Matched output valves?

[Evobassman]

Hey Audio Gurus,

I think one of my output valves (KT88) is on its way out. It causes a medium-high pitch noise about 15 minutes after being switched on and then won't bias properly. The noise goes away when the valve is tapped very lightly.

My question is this: If a push/pull amp has individual bias controls, is it important to have matched pairs each side? Or, can I purchase one new valve and replace the outgoing one?

All of the writing I have found so far relates to guitar amps with a single bias control. Even then, there is not a consensus.

Thanks in advance,

Evan

[Zaphod Beeblebrox]

The general answer to your question is yes.

The actual answer is somewhat less clear.

You should ask the manufacturer for their opinion.

That said, there are several issues to be addressed:

* Bias current is only one issue that needs to be addressed.

* Some amps cope well with unmatched valves and others do not.

* The number of hours on the valves will be important. Valves start wearing out, as soon as they are switched on. Mating a worn valve with a new one is not a smart idea. If the good valve is relatively new, then problems are likely to be less severe.

[Evobassman]

Thanks ZB! What would you consider 'relatively new' valve to have on the clock? >50 hours? >100 hours?

[LogicprObe]

100 hours isn't much, unless the valves are being hammered, like in a guitar amp.

Matched pairs are always better but with individual biasing, if your budget only allows you to buy one valve at a time, it'll be OK.

[Evobassman]

Thank you LogicprObe, I'm looking at options for purchasing new valve(s) at the moment. I would rather keep costs down at this stage as I just invested in my record cleaning machine.

Cheers!