High preamp plate voltage

[bluesguitar]

Been experimenting with the bias adjustment to check out any difference in tone on a '67 Super Reverb. The schematic calls for a plate voltage of 270v on V1 &V2. Although these have always measured a bit high I am now showing 300v on V2. How high is ok, and how high is not good for the amp? The schematic notes "voltmeter values shown + or - 20%". Does this pertain to all voltages on the schematic? Seems like 20% high @ the OT center tap would be a major concern.

[martin manning]

What is your wall voltage? If the PT primary voltage is off (it's usually high these days), everything in the amp will be about the same percentage above or below the listed value. Adjusting the bias voltage will change the idle current draw from the power supply, and that will affect the plate voltage to some degree too.

[bluesguitar]

What is your wall voltage?

122v. I remember reading that todays wall voltages are higher than they had back in the 60's.

[jjman]

All voltages should read somewhat high on a 1967 amp. 11% is probably close to the norm.

[bluesguitar]

All voltages should read somewhat high on a 1967 amp. 11% is probably close to the norm.

Thanks. That sets my mind at ease.

[martin manning]

"Nominal" was 117VAC back then, so expect everything to be about 5% high due to increased line voltage That's 20-25V at the power tube plates, and 10-12V on the preamp plates. In the preamp, there could be as much as 10 or more volts variation just from changing the tube.