6G12-A Concert clone behaving oddly

[Pin10]

Howdy all,
I have been chasing a problem in a scratch-build I've recently finished. I seem to have the problem sort of narrowed down, but not in a way that makes any sense to me. If I plug in and play at low to moderate volume, no problem. As soon as I play loud (volume knob at 5 or so through either channel) there is a nasty distortion/pop/crackle sort of sound that lasts for about the duration of a hard strum, in other words a pretty short time. Because the two channels have only the driver and power tubes in common, I've been looking there. Adjusting the bias up or down appears to have no effect. I have replaced plate resistors for the driver and the coupling caps between the driver and the power tubes. Here's the exciting part: the problem goes away completely if I have an ammeter (analog, I haven't tried it with a digital one) in line with either power tube, in other words in series with pin 8, which goes straight to ground on this amp. I see that the ammeter measures about 4 ohms. Does this make any more sense to any of you than it does to me? Thanks, and happy nearly new year!

[tubeswell]

Blocking distortion? Have you tried grid stoppers on the signal grids? 1k5-4k7 for output tubes, 10k-68k for pre-amp tubes. Also lowering cathode bypass caps in earlier stages might help, as might lowering coupling caps. But try 1 thing at a time.

[Pin10]

I have them in place, but changing the grid stoppers on the power tubes sounds like a good item to check...and I can work through the rest of the list if that doesn't get it...thanks!

[Enzo]

Offhand, it sounds to me like you are hearing parasitic oscillations on your peaks. And that is more than likely a problem with lead dress - where you run your grid wires relative to the plate wires.

[Pin10]

I'm sure my lead dress could use some work too, but 1500 ohm grid stoppers on the power tubes did the trick. when I said yesterday that I had installed them, what I meant was that I had not installed them...I just didn't know what I was talking about. At last I can get this behemoth off my bench and get the autoharp restrung! Hahahahahahahaha! I can't thank you guys enough! Happy new year!