Pre amp voltages should not be overlooked.

[jasonP]

Hey guys,

Lately I’ve done a lot of experimenting with pre amp voltages especially in a marshall jose type circuit.
I found in this type of circuit I like runing the voltage higher around 390v at the PI node and 155v in V1a after the 330k resistor. When I went down to around 330v I found the tone to become slightly darker, looser, grinder, and the bigger issue for me was how tone tone of the amp to have more of this scratchy midrange and top end that no matter what I did, bypassing plate resistors and using caps to ground I couldn’t help it not matter how much I rang the things neck. I just got further away from the tone that I was wanting. I thought it was interesting and wanted to share. As soon as I raise the voltages back to that 375+ range on the PI node it was like there it is.

 

[FourT6and2]

Yep, I shoot for a minimum of 160v on V1a plate. With the standard 10K droppers between each node, that means you either have to play with the first/main dropper to raise voltages, or spec a power transformer with higher secondaries. I always prefer higher PT secondaries, though, because it's easier to drop the voltage for the rest of the amp to whatever you want, rather than raise it.

 

[glpg80]

Yep, I shoot for a minimum of 160v on V1a plate. With the standard 10K droppers between each node, that means you either have to play with the first/main dropper to raise voltages, or spec a power transformer with higher secondaries. I always prefer higher PT secondaries, though, because it's easier to drop the voltage for the rest of the amp to whatever you want, rather than raise it.

+1

I spec’d a high B+ secondary PT for my 74 for this very reason.