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Failing KT88- what to do?

Hello, was listening to some streamed music this afternoon. The amp is a Separo amp that has 4xKT88 ouut valves. I was doing my ironing when my wife came in and said, “what’s that smell?”. At first Ithought it was the iron but I looked across at the valve amp and on of the KT88s was glowing much brighter than the others. The sides of the tube were going black too.

The volume was inly at 10 o’ clock bit I switched the power off immediately. So what do I do now? Is it likely to be one valve failing or is it likely to be something else? If it is just the valve, can I just replace it with another KT88 of the same make (Genelux) ordo I need to replace all the KT 88s?

How do I proceed from here to see whether it is just the valve failing or possibly the transformer or something else? I’m reluctant to even turn it on again before I receive advice. Thanks all.
 
Without knowing anything about the actual amp design, I would say it was caused by either a bad coupling capacitor to it's grid, or a bias voltage failure if it's fixed bias - It was certainly caused by excessive current through the tube and those are the only two failure modes I'm aware of that would cause it.

Try to find and link a schematic and it'll be easier to diagnose.

You won't need to replace all four tubes, but replacing the pair on the side that red-plated isn't a bad idea.
 
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If there are no bias adjustments, it must have cathode bias, with a large resistor and a bypass capacitor.
There could be one RC per tube, one RC per pair, or one RC per quad. Most likely there is one RC per pair.

If you see no bias adjustments, you can swap the two tubes of the pair, and see if the same tube still seems bad.
If the tube seems bad, it's likely the pair would have to be replaced. But you could keep the good tube of the pair.
If the same socket seems bad, the coupling capacitor could be leaking.

Also the internal cathode bias resistor for the bad tube should be checked for possible thermal damage.
 
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It is simple: the negative bias became less negative. If the output tubes use cathode bias, the bypass capacitor parallel to the cathode resistor failed. In case of fixed bias, the adjustment variable resistor (if there is any) or a nearby resistor failed. A leaky coupling capacitor to the g1 of the output tube could also cause weird things. The bias voltage can be measured on the grid pin of the socket, with the tube removed.
 
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Thank you all for your input. I have no schematic but attach photos of the bias points, the bias adjustment instructions and the specification page. Setting the bias instructions I think, may be incorrect hence the pencil annotations. Setting my digital meter to 200mv range gives me a reading of 1. So, I set the meter in either the 2000mv range and bias at approx 500mv or at 20v where the bias is .50.
I’ve turned the amp on this morning for 30 mins then tested the valves and reset the bias to 500mv. They’re all at or anout this now and there has been no repeat of the red plating valve. Touch wood.
By the way, the valves are Gold Lion Genelex, made in Russia. So getting a replacement matched pair to replace, if necessary, will be difficult at the moment. Until sense prevails and this war stops. But I do have matched quads of Brimar and Tube Amp Doctor KT88s.
 

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Last edited:
it's clearly the Meixing electronics factory.
I will be you, I will lower the bias a little (35/40mA) and I will go back to the basics and I will control everything from the power supply to the output transformers.
This amp must be around 15 or 20 years old and is probably 220V, you must be closer to 240/250v in England especially in summer, your amp may have given you a warning before sanction.
 
Perhaps one of the wipers on the bias pot went Open.

Perhaps, as mentioned by huggygood, your amp was made for 220V, but is subjected / abused with 240/250V.

Perhaps your KT88 g1 grid return resistance has more resistance than the KT88 data sheet specifies for a fixed/adjustable bias circuit.

Or . . .
Who knows?

Back to . . . needing the recommended schematic.
 
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