Questions about oscillation

[BoomTexan]

So, I have actually never played through an oscillating amplifier or heard what it sounds like, but with the amp I'm currently working on, the owner told me that it was oscillating badly. He took the tubes out, so I'm waiting on a set of 6L6s before I start working on it.

My question here is: what causes oscillation? What parts should I look at that could've failed or gone out-of-spec and caused the issue?

 

[Jon Snell]

Firstly, remove the negative feedback loop and if the oscillation stops, swop either the anode wires or the speaker wiring to reverse the polarity on or from the output transformer. If it is the wrong polarity, it will oscillate uncontrolably.
If that is not your cure, check to see what frequency it oscillates at and ask again with that information.

 

[Bendyha]

Firstly, remove the negative feedback loop and if the oscillation stops, swop either the anode wires or the speaker wiring to reverse the polarity on or from the output transformer. If it is the wrong polarity, it will oscillate uncontrolably.
If that is not your cure, check to see what frequency it oscillates at and ask again with that information.

You're right in what you say there, but the idea of swapping around the speaker wiring might be misleading.
It is not just changing over the wires on the speaker that's needed, but inverting the O.T. output, so that the NFB side and the grounded side are changed....which is not advisable if there are various impedance taps. Swapping around the anode wires is usually easier.
I had a silverface Bassman to fix that had this problem, after about 25 years of dissatisfying use, by several owners probably, and a quick wire swap solved the problem it had left the factory with.

Otherwise, what the amps' owner refers to as oscillation could be anything from low frequency motor-boating, to inaudibly high frequency feedback from bad dressing (that can manifest themselves in several ways), maybe just a ringing preamp tube, or any number of other howls that can come from all sorts of causes.
So, as Jon advises, checking to see what frequency it oscillates at will help with the diagnosis.

 

[Jon Snell]

By far the easiest way to reverse the nfb is to swop the drive from the phase invertor. Two capacitors.
But not knowing the extent of experience there is out there, I suggested swapping the anode leads after removing the nfb loop to check if that was the cause.
Obviously I added the note, about posting the frequency of the oscillation which will tell us straight away whether motor boating, (failed decoupling capacitors) or screaming, (ceramic capacitor etc), a noisy valve is also a possibility.

 

[BoomTexan]

By far the easiest way to reverse the nfb is to swop the drive from the phase invertor. Two capacitors.
But not knowing the extent of experience there is out there, I suggested swapping the anode leads after removing the nfb loop to check if that was the cause.
Obviously I added the note, about posting the frequency of the oscillation which will tell us straight away whether motor boating, (failed decoupling capacitors) or screaming, (ceramic capacitor etc), a noisy valve is also a possibility.

I'm just gonna be doing basic maintenance on the amp for the moment, replacing all electrolytics, out of spec resistors. I'll wait for the tubes to get here and check if it's still there. I'm really hoping it was a problem with the old tubes.

If it persists, I'll definitely do those suggestions.

The main issue is that there is no info about this amp online. It's a Japanese Twin-style clone. It's very different from an actual twin. It has an EL84 reverb driver along with 6 12AX7s, and a really weird filter cap section, along with the apparent lack of any bias capacitors. It also uses a lot of right angle terminal strip construction instead of eyelets or turrets, so there's a lot of potential crossed wire failure points. It's gonna be a job getting this amp back up and fully functional.

 

[Jon Snell]

Without knowing what the amplifier is, you don't mention it, we can only guess. Some of us may recognise it from the model number, after all, an amplifier is an amplifier is an amplifier. Not a lot can be done to change the basics, if anything.
Maybe helpful to you.

 

[Dacious]

Oscillation can be caused by a tube. Usually but not always this can be ringing feedback from a tube. Sometimes it's inaudible. I had a Laney 100 watt Plexi clone with a blown power tube that kept running away. If you kept all the controls zeroed it was fine. The moment you turned the volume up the tubes redplated and it started humming.

Replacing the first preamp tube solved the problem.