New Project: 6U8/6V6 amp

[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. :

[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.

[mrmudcat]

Nice looking stuff 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.