I've never the bias checked on this amp. Had to after I had a certified technician swap out the speaker and power tubes for the first time. He was getting very low readings. If he maxed out the bias adjustment knobs, he was getting a maximum of 31 mV or 32 mV on both sets of testing ports. He could not adjust any further.
Interesting, I thought, so he put back the old tubes and speaker back (and he tried a combination of new speaker/old tubes, new tubes/old speaker). ALL were relatively the same. The highest reading he could achieve was around 33 mV. He used multiple multimeters, so I'm quite confident that the readings were accurate.
The older tubes were the original Marshall EL34's. The new tubes are JJ EL34's. It has the original V-type speaker which I was trying to change to a 8-ohm Eminence Texas Heat (8-ohm jack was being used). The tech is still investigating, but I figured I would ask on the forum to see anyone had ideas as to the cause of low mV reading/adjustment on the bias.
Thanks!
Chris
Hi Chris
The Bias supply is a little generous. (You should see about -40/42V at the grids)
I’d be looking at the Bias supply in case a resistor has changed value with reference to ground, but it could be as simple as the valves aren’t drawing much current.
If the original valves are low gain/tired now, this may manifest itself the same and if the valves being fitted coincidentally don’t draw a lot of current, it will look like a fault with the amp that’s not actually there,
Regards
Marshall Support
