Tube Amps / Music Electronics
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|1/20/2005 9:49 PM|
|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..
|1/21/2005 12:00 AM|
|Stan||Nevermind...output caps it was!|
Twas the .068 output caps....all fine now..
|1/24/2005 6:38 AM|
Wish I found your post sooner, could have saved you some time. Orange amps were notorious for leaky coupling caps. The next time you 'know' the bias is correct and you're still 'red-plating' the power tubes, check for DC leakage on the coupling caps, with no siganl applied to the amp take a DC voltage measurement to ground on BOTH sides of the coupling caps. You should see B+ on one side, NOTHING (zero volts, maybe a few millivolts) on the output side. Any small DC voltage on the output side of those coupling caps will drop the bias voltage and run your power tubes REAL HOT! If I were you, I'd check the rest of the coupling caps in that amp. My good friend has an old Orange Head, the darn thing gets so hot, the caps just dry out! I had to replace them with military grade poly-props to keep the amp stable.
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