Audible oscillation while plugged into dummy load

[Gaz]

While trying to debug a new amp of 120 cycle hum, I heard an audible high pitched oscillation with all the controls maxed. Now Two Problems!

I don't know if it's normal to hear an oscillation without a speaker plugged in, but there is an oscillation nonetheless. What would be my first step to getting rid of it?

Here are link to pictures inside the chassis and the schematic as well:

Buzzing amp pictures by the_gaz - Photobucket

http://img69.imageshack.us/img69/9020/kt77amp.jpg

Thanks!

[MWJB]

It's not unusual to hear a whine when running an amp into a dummy load.

It's also not unusual for Marshall style amps to oscillate with treble & presence maxed out.

You have all your filter caps daisy chained together, rather than main & screen caps grounded at one end of the amp, preamp & PI grounded at the other.

[Gaz]

You're sure right about a Marshall. I guess I thought mine would have to be 'better' than a Marshall I suppose I was wondering if I could stabalize the amp somehow to cure the oscillation. I'm only aware of the cap-across-the-plate resistors, or putting caps to ground at the output tube grids.

I used the "galactic ground scheme" advocated by Kevin O'Connor and others, where the many localized stars are daisy-chained (as you noted) and grounded only at one point, in my case, the input jack.

Everything appears to be grounded in its right place according the that scheme, but I'm still getting a bit of a hum with the volume controls down and the master up. It disappears when the 2nd tube is removed.

[Amp Kat]

What I do in those situations is take the output from the plate after the coupling cap and send it to the PI in. Move on down the line of gain stages plate to plate. When the hum shows up then you know that's where the problem is. If it's humming from the first tube it's going to hum downstream as well. If you can make sure that first gain stage is hum free at least you know where you stand on the grounding points.

[loudthud]

I'm not seeing the 47pF cap you typically see in Marshalls and 5F6As between the plates of the phase inverter. Have you tried a cap there?

[Gaz]

I'm not seeing the 47pF cap you typically see in Marshalls and 5F6As between the plates of the phase inverter. Have you tried a cap there?
Today 04:50 AM

I tried a few values there, and it lowered the oscillation in intensity and frequency, but did not get rid of it. The amp doesn't have that much gain, so I'm troubled by the oscillation.

What I do in those situations is take the output from the plate after the coupling cap and send it to the PI in. Move on down the line of gain stages plate to plate. When the hum shows up then you know that's where the problem is. If it's humming from the first tube it's going to hum downstream as well. If you can make sure that first gain stage is hum free at least you know where you stand on the grounding points.

If I jump from the mixing resistors of the 1st and 2nd stage to the CC before the PI, the hum goes away. It may be that the gain is so low that buzz becomes inaudible, however.

[Bruce / Mission Amps]

I tried a few values there, and it lowered the oscillation in intensity and frequency, but did not get rid of it. The amp doesn't have that much gain, so I'm troubled by the oscillation.



If I jump from the mixing resistors of the 1st and 2nd stage to the CC before the PI, the hum goes away. It may be that the gain is so low that buzz becomes inaudible, however.

I'd suspect the layout with respect to the negative feedback, "fixed" presence circuit and your preamp stages.
Too see if it is the presence/NFB circuit causing the hi-freq oscillation, disconnect the NFB for a test.

[Gaz]

while troubleshooting the buzz, i disconnected the NFB, and that's when i noticed the oscillation. i just realized that after you mentioned it. i'll have to see if it persists with the NFB hooked up. thanks.

[Gaz]

still does.