Weird feedback ?

[Vaughan]

Hi ,
I'm a relative newbie to tube amps, so please bear with me, my knowledge is limited. A while ago I purchased a couple of tube amp chassis off a well known auction site. They are Chinese built I think with the name "Tube30" on the face-plate.
They were both faulty, but only with minor problems. These were fixed and the units tested on the bench with a good speaker cabinet. There were no apparent problems, even when run flat out for extended periods.
I have now built new 1x12 combo cabinets for them using Celestion speakers. NOW I have a Probem!!!
The amp sounds really good until I hit a bottom E string when it goes into feedback, at the same frequency as the E string. The only way to stop it is to kill the power! I've tried a new set of tubes, checked the voltages, which I think are OK (see schematic). Could this be some sort of resonance effect? Has anyone any ideas???

[Chuck H]

That is wierd. It could be a microphonic tube. Try a preamp tube swap. You could also try this: Get the feeback going and then lightly grab each preamp tube. Sometimes this will stop a microphonic tube temporarily, but not always. But it will give a clue if that is your problem.

Chuck

[Vaughan]

Weird feedback

Hi Chuck
I've tried all the tubes - changed the whole lot. Strange thing is, when it starts, you can zero all the controls and pull the guitar lead out, but it still continues. It CAN be stopped by putting hand onto the speaker cone when it stops instantly, so could it be some sort of acoustic feedback?
I'll try moving the amp chassis out of the cabinet tomorrow. Maybe it's a mechanical linkage.
If it works I'll post again tomorrow.

Vaughan

[52 Bill]

...Strange thing is, when it starts, you can zero all the controls and pull the guitar lead out, but it still continues. It CAN be stopped by putting hand onto the speaker cone when it stops instantly, so could it be some sort of acoustic feedback?

There can be other discrete components that will act like transducers, such as ceramic caps, inductors, etc.

If it doesn't shut down with any of the controls, the problem lies in the circuit after these controls. Try the tap test on all of the components in the power amp section to see if any have become microphonic.

[Chuck H]

+1

I've seen this before. One time it was a head that I mounted in a combo cabinet. Out=good, in=bad ?!? Turned out to be a ceramic disc cap.

That you can 0 all the knobs, unplug and then stop it by muting the speaker tells the whole story. Something from the last control foreward is resonating acoustically and feeding back with the speaker.

Sometimes the "tap" test is difficult because it seems like no matter what you tap, you get a thud or you get the feedback. In this case you would get the feedback started and then use something soft (not your finger) to damp vibration from individual componants until it stops.

Chuck

[Mars Amp Repair]

You might be certain you have the output transformer primary leads properly phased. IOW, just try swapping the OT primary leads between the power tubes.

Although this usually causes a very high frequency oscillation, I've seen weirder results depending on the negative feedback setup.

Hey, worth a try anyway for a werid problem...glen

[Vaughan]

Hi All
I've been tinkering with this problem today. The feedback starts with the volume and gain at about 5. A very gentle touch on the input sockets will start it going. I've checked the socket, all the components past V1 including resoldering all the joints.
Just as I was about to give up for the day I had a flash of inspiration ;-) I disconnected the 100K NFB resistor. Now, the feedback has virtually disappeared, even with the Voume/gain maxed. There is just a hint that it may startup again if the bass control is maxed.
Is it possible that the bass response is way too high??
I'll test with guitar tomorrow to see what it sounds like.

Vaughan

[Mars Amp Repair]

Sounds like you have the transformer primary wires to the output tubes reversed. Swap them & see if that resolves you issue. BTW with the primary wires swapped, the negative feedback designed to increase bandwidth & as a result reduce gain, becomes positive feedback & does just that. glen

[Vaughan]

Hi Glen
You were absolutely correct. I reversed the O/P tranny wires and Hey Presto, the feedback stopped. In actual fact there were two faults, the other one being a very microphonic capacitor in the V1 circuit. Not having too much experience with tube amps, I assumed that a phase reversal always gave a high frequency howl. I've learned something new today. I find you always remember the nasty ones.
Many thanks to everyone who came in with valuable advice.
Vaughan