2 out of 4 EL34 pulsing with guitar signal

[schwarzerkaffee]

Good day everybody,

I have a question concerning the EL34 output tubes of my Marshall 1959 SLP RI (1994). I bought the amp in the mid 90s and changed the output tubes (Tesla -> JJ) recently. I bough a matched quartet from TAD and biased them at around 30mA. The plate voltages on all 4 sockets are 450V (very precise) and the grid resistors (1K5W) measure well within spec.

When I run the amp at high volume (> 7) I observed that the tubes get the brighter, the harder I hit the guitar- in both channels. To be more precise:

a) The two output tubes closer to the OT (left pair as seen from the back) seem to glow in a brighter orange than the other two.
b) All of them show a blue-ish coloring dependent on the signal, which I think is ok.
c) The pattern in a) is independent of the arrangements of the tubes. Its always on the left two sockets.

Here is a video, where I tried to catch it on camera (I apologize for the horrible playing and the stupid cell phone sound) 

Since I never played the amp out of the cabinet, it might be that this effect has always been there unnoticed. I just want to understand what's going on.

Best
Klaus

[pdf64]

It's probably normal, in that the circuit feeding the output valves probably has an asymmetric overdrive character. The DCCF circuit in V3 may be mainly responsible for it, see http://www.valvewizard.co.uk/dccf.html
If you pushing it that hard, it may be a good idea to swap the output valves around once in a while, to even up their wear.