣

Happy New year! Can't wait for part two. I have that same transformer I picked up from a vintage radio repair guy. Got it for $10! Blew my mind when I found out the B+ voltage. Please post schematics if you can

I can’t wait for part 2

Happy New Year Terry

Love this style of Video Terry, this is what I want to learn how to do, keep it up!!

This is a great series Terri. I hope you and the family had a great holiday season and y’all have had a great first day of the new decade.

Ever thought of a Patreon page? Maybe offer some of your builds or test equipment for sale to Patrons? Excited for the 2020 videos!!

Fabulous, thankyou and Happy New Year!

Really great video.

Very cool Terry! I'm working right now on converting an old Lafayette police radio into a push pull amp and we are following your build and its working out very well. Do you plan on attending the 2020 Vintage Electronic's expo in Farmington Hills?

Good one Terry. And yes, I did like it. Very informative.

Awesome video

Hi Terry, great work! But personally, I would highly recommend to balance those filter caps with resistors... Looking forward to the result! Happy new year from germany!

I enjoyed this entire series but just had a nagging thought in my head that wouldn't go away. "Why is the B+ so high? This seems to be a high quality piece of military grade equipment so why would the manufacturer use a 5Y3 and a 450Vdc multi-cap can in a circuit that is producing 500Vdc? Then I found the answer when I rolled back to the beginning of part 1. The B+ never was 500Vdc because the manufacturer used a choke input filter in the power supply. The rectifier output had to pass through the choke before arriving at the first cap. This arrangement produces a much lower B+ output than the more common cap input filter. The choke input filter produces a peak DCV that is only .45 X secondary ACV. And the more common cap input filter produces a peak VDC that is 1.414 x secondary ACV. Wish I had seen this video 3 years ago. Probably doesn't mean too much this late in the game but at least my nagging head is happy. ;>)

Just so you're aware, "mil-spec", or "military-grade" doesn't automatically mean it's a quality build. It just means that it's built with standardized parts as cheaply as possible while still retaining a semblance of real-world functionality. Note the lack of grommets on chassis pass-throughs. Industrial and lab-grade components and gear are typically of much higher quality. Just look at the Bradley IFV. That colossal dumpster-fire of an Infantry-support vehicle should never have made it into service, yet it's "military-grade", right down to the extremely lethal Halon fire (and crew) suppression system. Halon is like Carbon Monoxide on steroids, as far as my understanding goes. When it comes to procurement, sometimes entire contracts depend on who's greasing the wheels. Bad gear makes it into the chain all the time. The AR-15 also comes to mind. It has good ergonomics, but it's entirely inadequate. The argument that lighter ammunition means you can carry more of it goes out the window when you factor in that due to its reduced size (and power), you now need to use multiple rounds to do the same job that one used to do, which means troops are now blowing their load even more quickly than they did back when the M14 or FAL were in use by America and Canada, respectively.

Don't forget to give the fairy Yak a carrot . Happy New year.

Terry, dig the flea market tours...and of course all the posts! Happy New Year 2020! All the best from Santa Clarita California!

Great content as always, thank you.

Terry, you're the BEST! Can't think of a better way to start the new year and a new decade than watching one of your excellent presentations. Happy New Year to you, and your family, and wishing you a prosperous 2020! Love your candor and you often give me reason to smile. Go to 4:45 and hear vintage "Terry" at his best. "that filter cap is probably leaking, loading down the power supply and pissin off the transformer".....hahaha. Thanks for all you do. I've learned a lot and greatly appreciate your dedication, talent and abilities as it applies to "electronics".

ARL is the Army Research Laboratory which is headquartered in Adelphi, Maryland.

This is like candy to a baby.... 👍

Happy New Year Terry & family. 🤘🏻

Happy new year, Terry. I've got a crazy idea: Why not use an 807 running at about 50 mAmps and take two of the miniatur 7-pin sockets for stabilizer tubes of type OB2 to get a fixed screen voltage of 216 Volts? Of course, in this case the power transformer should be capable of delivering the extra stabilizer crosscurrent...

This will be an interesting year of projects for sure. I'm looking forward to it. Any concern over how close the 5U4 and the 6L6 are to each other? I know most of the heat will rise but that glass is going to get pretty hot.

Excellent idea. Can't wait for part 2. Question- are you using 5u4 due to accessibility? Does the 5 volt winding have enough amperage or would a 5r4 be a better idea, similar voltage drop but only 2 amps on 5 volt. And I would definitely not use a 5ar4 cuz that would probably make it closer to 600 volts.

Good thing your getting back to amps. Looks like I subscribed at the right time haha

Good Stuff Terry... this is going to be extremely useful series.. love it.. thanks man

Great stuff Terry. I've been inspired to learn more about electronics. Happy New Year!

Terry happy new year!!! A big embrace and thanks for what you do.

RCA receiving manual RC-24 gives 6L6GC a max of 500 pV in class A and 450vdc in class AB?? More pV in class A PP than class AB PP?? Seems backwards to me. Anyway,...I’d try to loose some B+ if I could.

Love it!

Could also be a nice hifi amp too. Why not do one that is a dedicated hifi amp? No one seems to do that which i've seen. If anyone knows...

Happy New Year, Terry! I love these videos.

Loving it already!! Happy New Year Terry.... When the Zombie apocalypse hits, you can go back to the ham radio experiments. LOL! Make a series with converting one of those puppies into a Guitar amp/ zombie slayer...

Great buold looking forward to seeing the finish project. Is it ok to have those twotube so close to each other. Love the channel. Happy New Year.

Nice!!

4:26 - Terry, Is the ammeter working on that new Variac? I see no current draw when the transformer started humming (?)

Hi Terry, Happy New Year! I always enjoy when you repurpose stuff. FYI, 5U4 draws 3A of heater current vs. only 2A for 5Y3. I guess you’re assuming that transformer can handle the extra load? Somebody had to point this out. 😉

I am a tube amp tech also and really enjoy watching your channel, happy New year.

Always good content. Thanks alot and all the best for 2020 and beyond.

This looks a beautiful project indeed. Just wondering if you have any info on the specs for the power transformers and choke. Their humongousness makes me wonder if they wouldn't be fit for a push-pull design.

HI Ter.. I was wondering if you need resistors across the Caps when they are ran in series for High Voltage?

Hi I was wondering if maybe you could help me with something. I bought a used tube amp and want to do some bias work after changing tubes. The problem is the company uses a wonky test point system. The variable control turns up and down the bias negative voltage and allows two diff ranges one listed as EL34 range and one listed as 6L6 range. I have 4 6L6 in the head now. The problem is the test point they provide just shows the negative Bias Voltage and NOT the actual current draw from the tubes. I am going to just order one of those tube socket things you plug one of the tubes into and then plug that into the socket to read the current draw. But I have NO CLUE at all how to figure out how much current draw the tubes should be drawing. I know its some kinda math lol. Could you tell me the formula on how to come up with the proper ma current draw for this amp? Thanks :)

What are the difference between an guitar amp and an audio amp? Could this just be used as an audio amp if I wanted to? Please try to keep in mind there are no dumb questions so they say.

Hi; Since you are using a 5U4 did you make sure that the 5V filament supply is capable of 3 amps? 5Y3 only needs 2 amps.