New Project: 6U8/6V6 amp

[tunghaichuan]

I got a start on my newest project: a single ended amp using a 6U8 triode/pentode for the preamp tube and a single ended 6V6 for the output.

Here are some pix (http://www.diycustomamps.com/images/6U8_6V6/).

The amp uses a 6U8 triode/pentode for the preamp. The pentode section is the input tube; the triode drives the 6V6. I have quite a few NOS 6U8 tubes as well as 6GH8 and 6BL8 (http://www.diycustomamps.com/images/6U8_6V6/6gh8_tubes.jpg), which have the same pinout but slightly different ratings.

The output is based on the tweed Princeton/Champ. I decided to use JJ 6V6S tubes as I got a batch with the skinny pins. The ceramic tube sockets I got a while back grip the pins very tightly, which is good because the JJ 6V6S tubes are loose in the NOS sockets I have :(

One interesting thing about this circuit is that it has a built in compression control. More on that later.

I also plan on installing the VVR circuit (http://yeomansinstruments.blogspot.com/2008/07/vvr-greatest-thing-ever.html) using this layout (http://bp3.blogger.com/_4u9l5q2yFBA/SHBQA55CwPI/AAAAAAAAAbY/R48Wf3boj7o/s1600-h/MyVVRTop1.gif). The VVR is a type of power scaling which uses a MosFet to control the anode voltages, and thereby reduce the ouput wattage. I've been wanting to try this for a while because it is supposed to give good OD sounds at whisper-quiet volumes. We'll see. The one bummer about using the VVR is that the MosFet costs about $12US. It really pains me to pay that much for a solid state device. :D

So far I've generated the schematic and the board layout. Next up is building the board, and then generating a chassis layout.

I've been wanting to do this project because eventually I want to build a push pull version of this amp using the 6U8, a 12AX7 phase inverter and a pair of 6V6 tubes. Maybe even a tube rectifier.

Stay tuned...

tung

[duhvoodooman]

Cool! Waiting to see (and hear!!) how this one turns out! :AOK: :drool:

[tunghaichuan]

Got the circuit board built this morning. As usual, I made a master drilling board so all I have to do is cut the blank to length and put the master on top and trace out the holes. I then drill the board using a layout guide and a fence on my drill press so all the holes line up evenly. Next, installed the eyelets.

Next step is to drill the chassis. Since this is a prototype, I'm not going to spring for face plates. There will only be four knobs on it anyway: compression, volume, tone on the front and a VVR level on the back.

Some pix:

Drilling pattern taped on master and blank board (http://www.diycustomamps.com/images/6U8_6V6/board_01.jpg). The bag to the right is full of 1/8" eyelets, approximately 1,000.

Closeup of pattern taped on master board and blank board (http://www.diycustomamps.com/images/6U8_6V6/board_02.jpg).

Drilled out blank board, master and extra pattern (http://www.diycustomamps.com/images/6U8_6V6/board_03.jpg).

Finished board (http://www.diycustomamps.com/images/6U8_6V6/board_04.jpg).

tung

[tunghaichuan]

Got the chassis done yesterday.

Blank Bud AC-409 chassis (http://www.diycustomamps.com/images/6U8_6V6/chassis_01.jpg) It's even RoHS. :rotflmao:

Chassis with blank circuit board (http://www.diycustomamps.com/images/6U8_6V6/chassis_02.jpg)

Chassis clamped to bench (http://www.diycustomamps.com/images/6U8_6V6/chassis_03.jpg)

Control panel holes drilled out (http://www.diycustomamps.com/images/6U8_6V6/chassis_04.jpg)

Finished chassis (http://www.diycustomamps.com/images/6U8_6V6/chassis_05.jpg)

Front of finished chassis (http://www.diycustomamps.com/images/6U8_6V6/chassis_06.jpg)

Board mounted in chassis (http://www.diycustomamps.com/images/6U8_6V6/chassis_07.jpg)

Parts mounted on chassis (http://www.diycustomamps.com/images/6U8_6V6/chassis_08.jpg)

Board stuffed with parts, not soldered yet (http://www.diycustomamps.com/images/6U8_6V6/board_05.jpg)

So far this is shaping up nicely. The next step is to solder the flying leads to the board and mount into the chassis. After that, I can begin to wire the circuit.

More later.

tung

[piebaldpython]

I gotta tell ya Tung, I am totally in awe; :master: :master: :master: ; of what you and DVM do with your skills in making amps, pedals, etc.

OMG, I would be ready for the loony bin for sure.

