6L6GC bias resistors in a 78 Fender Bassman Ten

[kingswayguitar]

Searching the internet and looking at other schematics, I don't see why my amp has unequal resistor values from the negative grid bias supply to the grids (just before the 1500 ohm stoppers). They are 47k and 68k. Does this make any sense? Thanks.

http://www.el34world.com/charts/Sche...MAN_10_75w.pdf

PS it's silverface with tube matching pot, not the real bias adjustment pot.

[nickb]

I think this may be done to balance the gain of the PI. The PI plate resistors have the same value.

[kingswayguitar]

indeed - and that's not very common is it...tempted to play around with the values

[pdf64]

I've just serviced a SF TR UL model which has that arrangement; it seems to work pretty well at evening up the outputs from either side of the LTP.
From the perseptive of the LTP tube, the load resistor and the plate resistor are in parallel (with the tube's plate).
So to get good balance, tweaking the load resistor value is no different than tweaking the plate resistor.
Making the plate resistors the same value may be intended to make the dc operating conditions the same for each side.
Really. many of the schematic revisions of the SF era were well thought out and shouldn't be tossed aside.
Pete

[JoeM]

....

Really. many of the schematic revisions of the SF era were well thought out and shouldn't be tossed aside.
Pete

+1

[kingswayguitar]

Really. many of the schematic revisions of the SF era were well thought out and shouldn't be tossed aside.
Pete

when you say schematic revisions, do you just mean the design of the silverface vs blackface