New Project: 6U8/6V6 amp

[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. :

[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/...chassis_10.jpg
http://www.diycustomamps.com/images/...chassis_11.jpg
http://www.diycustomamps.com/images/...chassis_12.jpg
http://www.diycustomamps.com/images/...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! :


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

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.

I tested it with a bunch of pedals, a Weber Mini Mass and my Hohner and Warmoth 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!!!

[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!!!

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.