[duhvoodooman]

Comin' along nicely there, Tung! :AOK:


I gotta tell ya Tung, I am totally in awe; :master: :master: :master: ; of what you and DVM do with your skills in making amps, pedals, etc.
Don't be so impressed, Pie. If Tung and I can handle this stuff, how hard can it be?? ;) :D

[tunghaichuan]

Comin' along nicely there, Tung! :AOK:

Don't be so impressed, Pie. If Tung and I can handle this stuff, how hard can it be?? ;) :D

Yeah, really. :rotflmao: :dude:



I gotta tell ya Tung, I am totally in awe; :master: :master: :master: ; of what you and DVM do with your skills in making amps, pedals, etc.

OMG, I would be ready for the loony bin for sure.

Thanks for the kind words, Python.

Update:

I spent the afternoon attaching the flying leads and installing the board. I spent quite a bit of time soldering down the leads from the transformer as they can be a real PITA to deal with.

Here are a couple more pix:

Flying leads attached to board (http://www.diycustomamps.com/images/6U8_6V6/board_06.jpg)

Board installed in chassis (http://www.diycustomamps.com/images/6U8_6V6/chassis_09.jpg)

I've got quite a bit more wiring done than shown in the above picture. More pix to follow.

tung

[duhvoodooman]

Man, you are flyin' along on this one, Tung! :master:

[F_BSurfer]

looking good.......what kind of transformers you using on this one

[Ch0jin]

Nice work as usual Tung :)

I'm especially interested in what you think of the VVR in practice. I might have a lot of spare time in the near future, and I'm thinking of rebuilding my old frankenstein amp to sort out a few niggling issues. As it's probably going to remain around 50W (don't want to change trannies etc) being able to dial down the voltage and subsequent volume is appealing.

I'm also waiting eagerly to hear about the compression control :)

[tunghaichuan]

looking good.......what kind of transformers you using on this one

Thanks!

The power transformer is a vintage Triad R6A. I've used it for several tweed Princeton builds, and it works great. The output transformer is a Fender Champ 12 unit. 5K:8 ohms, rated at 10 watts. It should match well with the JJ 6V6S.

tung

[tunghaichuan]

Nice work as usual Tung :)

I'm especially interested in what you think of the VVR in practice. I might have a lot of spare time in the near future, and I'm thinking of rebuilding my old frankenstein amp to sort out a few niggling issues. As it's probably going to remain around 50W (don't want to change trannies etc) being able to dial down the voltage and subsequent volume is appealing.

I'm also waiting eagerly to hear about the compression control :)

The VVR is the last circuit I'm going to put in. I still need to order some of those NTE mosfets, which are pricey. The VVR shown previously only works with cathode bias amps. There is another VVR circuit for push pull amps which automatically tracks the bias voltage.

The compression control is merely a pot to regulate the screen. Randall Smith has a patent on it, so I would never be able to use it in a production amp :(

I'm attaching the preamp schematic I'm working from so that you can check it out yourself.

tung

[Jipes]

That's fascinating Tung I'm really curious to her and see the final result I have a friend who loves to play around with Tube amp but you seems to be kind of a "Tube Guru" isn't it ;)

I have unfortunately two left hands (I'm right handed) so I will never ever give it a try on this type of construction but I'm really curious and admirative for your work :master:

[tunghaichuan]

That's fascinating Tung I'm really curious to her and see the final result I have a friend who loves to play around with Tube amp but you seems to be kind of a "Tube Guru" isn't it ;)


Thanks, but I'm really just a hack. :D I like messing around with tube circuits and trying new things, but I really could stand to learn some theory.



I have unfortunately two left hands (I'm right handed) so I will never ever give it a try on this type of construction but I'm really curious and admirative for your work :master:

Tube electronics isn't something you should mess around with unless you're really committed to learning safety procedures. It is a lot of fun for me. And I'm looking forward to finishing this project to see how this amp sounds. I have high hopes. OTOH, it could sound like crap. :rotflmao:

tung

[piebaldpython]

C'mon Tung.....you can do it! :D SHINER set the bar (the limbo bar, not the drinkin' bar :D ) kinda high with his FIREFLY amp. :poke: :beer:

Seriously, you (along with DVM) are much too modest about your skills and knowledge about amps, pedals, etc. :master: :master: You are NO hack, that's for sure. I look forward to reading/hearing about your amp.

A long time ago, I installed a couple of ceiling fans in my bedrooms. I was scared half to death when I turned the electric back on. I half expected to see SMOKE and SPARKS coming from the outlet. :rotflmao:

So, when I read about what you guys are able to do, it just earns all of my admiration for the time and talent it takes to do all that stuff. KUDOS.

[tunghaichuan]

Again thanks for the kind words.



