Marshall 1959 100W top power amp problem

[Michael Tibes]

My beloved 70s 1959 is having a problem I am stuck with: while everything seems fine when idling or at low volumes, one side of the power tubes runs hot (glowing red) when it is pushed hard.

I found that without input the bias is stable at -42V at both pairs of EL34, but when it gets pushed it might get uneven, roughly around -50V / -70V if I remember right. I say 'might' because  I checked it several times on the bench, but didn't write down the exact numbers - and now that I want to post it here the problem seems to be nearly gone - now it's -57V / -61V and the tubes seems roughly equally hot! So the problem is also intermittent

Any thoughts on that are much appreciated - the amp is still in nearly original condition I don't want to simply swap parts to see what happens but keep it as original as possible.

Thanx,

Michael

 

[PRR]

> roughly around -50V / -70V

It is normal for grids to dip negative when an amp is over-driven. They go negative easy, they won't go positive of Zero without charging the coupling caps, so the average runs more-negative while you beat on it.

Redplating could be a bad tube. IMHO it is not impossible a heavily-loaded Marshall red-plates "normally" in heavy overdrive. OTOH the price of good EL34 is so low (for the power you get!!) that you should try a new set.