I'm stumped!

[H Chandler]

I've been building a single ended 6L6GC amp and it finally got finished to the point of making sounds. It sounds OK-- well at first it did, but when I crank it up the sound starts to get (for lack of a better term) fizzy/fuzzy and the output signal collapses (gets real quiet). If I continue, pushing it, there is no sound at all. Then, after backing down the volume and waiting for sometimes a real long time, the sound starts to slowly come back, at first fuzzy, like it did going into problem, and then eventually everything is back and it sounds good again. That is, unless I crank it again. I did some reading, and thought it might be HF oscilations, but I connected a 200Mhz scope to it and it's just a flat line?

What can cause something like this?

Thanks,

Howard

[Tom Phillips]

Howard,
Some thoughts:
1) Darken the room and look for any tube heaters going out when the problem happens. If so check the wiring and tube sockets. Visually inspect for bad solder joints.
2) A cap could be charging up and changing the bias of a stage. Can you monitor the output tube bias current when you play the amp and make the problem happen? See if it shifts.
3) Look for missing grid leak resistors
4) Monitor various power supply voltages and watch for changes when the problem happens. Including the screen voltage of the 6L6. Any strange changes will give you a clue of where the problem could be.
Could be other things. Hope the above helps.
Tom

[H Chandler]

I don't think it's filament related.

Next session, I'll clip my DMM on the output tube's cathode resistor. The cathode resistor is 500V 20watt bypassed with a 25/50V ecap. After reading about grid blocking, I increased the grid resistor to about 60K and with the standard 220K resistor from the preamp side to ground. The PS uses a 40-20-20-20 (525V- CEdist) can cap and uses a choke. B+ is about 435V The screen voltage was really high connected to the choke, so I dropped about 50V with a resistor. The scope shows that the signal is stable as far as the PT grid, so it's something in the power section.

Thanks

[Enzo]

What kind of output transformer did you use for this single ended amp? Intended for single ended, not just half a push pull tranny?

I'm with Tom, monitor the DC voltages around the power tube.

And I really like the missing grid resistor call, since the symptom does sound like a cap charging up.

When it goes silent, scope the control grid, is the signal still arriving there from the preamp?

Isolate the problem - it is the power amp or it is the preamp or it is the power supply or it is something else?

Give it a load and apply a signal on the bench. Now crank it until it fails again. Now scope each stage for signal, looking for the stage that loses it.

[H Chandler]

You guys are great!

You nailed it, it was a missing grid leak resistor!!

I had cascaded two preamp stages after the tone stack and having had the same problem I thought it might have to do with overdriving the 6L6 grid, I moved the power tube's grid stopper back to the previous stage's coupling cap, but forgot to move the grid leak resistor. Come to think of it, I probably needed to have a grid leak resistor on the 12ax7 gain stage too, yes? 1 meg sound good?
Also, just to figure out in my mind what was happening; without the grid leak resistor, the grid builds up a charge in the coupling cap and the Miller capacitance of the output tube, right? This charge drives the DC bias on the tube's grid so positive WRT the cathode that the tube loses bias, pinching off the signal? Yes?

Thanks,

Howard