Low-Frequency Feedback

[harry]

Hi there,

I'm working on a tube amp that has some issues, it's a new amp but it needs some tweaking. It was working fine until just recently when it started feeding back massively when a note on the lowest three strings is played.

This feedback is low pitched and happens almost instantly. It is uncontrollable, turning neither the eq/gain nor volume knobs stops it. The only way is to put it on standby.

Also, the pilot light flickers, although I don't think it's due to power supply sag, because it's not necessarily when I'm playing. The flicker is just a slight dimming of the light. Does anyone have any ideas what that might be?

Any help would be much appreciated!

Harry

[Alex R]

Sounds like oscillation, often caused by internal feedback between stages. Preamp filter caps are often the culprit. This happens to the modern Fender range a lot. I take a 33uF or so cap, clip the -ve to the chassis, and the +ve wire to the +ve of a preamp filter cap. Then try again with the note. If the oscillation disappears you've found the problem. Doesn't really eliminate the caps if it doesn't go away...

Move some signal leads around with a wooden chopstick. Sometimes you get induction plate to grid that way, or via other more arcane routes.

Oh do try swapping in a good preamp valve along the line first - they can go microphonic.

[loudthud]

More info about the amp would be helpfull. Is it a home built or a store bought amp? If home built, is it similar to anything? Describe the power amp (tubes, transformer, coupling caps and phase inverter) and power supply (rectifier, caps, choke, resistors).

There are two possible causes that I can think of. The power amp is unstable because there is too much gain at sub-sonic frequencies such that there is excessive phase shift at the unity gain crossover point. This applies to power amps with feedback loops. Disconnect the feedback loop to see if it goes away. The other cause would be feedback through the power supply. Could be a bad cap or inadequate filtering.

[harry]

Thanks for the tips!

It's a production amp, made in China but suffered from poor soldering and a few issues (heater wires too thin, multiple ground points).

It's got 2x6L6s, 2x12AX7 with one gain stage, one PI.

I'll have a look later for more info, in the meantime, there's a schematic here:
http://i81.photobucket.com/albums/j2...e/CIMG2103.jpg

[Alex R]

Check C43 by bridging it with a new cap or replacing it, and check the two resistors that connect to it, then check C42. Also try a new preamp valve.

[Enzo]

When it happens, grasp the tubes firmly and see if it stops - you could have a microphonic tube feeding back.