KT88 failures

[harry]

A friend's amp has been sat unused in a shed for a good 18 months. He took it out recently, and played at low volume it blew the mains fuse.

I've opened it up to take a look, changed the fuse and replaced a dodgy power tube grid resistor.

It powered up and passed a signal OK, but the output was heavily mismatched, almost flat on one side.

I noticed that two KT88s (EH) were much cooler to the touch than the others. All pin voltages seemed the same, so it suggested to me that these were dead.

Removing the two left KT88s and putting one of the right KT88s in its place meant the output was a decent sinusoid, although one side took about 10 seconds and some signal to amplify that half of the signal.

I think the KT88 quad was new to begin with. Are these others dead? What is this failure mode? I am new to KT88s and I don't think I have come across a valve that fails in such a polite manner, i.e. just sits there and doesn't do anything. I am used to EL34s redplating and causing problems.

[pdf64]

There's been a couple of harsh winters, oxidation of contacts likely to occur.
Maybe the heater or screen grid circuits were open due to bad contact between socket and pin?
Perhaps the KT88 are ok really, just needed re-seating?
Best to confirm voltages on socket are good before powering up with valves in.
Pete

[harry]

Best to confirm voltages on socket are good before powering up with valves in.

Noted, that is a more sensible approach. Thank you for your advice. What you suggest seems entirely feasible. I will give the sockets and pins a good clean.
Thanks!

Harry

[defaced]

I noticed that two KT88s (EH) were much cooler to the touch than the others.

Were the heaters operating (glowing)? You can check to see if the heaters are still connected internally by doing a resistance test of the pins (outside of the amp). If they're open, that's your cause of failure.

[harry]

Were the heaters operating (glowing)? You can check to see if the heaters are still connected internally by doing a resistance test of the pins (outside of the amp). If they're open, that's your cause of failure.

Good idea! I will check. They were glowing - visually all valves looked the same, but maybe I will look with the lights out.

I had a clean, and the suspect valves still don't seem to work. The good valves work in any position, the suspect valves produce no output in any position.

[Jazz P Bass]

If you are able to monitor the mains current to the amp, this will help tell if the tube is conducting or not.
Try it.
One tube at a time.
At idle, for a good tube, you will see the current rise as the cathode starts to emit.
Then the current will level off.
You will see the bad tube for what it does not do.
This is also a useful way of 'matching' & /or 'grading' tubes.

[Enzo]

I like that approach too^^^^

Pick one socket, and test each tube in that same socket. That way any differences will be due to the tube itself, not the circuit. Then, once you have weeded out any bad or marginal tubes, now take ONE tube, and do the test again in each socket. By using just the one tube, any differences must be due to the socket and its circuit as you move down the row.