Valves: might need to replace one of mine...dunno though?

[Gazjam]

One for the glass bottle experts...

As part of keeping on top of the system, the other night I decided to clean and deoxit the valve pins on my amp.
Hadn't done it since I got the amp over a year ago.
Basically I "dry wiped" the valves in the pin holes a couple of times and used Deoxit to clean the pins.

On switch-on, after about 5 minutes, one of the KT88 Gold Lion valves in my amp went into "runaway" and the amp started humming on one channel. :doh:
Its plate started glowing redder than it should have so shut the amp down right away.

I had this problem with the same valve before so I think I've used up two of its lives.
Still sound OK, not different than before, but I notice the heater gives off a fainter glow than the other 3 valves.
Its not the valve socket, as the problem with the valve is the same no matter which socket the valve is used in.

I suspect dry wiping the valve has maybe displaced the socket a pit causing a momentary loss of bias to the valve which caused the valve to draw more and more current.
Just a theory mind, sound reasonable? :scratch:

Time to replace the valve you think?
If so, do I REALLY need a new "matched" pair or quad set?
Can I just buy one of the same type and swap it?

Just wonder if the valve has not been matched in exactly the same circumstances as my own kit how accurate (or needed) it actually is.


Thanks!

[lurcher]

Its plate started glowing redder than it should have so shut the amp down right away.

the plate should not be glowing red at all. There are some valves where the plate runs that hot, but a kt88 is not one of them.

I would replace the valve, its a disaster waiting to happen. if it follows the valve, its not the sockets.

As to if you need matched pairs, is the valve autobias or do you set it for each valve?

[Gazjam]

Cheers Nick. :)

There's a pot for each output valve, first thing I did when the amp looked "stable" was to check bias was 0.4mv.
All seemed good, checked every 20mins or so, bias stable.

Think its the valve for sure, dimmed heater glow follows the valve no matter what socket its in.
Does this mean reduced output for that valve?

Had a couple of lucky escapes i think, don't want to temp fate.
Replacement imminent: If I can get way with one new valve instead of more all the better!

[lurcher]

well if its fixed bias, you can just replace the one. but if they are a year old and well used, it might be worth considering a new set.

[Gazjam]

Cheer Nick, good info.