Question about biasing Fender 94 Twin...?

[acorkos]

http://www.fender.com/support/amp_sc..._Schematic.pdf

This amp has only 2 of the 4 power tubes installed, and I'm wondering if that's what's confusing me....

2 power tubes installed, high power setting....when i measure bias voltage at the amp's back panel test points, i can get 80mV and I can adjust the bias balance to -zero-. all this as per manufacturer's recommendations on the schematic. but when I put my bias probe on one of the tubes and measure current, i read 80mA....and there's a bit of redplating visible.

is this because of two power tubes installed instead of 4? I'm thinking i need to raise the value of the 100K R95 bias resistor to bring the current down, whether i've got 2 tubes in or 4...

thanks for your help

[52 Bill]

2 power tubes installed, high power setting....when i measure bias voltage at the amp's back panel test points, i can get 80mV and I can adjust the bias balance to -zero-. all this as per manufacturer's recommendations on the schematic. but when I put my bias probe on one of the tubes and measure current, i read 80mA....and there's a bit of redplating visible.

The 80mV setting is for 2 power tubes! The amp has 1-1ohm cathode resistor shared by two 6L6s. So the 80mV equals 80mA for both of the 2 tubes, 40mA per tube. If you set the amp for 80mV, you are causing one tube to run at 80mA, not the original specified 40mA.

[acorkos]

thanks for the reply, 52. Does that mean I can set the bias voltage at 40MV and run only 2 tubes? or are 4 tubes required? The low power setting would seem to accomplish more effectively what pulling a pair of power tubes does.

[52 Bill]

thanks for the reply, 52. Does that mean I can set the bias voltage at 40MV and run only 2 tubes? or are 4 tubes required? The low power setting would seem to accomplish more effectively what pulling a pair of power tubes does.

Yes, you should be able to set it up for two tubes, but as you noted the low power (25 watt) setting will do a better job with all four tubes installed.