Help please! 1973 Marshall Super Lead - EL34 to 6550s bias question

[Greg_L]

Sure, why not?

I used this type of MV in my Vox AC30 in the early 70s, not knowing that it would later be called "Type 3".

That's cool! An AC30 has no negative feedback though right?

My goal with this is to just knock a little volume off the top. I'm not trying to turn this into a bedroom amp.

[doombass]

This wiring diagram got me wondering in addition to the pic below. I see no mentioning of grid stopper resistors. If you have, like in OP's case a 100W four output tube amp, does it mean you'll have to use a dual gang pot? The way I look at it is if you connect a single pot directly to pins 5 you'd be shortcutting the control grids on each pair of tubes. They'd basically share the parallelled resistance of both grid stoppers? Or no?

[Helmholtz]

I think the added (grey) legend in the Trainwreck drawing is wrong. The upper 2 resistors should not be the grid leaks but the bias distribution resistors (220k).
If the amp has and probably needs grid stoppers (like the Marshall), the MV should be connected before them and not directly at the grids for stability reasons.

Edit: Just realized that the bias distribution resistors are sometimes called grid leaks as well. So the Trainwreck drawing is ok. Also I had mixed up grid leaks with grid stoppers (language barrier) in my last statement above.

[doombass]

I think the added (grey) legend in the Trainwreck drawing is wrong. The upper 2 resistors should not be the grid leaks but the bias distribution resistors (220k).
If the amp has and probably needs grid leaks (like the Marshall), the MV should be connected before them and not directly at the grids for stability reasons.

Yes, that makes sense. Thanks!