1963 Blond Fender Bassman matched output tubes measure 10ma appart in the Amp

[tonefiend]

Hello,

I was at Band rehearsal last night and started getting nasty cross over distortion from my amp, which has a new set of NOS sylvania power tubes, matched to w/in 2 ma. When I measured
this morning with the tubes in my amp. There was a 10ma difference in current between both tubes! Output Tube 1 (closest to the PI) is always 10ma greater than output Tube 2. I confirmed
this with another set of Tubes, Tube 1 is always 10 ma greater than Tube 2. Any ideas what to look for here, to diagnose where the problem is? Im pretty sure this is what is causing the cross
over distortion.

thanks in advance for your help

[tonefiend]

Fender-Bassman-6G6B-Schematic.pdf


Attached Schematic

[Mick Bailey]

Did you establish it is crossover distortion by scoping the output?

My initial thought is to measure the plate, screen and grid voltages at idle on both power tubes for comparison. A significant voltage difference would indicate a possible fault. A leaking PI coupling cap (for example) can cause an output tube to draw more current. Fender used to specify up to 25% mismatch between output tubes.

[Jon Snell]

If you swop over the output pair and the 10mA follows the valve, it is a poorly matched set of valves. If it stays on the same socket, as Mick pointed out, check the coupling capacitors.

[tonefiend]

Excellent, thanks very much for your responses, I will try those suggestions, and report back! Im now begining to think its the tubes, and not the amp, stay tuned....