Putting EL34s in a 6L6 amp (or KT66, 5881) is a rollof the dice. My '67 Showman LOVED them but not my friend's Bandmaster. It just depends on the amp individual model. I'm glad it worked well for you, as I've been curious about that same set-up, of course the difference being I'd be working with a reissue. But good to know.
Could you confirm whether those sections are linked across, and what that cap's values are, eg 2x16uF, 2x32uF, 16uF & 32uF etc?
That screen grid arrangement seems weird, it may have originally been a 1k (or 1k5 or even up to 8k2) flying resistor and 2 x 470ohm resistors mounted on the valve bases. I don't think those 1k square section resistors are original, though it's feasible that the 150ohm flying resistor is. Certainly the 8k load puts more stress on the screen grids, especially so with EL34 'true' pentodes, and they'll draw more current when overdriven. Hence after a couple of years of the hard riffage you gave it, the original screen grid resistors could have been shot.
I thought that of the big 'pro level' manufacturers, it was only Vox that used wax potting, as a cost cutting measure; I'm surprised to discover that Marshall did too. But I suppose that if you hadn't seen the wax, you might not have checked the bias, so perversely it could have saved the PT
Is this amp unusual, in that it looks like the sections of its reservoir cap (ie the can that sticks out of the chassis between the power and rectifier valves) are linked across?
Could you confirm whether those sections are linked across, and what that cap's values are, eg 2x16uF, 2x32uF, 16uF & 32uF etc?
That screen grid arrangement seems weird, it may have originally been a 1k (or 1k5 or even up to 8k2) flying resistor and 2 x 470ohm resistors mounted on the valve bases. I don't think those 1k square section resistors are original, though it's feasible that the 150ohm flying resistor is. Certainly the 8k load puts more stress on the screen grids, especially so with EL34 'true' pentodes, and they'll draw more current when overdriven. Hence after a couple of years of the hard riffage you gave it, the original screen grid resistors could have been shot.
Regarding the different branding of the two EL34s, that doesn't matter to electrons. It's their technical characteristics that affect the tone. Though if their performance is too dissimilar (ie rather unbalanced), the amp will be difficult to bias as one would be idling too hot and the other too cold, and that tends not to sound good anyway.
As the gain of a valve increases with anode current, it may be worth tweaking the bias, though it probably wouldn't be a good idea to set idle current too much higher. Check the HT voltage and work out the anode dissipation.
I thought that of the big 'pro level' manufacturers, it was only Vox that used wax potting, as a cost cutting measure; I'm surprised to discover that Marshall did too. But I suppose that if you hadn't seen the wax, you might not have checked the bias, so perversely it could have saved the PT
Yes, given the symptoms, that’s the most likely suspect.
