Noise upon playing

[J-Sty]

Alrighty Metroampers... I have a doozy for you guys. No one seems to have any answers on this one. Maybe someone can offer a jem.

I'm rebuilding a 100W Super Bass clone and so far very few hiccups, things were looking really good. However, I've ran into a problem that no one seems to be able to help me with which is why I'm resorting to an emergency post!

I get proper voltage readings in my amp and everything appears to check out great. When I take the amp off standby it makes that nice slight hiss when I crank it all the way but besides that no ticks, noises or anything else to speak of.

When I plug into any of the input jacks everything goes good and no noises or bad sounds. I can turn up the amp volume all the way with my guitar plugged in, still no noise. I can also play single notes or some very simple two note chords and still no noise. However, whenever I turn up my guitar all the way, or play a full open chord I get this terrible diesel truck noise through the speakers. The noise starts medium/low pitch and then gets lower and lower. After about 15-20 second the noise stops. It's quiet again until I try to strum a chord again. Then the same thing happens. I've tried moving wires and checking the ground to see if it's some sort of ground issue and nothing seems to make a difference. Unplugging or turning the volumes all the way down makes no difference once the noise has started.

No answers so far... Any info at all would be very greatly appreciated.

[elronhoover]

If you go thru the amp, starting at v1a, and touch your meter probe to the grid on the tube socket directly (dont crank the vol way up), do you get a pop noise in the speaks (you should if all OK)? This can eliminate per section any wiring/connectivity issues.

Possibly a bad ground or a cold solder joint somewhere is giving you an intermittent and/or poor signal path.

Have you subbed any tubes yet?

Dave

[J-Sty]

Thanks for the reply Dave. I have swapped all the tubes, and no change. I'll definitely go through the amp and check the grids as you suggested. I don't really have a reason for this, but I have a sneaking suspicion that it's a ground problem... Anyways I'll post back when I get it done an hopefully we can knock one more thing off the list.