|Stan||Orange OR80 biasing problem|
Hello, I have a '73 Orange OR80 head here on the bench that after biasing, (conservatively) still redplates when driven a bit. Plate voltage is between 530-540 depending in the wall voltage. It came in with the bias around -36 and wouldn't go much higher than that, it's an unusual one with no bias pot, just a fixed resistor, I changed it to get -50, the most it would give.) At-50 I get between 25-30ma per tube. I've been using JJ e34L or SED El34's, both of which have held up to the brutal Orange voltage in many I've worked on with no problems. There seems to be a lot of drive coming from the phase inverter, almost like there's a boost pedal in front of the amp, which may be the problem. I also suspect the output caps, but they are difficult to remove, so that will be a last resort. I'm thinking of possibly putting a voltage doubler scheme on the bias supply, it's worked on Sound City and Hiwatt amps I've worked on in the past, the problem I see is the bias winding is center tapped (grounded) and on the Hiwatts and Sound City, it's not- one side is grounded the other supplies the voltage, would the common (from the groove tubes book) voltage doubler work on the center tapped bias supply? Any other ideas? Thanks and sorry for the rambling PS: the amp doesn't appear to be modded or anything, fairly true to the schematics, sort of a mix of the two schems you see around. It's been recapped recently too,including bias caps, that seems to be fine. Thanks again..