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Guitar amp oscillating

Postby yaire » Thu Nov 28, 2019 9:27 pm

Hi people,

Seeking some advice here.

I have a Hughes & Kettner Tubemeister 18.
Today I noticed that the lead channel oscillates heavily when I switch to it.
The clean channel works fine.

A few weeks ago I spilt some coffee and some creeped into the bottom of the amp.
I cleaned that up from the inside of the chassis. I did not see any on the circuit board or near pots, switches or jacks.
At that point I did not notice any strange behaviour. Can't remember whether I switched to the lead channel since then.

I am only using the amp on the clean channel.

I already tried swapping out all the tubes, no difference.

Now I am wondering is there any danger in using the tube amp at home like this?
There will not be any fluids inside anymore.
But the high voltage inside does bother me a little.

I would prefer not to get it repaired yet since I don't use the lead channel and money is a bit tight.
But I don't want it to be a health hazard either.
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Re: Guitar amp oscillating

Postby yaire » Thu Nov 28, 2019 10:35 pm

yaire wrote:At that point I did not notice any strange behaviour. .

Sorry, could not find an edit button.
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Re: Guitar amp oscillating

Postby Wonks » Thu Nov 28, 2019 11:18 pm

What's the gain setting on the lead channel when you've usd it. Does it always oscillate or only when the gain is up high?

You've changed all the valves - both preamp and power amp? If it was down to a valve, it would be almost certainly be a preamp one.

I've only used one amp that self-oscillated, and that was a Mesa Boogie 22 back in the late '80s. I have used a Tubemeister 18 combo, and that didn't show any signs of self-oscillation on the gain channel, so it's certainly not a typical behaviour of the amp. They aren't that old an amp, 8 years at the most, so it's unlikely to be a dried-out cap, though it could still be a faulty one.

You're right to be concerned about the internal voltages, so leave any investigation to an experienced valve amp repairer. Don't remove the chasis cover unless the amp has been unplugged for at least a full day.

Hopefully ef37a or Folderol will be along soon to suggest what could cause oscillation in an amplifier. I have a feeling the coffee spill was just coincidence.

But could it be that a guitar pickup has gone microphonic, rather than the amp? This would certainly show up far more on the lead than the clean channel. So does it oscillate with all your guitars (if you have more than one) and all pickup selections? If you plug just a lead in, the use a bit of wire to short between the tip and screen connections on the jack, does the lead channel still oscillate? With no lead in, does the lead channel oscillate?
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Re: Guitar amp oscillating

Postby yaire » Fri Nov 29, 2019 12:01 am

Thanks for your reply!

I have the head version with a 1 x vintage 30 cab.
I have it set on 1 Watt output with the attenuate switch on the back.

It happens on all gain settings on the lead channel (unless fully down which doesn't create any sound).
Pushing the lead boost button increases the frequeny of the feedback (changing the gain or volume knob does not change the feedback frequency).
It also happens without a lead on the input jack.

With the same guitar, the same behaviour is with any pickup configuration (P90, humbucker, single coil) .
Just plugged in another guitar; no change.

I have changed both the preamp and power amp tubes. Same behaviour.

On the clean channel with an overdrive or fuzz there are no problems.
The clean channel on maximum gain is fine too.

I agree that it is not typical behaviour of the amp even with the lead boost and full gain.
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Re: Guitar amp oscillating

Postby yaire » Fri Nov 29, 2019 12:04 am

Ok, now I am puzzled...

Just rechecked.

The lead channel still does the oscillation.

but..

When pushing the lead boost button then the sound is back to the normal overdriven sound as it should and no oscillation.
Disengaging the boost gives the oscillation again.

Two hours ago there was oscillation with the lead boost on. So now not anymore.

I thought the lead boost just increases the gain of the lead channel but something else is happening here.
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Re: Guitar amp oscillating

Postby yaire » Fri Nov 29, 2019 12:19 am

discovery:

Setting the gain of the clean channel to maximum
then the oscillation on the lead channel disappears.

Setting the gain on the clean channel to about 4 o'clock
then there is no oscillation on the lead channel with the boost engaged.
There is oscillation on the lead channel without the boost engaged.

When the gain on the clean channel is at 3 o'clock
then there is oscillation on the lead channel with and without the boost.

So there is some interaction between the 2 channels that triggers the oscillation...
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Re: Guitar amp oscillating

Postby Hugh Robjohns » Fri Nov 29, 2019 12:26 am

When you cleaned out the coffee spill, might you have disturbed any of the internal wiring, or components? Perhaps pushed wires or components closer together than they were originally, or altered the cable dressing?
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Re: Guitar amp oscillating

Postby yaire » Fri Nov 29, 2019 12:40 am

Could be, but I opened it up tonight to check and I did not see anything different or strange.
All cableshoes looked fine and nothing else caught my attention.
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Re: Guitar amp oscillating

Postby ef37a » Fri Nov 29, 2019 7:33 am

This is, I am sure what is called "Parasitic Oscillation" and typically shows in amps with certain channels and certain settings of those channels. The spill may or may not have a bearing but Hugh's idea of lead dress is a good one. Sometimes high gain valve circuits can be the very devil to keep stable and only a certain routing of signal wires and earth returns will make them so.

The problem is you cannot diaganose a lead dress problem unless you work with a 'live' chassis and I do NOT recommend you do that!
I fear you are going to have to live with the problem until you can afford a tech investigation.

BTW, don't leave the amp squarking for too long. The oscillation is probably ultrasonic in origin and after a few minutes the OP valves will redplate then die.

Dave.
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Re: Guitar amp oscillating

Postby yaire » Fri Nov 29, 2019 12:18 pm

Thanks Dave,

I can live with the problem as long as it is no health hazard to play the amp.
Will take it to a tech next year.
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