Advice on Croft pre-amp issue.

[fiddlemaker]

Hi
I have a lovely 2 box Croft preamp (micro A upgraded to 25RS+ spec by Glenn about 5 years ago). It has developed an intermittent issue whereby after being powered up for a while it makes a noise that I can best describe as wind blowing through trees. It stops when the preamp is switched off and then on again.
I was wondering if anyone had any idea of what could be the cause of this? Faulty valve maybe?

Thanks for any thoughts.....

[Jimbo]

I had that occur with a faulty valve John, it is a symptom of a valve running out of spec and starting to get noisy.

[Firebottle]

Easy enough to change a valve to see if it is that, assuming you have some spares.

[fiddlemaker]

Easy enough to change a valve to see if it is that, assuming you have some spares.

The issue is so intermittent (it can go for days without occuring) that trying to solve by random valve swapping could be a very drawn out process.
Given that it occurs on all inputs, is it safe to assume that the line stage valve is the likely culprit?

[Firebottle]

That would be a good guess.