Tweaker 15 Head volume fizzles, not a tube issue HELP!

[12growon]

I bought a tweaker demo model from GC (bad idea I know) and it played beautifully for a couple weeks (didn't get to play it too much though), but the volume now cuts down low or off randomly especially if you play lightly, but when you stroke the strings hard it blares back to life. Play softly for a few seconds and it fizzles back out again. Any ideas? It's past the 30-day return or I'd take it back to GC. I'm good with an iron and have built an 18 watt before, but I've run out of ideas on this one, three different guitars, couple different cables, all switch positions and all new tubes, the same thing still happens. Searched all the forums for the last few weeks can't find anything other than my cathode resistor is 500 ohm and almost everyone else seems to have a 225 or 330 ohm, would that cause the issue? Any suggestions would be much appreciated. Just don't want to pay and wait for shipping if it's something I could knock out myself.

 

[12growon]

The plot thickens.... changed the bias resistor to 330 ohm and the cap to 100uF, problem still exists. Crazy thing is, now I was able to get it to drop volume on me to low (but signal was getting through). Put an O/D pedal to crank up the input, still barely any volume, pedal off and out of circuit, same thing. But...... I flipped the amp to standby while a chord was playing, the sound of course died out, and there was a slight pop in the speakers, flipped the standby switch right back to play position and the volume came back! Sounds like the problem was momentarily corrected when a capacitor discharged? (Or when power was drained off the tubes.) Of course it came back a few minutes later... Is there a built-in overload protection on these? Almost seems like it's something early in the circuit maybe but I'm completely stumped... Anyone out there with a bright idea? Hate to give up on it, it's awesome when it works.

 

[Buckeyedog]

best bet is to e-mail Bruce direct.