A long time ago, I installed a couple of ceiling fans in my bedrooms. I was scared half to death when I turned the electric back on. I half expected to see SMOKE and SPARKS coming from the outlet. :rotflmao:


About 3-4 years ago I replaced all the switches and AC receptacles on the first floor of my house. I did check out a book on electrical wiring to make sure my job was up to code. Haven't burned down the house, yet :AOK:

tung

[tot_Ou_tard]

Really clean work Tung!

I'm all eyes. My ears are getting jealous.

[tunghaichuan]

Really clean work Tung!

I'm all eyes. My ears are getting jealous.

Wednesday and Thursday were my days off, so I had a lot of free time to work on the amp. I probably won't get a chance to work on it again until tomorrow or maybe Tuesday.

One of the reasons I'm working on this circuit is that it would be a direct replacement for the Champion 600 amp. With a few mods it may even work for the Little Giant amp.

I became interested in pentode preamps after building the EF86 amp which is a hybrid Marshall 18W and Vox AC15. The 6U8/6GH8/6BL8 and a few others in that family were used in TV sets and made by the millions. They never were used in guitar or hifi amps with one or two exceptions so there is lots of NOS stock available inexpensively.

The other reason I'm building this prototype is to see if the 6U8 pentode/triode sounds good in a guitar amp. I have a push pull version on the drawing board that I plan on building for a friend. This is the friend who sent me the MI Audio Tube Zone pedal, so I wanted to return the favor with a well made boutique style amp.

Stay tuned, more on the prototype as I have a chance to work on it.

tung

[tot_Ou_tard]

Has the 6U8 been used in guitar amps before?

[tunghaichuan]

Has the 6U8 been used in guitar amps before?

I don't believe so. At least not in a production amp. It was used in this amp:

http://www.schematicheaven.com/newamps/sjb_ant.pdf

but as a reverb driver/recovery circuit and not for gain in the preamp.

The 6GH8 was used in an HH Scott hifi amp.

tung

[tot_Ou_tard]

Yow, forging some new ground then.

Here's hoping for some cool sounds. :AOK:

[tunghaichuan]

Got the wiring finished this afternoon. I checked my work over very carefully against the schematic and the layout and against the tube pinout.

It lives!

I always plug the amp in without tubes and turn it on to measure the voltages just to make sure I wired everything correctly. I plugged in a JJ 6V6S and a used 6U8, let them warm up, and took some measurements.

The power amp was right where I want it to be. With the JJ 6V6S in the socket, it was putting out about 12W static dissipation and was drawing about 40mA. The JJ is a pretty tough tube and can dissipate more than the 14W max for a 6V6GTA, but I didn't want to push it as the power transformer is only rated for 50mA.

The preamp voltages looked a little low, but I thought it was due to the oddball 6U8 in the socket.

I plugged the amp into my 1x12" cab and let it warm up. The amp sounded good, but not very loud, not nearly as loud as I expected. Not nearly as dirty as I expected.

I pulled the used 6U8 and put in a NOS GE 6678/6U8A tube. Much better. Tons of gain on tap, very dirty and nasty when cranked all the way up. The tube is microphonic though, but a tube shield took care of that. The amp is now very loud.

Just for kicks, I installed a NOS GE 6V6GTA, but to my surprise, the amp sounded much better with the JJ tube.

I hooked up an old DOD FX50B OD pedal. The amp seems to love pedals! I need to test it out with some of the other pedals I have, though.

Some more pix of the finished amp:

http://www.diycustomamps.com/images/6U8_6V6/chassis_10.jpg
http://www.diycustomamps.com/images/6U8_6V6/chassis_11.jpg
http://www.diycustomamps.com/images/6U8_6V6/chassis_12.jpg
http://www.diycustomamps.com/images/6U8_6V6/chassis_13.jpg

Just a couple more things to complete the amp:

1. I need to get a 500k linear pot for the compression control. I used a 250k linear as it was all I had on hand. I may try a 1meg for grins.

2. Obtain the parts for a VVR circuit and install them. This is a type of power scaling that allows the voltages to be dialed down to get cranked tone at whisper quiet volumes. The original MosFet specified runs about $12US, but I just learned of an alternative MosFet that runs about $8US.

3. Remove the hookup wire to the grids and replace with shielded cable. The amp has quite a bit of hum, more than I like so the first step is to install shielded cable.

Stay tuned...

tung

[duhvoodooman]

Nice pics. I am S-O-O-O-O dying to hear this thing.

[just strum]

Comin' along nicely there, Tung! :AOK:


Don't be so impressed, Pie. If Tung and I can handle this stuff, how hard can it be?? ;) :D


You and tung are too modest. Through my eyes, you two are artist.

