Opinions on momentary oscillation in tube preamp

[lassoharp]

I'm looking for some opinions on what may be causing a momentary oscillation (low freq howling type) in the amp shown in the attachment.

The amp has a load on both input (150r source) and output (10K load) for the following:

What happens:  

The oscillation only occurs under the following conditions:

When the amp is first powered up - right as the tubes begin conducting current - ONLY when the interstage volume control is turned completely down.  The howling lasts for a second, then vanishes. (It also reappears as the amp is powered down).

If the volume control is turned up past a certain point** before turn on, the amp powers up without issue - no howling.

** Once the amp is on, there is a click/thump/pop that occurs at precisely the point of rotation on the volume pot that corresponds to the point where oscillation will occur at start up.  The pop sounds like DC type thumping.  The preceding coupling cap is not leaking though.

Best I could figure, the amp wanted to see some resistance between the lo side of the vol pot and ground.  I installed a 1K resistor there and it 'cures' the howling and thumping - I can power up with vol control all the way down without issue, and there's no pops when rotating vol control through the critical point.

The concern is that the amp may be too close to an unstable condition as is, even though the oscillation  at this point is conditional.  Other than the momentary osc, the amp works and sound fine.

I'd greatly appreciate if someone could run this circuit through a simulator and see if anything shows up.  


Much Thanks,

Alan



EDIT - The full PS chain is:  275-0-275 > 5Y3 > 470r > 40uf > 1K > 80uf = B+

 

[emrr]

What happens if you directly ground V2 grid?

 

[lassoharp]

same thing.

Also tried several combinations of coupling cap values - same.

 

[ruffrecords]

This sound like the B+ impedance is too high. Can you post a PSU schematic??

Cheers

Ian

 

[pstamler]

Agreed on the too-high supply impedance. I'd try boosting the values of the decoupling caps.

Also, though this may not be related, try direct-coupling the cathode follower's grid to the previous tube's plate. It'll take one time constant out of the picture.

Peace,
Paul

 

[lassoharp]

Apparently the oscillations, pops, and several other erratic voltage issues were being caused by a bad decoupling cap - specifically the decoupler for the output stage.

Though not shown in the above schematic,  I had been experimenting with a 1uf cap in // with the output stage decoupler (20uf).  The 1uf was an oil type I've been using on the test bench for several years but it seems to be on it's last legs.  Removing it seems to have cleared up all issues.  This has been another good learning experience on the widespread ill effects of dying capacitors.

Here's the final, full PS schematic:

 

[lassoharp]

Also, though this may not be related, try direct-coupling the cathode follower's grid to the previous tube's plate. It'll take one time constant out of the picture.

Thanks Paul - I'd been wondering whether that would work well in this circuit.  I haven't worked much with cathode followers.  One less pole in a high gain circuit makes good sense.