High frequency oscillations when diming controls

[dave g]

I was just wondering, is it normal for D-style amps to oscillate when you, for example, max out the volume, treble, OD drive, OD level, and master volume controls all at the same time? I know these amps are not meant to be used with the controls turned up that high, but I was just wondering if I was the only one to experience this. All my amps work fine under normal conditions, but they all seem to start oscillating when you max out the treble, drive, volume, etc all at the same time.

[mlp-mx6]

If you're talking about in OD mode, perhaps you should lower your OD entry trimmer.

I really do not think that any D-style is SUPPOSED to oscillate at any setting.

[heisthl]

A good tip when building is to put the most microphonic tube you have in V1 and work with the lead(s) layout for the least oscillation. Then when you put in a good tube you have bought yourself a little insurance for when a tube starts to go microphonic.

[dave g]

Thanks for the tips, guys. I really haven't had any trouble at all with oscillations at any reasonable settings that anyone would ever play the amp at. Like I said, things only get weird when you're on OD with the PAB on with the preamp volume maxed, the drive maxed, the OD level maxed, the presence maxed, and the master volume turned all the way up. I can't imagine why anyone in their right mind would ever turn the knobs like that, but any oscillation at all makes me uneasy. I played with the lead dress and swapped a better tube into V1 and it made it better, but I still get an oscillation with everything turned up with OD and PAB on. I'll take a look at it on the scope when I get a chance.

[heisthl]

anything in the swath between the front panel and v1&2 could cause this including bright cap lead dress. Be especially suspicious of wires running under the board in that area.

[burnt fingers]

I've been running into this one often.

[dave g]

Ok, last night I took the amp down to the basement and hooked it up to the scope.

Nothing whatsoever.

Down in the basement surrounded by florescent lights, I could not get the thing to oscillate no matter what.

I take it back upstairs, and boom - it's oscillating again

[Structo]

Sounds like something in the circuit you have it on but I don't know what it could be.
I know I had my B+ wires a little too close to the ac primaries and I was getting some noise on the OD channel. Kind of a switching noise, possibly from the diodes.
I chop sticked around until I found the are that had the noise.

I re-routed those away from them and it's very quiet will all the controls dimed. ( I don't play it that way, I just wanted to see if I cured the problem)

[greiswig]

I just tried mine with the controls dimed (I'm trying to figure out if an oscillation is causing my OD fizz), and it didn't do anything other than make some nice white noise through the speaker.