To put it in perspective:

After doing a simple mod, you two are to me as:

My golf game is to Tiger Woods (play one round with me and you will realize how big of a compliment that is).

My guitar playing is to Buddy Guy, Clapton, SRV, Townsend....

[tot_Ou_tard]

Fantastic tung!

Is there any way we can get clips?

Vood, when are you gonna build an amph?

[duhvoodooman]

Vood, when are you gonna build an amph?
You must mean, "when are you going to build another amph":

http://www.thefret.net/showthread.php?t=5702

Answer: I dunno. Won't be real soon, since I have the new Egnater to play with for a while, at least. But I still want to do a "proper" amp build some time--something in the 15 - 20W range probably. A Marshall 18W clone or a Fender 5E3, most likely, but who knows? These recent projects of Tung's look REALLY interesting, so maybe one of those. Choices, choices, choices....

[tunghaichuan]

Update:

I had a chance to play my amp last night for a while. Mrs. Tung was out so I could crank it without annoying her. :D

I tested it with a bunch of pedals, a Weber Mini Mass and my Hohner (http://www.diycustomamps.com/images/Rev_RTS_1.JPG) and Warmoth (http://www.diycustomamps.com/images/warmoth_strat_1.jpg) guitars. The Hohner has true single coils, but I don't know any of the specs on them. The Warmoth has stacked humbuckers, SD Hot Stack Strat pups which are very bright for humbuckers.

Observations:

1. The amp is dark. Even with the Tweed Princeton tone control all the way up, it is not very bright, even with single coils.

2. The tube I have in there is microphonic and while the IERC tube shield helps, the amp still squeels a bit with all the controls dimed. Probably needs some shielded wire to the grids. I've only tried a 6U8 tube in the amp. I need to try some 6GH8 and 6BL8 tubes.

3. The amp does not like pedals as well as I thought. I tried a BBE Green Screamer (TS808 clone) a Washburn Soloist OD, a Rocktron Blackjack booster/distortion and my BYOC Triboost. Most of these sound very good through my Little Giant and Valve Junior heads. They only sounded so-so with the 6U8 amp.

4. The natural, cranked OD sounds pretty good, but the tone control has to be turned up all the way to keep the bass notes from farting out. The Triboost sounded the best on the germanium setting, with treble boost sounding the best and mid booste a close second. The amp did not like the full range boost settings on the pedal. I also tried the amp through a Weber Mini Mass. This is the older style without the bright settings. With this amp the MM could really use them.

I need to get the compressor circuit sorted out first. Then I may try reducing the coupling caps a la Vox to get a brighter, chimier tone out of the amp.

I'm going to try to get some sound samples up in the next few days so you can hear what the amp sounds like now.

Stay tuned...

tung

[BigDaddy]

I am about to start on a similar build. It's a 6V6 SE power section with a 6U8a preamp tube. I do not know exactly how to approach this. Do I use the triode as the first stage then TS then pentode into the 6V6 or the pentode first. Or should I add another stage from another pentode or triode before or after?

So I was wondering if you have sorted out the amp and have a schematic to share.

I was thinking of possibly using a 6AT6/6AV6 then into the 6U8a. Maybe using the triode part to drive a CF TS into the pentode.

[mrmudcat]

Nice looking stuff Tung!!!:D

[tunghaichuan]

I am about to start on a similar build. It's a 6V6 SE power section with a 6U8a preamp tube. I do not know exactly how to approach this. Do I use the triode as the first stage then TS then pentode into the 6V6 or the pentode first. Or should I add another stage from another pentode or triode before or after?

So I was wondering if you have sorted out the amp and have a schematic to share.

I was thinking of possibly using a 6AT6/6AV6 then into the 6U8a. Maybe using the triode part to drive a CF TS into the pentode.

I built the amp using Kerry's schematic from the AX84.com site. Post or PM your email and I'll send it to you.

I used the pentode section as the input gain stage. The triode was used as the driver for the 6V6.

You could reverse that though, use the triode as the input gain stage and pentode section to drive the 6V6.

The 6AT6/6AV6 will give lots of gain cascaded into the 6U8. Be prepared to use a voltage divider circuit to pad down the gain.

tung

[tunghaichuan]

Nice looking stuff Tung!!!:D

Thanks, man. I need to dust off this circuit and experiment with it some more.

tung

[BigDaddy]

I sent you my email addy. Thanks a bunch.

I only build SE amps in the 5-10+ watt range with a variety of preamps and power tubes. It seems every time I think I come up with an idea I end up reinventing someone else's wheel.

But I try to take it to another level. I have just finished a dual cascaded 6SJ7 and a dual cas EF-86 both into similar 6V6 SE power sections.