Symptoms of worn out Power Valves - low mA??

welshboyo

Lost a bit of volume at the weekend at a gig, not enough to be totally drowned out by the rest of the band but enough for me to notice I couldn't hear myself as good as 1st half!!! and it wasn't hearing fatigue as I was IEM'd up!!

So, this morning I decided to give the amp a once over, check valves etc and upon checking the bias I noticed that the valves in there (JJ's) were only pushing out 16mA per side with the bias pot maxxed out - the plate voltages on this amp (Shiva) are high so the idle should be around 26-29!!

So, I chucked a pair of brand new Ruby's into it and instantly the idle came back up into this range with plenty of range left on the bias pot??

Is this normal for worn valves to slowly start showing their wear by giving lower readings?

Have I just caught these before they became totally useless and gave up the ghost big time on a gig (they are about 18 months old with at least 200 gigs on them)

ecc83

But do you know where the JJs were at when new? They could have been banged in and the bias set in a hurry and no time given for grid current to fall, takes about an hour in some cases. Then, depending on how the bias is derived it may or may not track well with incoming mains V.

If you can do so safely, make a note of the anode and G2 voltages then after the first gig check bias again and note Va/Vg2.

And! What you DID not say me boyo is if the amp now SOUNDS back to normal?

Dave.

welshboyo

@ecc83 ;

Cheers, yes, the JJ's were a bit of a unknown and came with the amp, they were new but I had no idea of how/where/who/why fitted them etc etc.

And yes, amp is back to normal!!

Had the amp running for over an hour now on new valves and will give it a good run tonight to see where we are.