parasitic oscillation ... now what?

[PRNDL]

I think I have parasitic oscillation in my latest amp build - a 6AK6 push-pull amp.

It has a fairly loud hum - not tons, but enough to be annoying. The amp played fine for several months and then began to have scratchy distortion on top of notes played with the volume turned up.

I hooked up the scope and found a 40 mV PP sine wave (almost) at about 100 Hz (5 x 2 ms) on the cathodes of the output tube (fixed bias), and a strange 6 V P-P high frequency (2.5 x 2µ) sin wave with varying phase before the grid resistor of one of the output tubes. This happens only when the volume is at about 8, which is about where the scratchy distortion occurs.

I've attached a schematic.

I used the chopstick test and re-soldered almost everything, but it's still there.

I've also ordered 6AN5's to replace the 6AK6's, which many say can't take more than 180 volts (this amp has about 270).

It's also been mentioned that the power supply could use one more cap to separate the PI and preamp supplies ... could this be it?

It might also be the grounding.
The AC ground (green wire), power transformer (CT and heater CT) and power supply ground from the board (black wire) are all soldered together to a standoff on one of the transformer bolts. The preamp side of the board is grounded through the pot to the chassis.

I'm not sure how to proceed and am seeking your advice!

Thanks

Ron

Here's a photo of the chassis (it's the Dyno Tweed)

[PRNDL]

I changed the grounds and the parasitic oscillation seems to have disappeared, although the amp still has hum and scratches when played with the volume over about 8.

Here are the changes so far
- the PI ground is now with the PS cap ground
- the volume/tone pots are grounded to the preamp ground (cathode RC)
- the heater CT is now grounded to the chassis
- the output transformer return is grounded to the chassis

The scope shows power supply ripple on the preamp tube output side, which goes through the rest of the amp. It's not tons (20 mV or so, if I recall correctly).

I'm wondering if that first preamp tube is weak, which would be a problem since this amp has minimal PS filtering. Adding another filter cap is possible, but its going to take a lot of work. Plus it will make the amp less responsive. There's room to add another cap in parallel to the second or third one, or put in a larger value. What do you think?

Any suggestions would be greatly appreciated!

BTW
The power supply caps are 22µ - 1K - 10µ - 400 - 10µ
I have some extra 10µ caps handy.
I'm also wondering whether that 400 ohm resistor should be increased to 5K
(You may have guessed that design calculations are not my forte).

[PRNDL]

I replaced that 400 ohm resistor with a 22K, which reduced the hum to reasonable levels.

The scratching at high volumes is still there. It's strange - the hum is fairly silent, then increases dramatically at one point of volume (8 to 9) and then drops off again at 10. It's the same point that the scratching occurs.

I'm wondering if it might be a bad volume pot, although I'm hard pressed to say how it could do that. It's a volume and on/off switch combo and I vaguely recall having a weird issue with another one purchased at the same time.

Has anyone else experienced issues with volume/on/off switch pots?

At this point, I'm trying any and everything.

[pdf64]

Scratchy pot can indicate dc on it, so check for that. Or it could just be bad pot which maybe is letting the wiper go open circuit at certain places.
I wouldn't have thought that there was enough gain on that amp for parasitics.
Ground the A and B node caps at the transformer B+ CT, but keep the C node cap ground seperate, at input jack or vol pot case.
Good luck - Peter.

[PRNDL]

Scratchy pot can indicate dc on it, so check for that. Or it could just be bad pot which maybe is letting the wiper go open circuit at certain places.

What's unusual is that it's not scratching when you turn it. The notes have a scratchy distortion only when the volume is high.

I wouldn't have thought that there was enough gain on that amp for parasitics.

It may have been the grounding.

Ground the A and B node caps at the transformer B+ CT, but keep the C node cap ground seperate, at input jack or vol pot case.

Done!

Thanks for the suggestions!

Tomorrow I'll most likely be replacing that pot.