Low Bias Current - Carvin X Series III 60W

[BungleSim]

I'm repairing a Carvin X Series III 60W amp, which I guess is supposed to be a Mesa Mark III copy. I'm using the OT shunt method to bias the tubes and I'm reading a bias current range of about 18-30mA when I turn the bias pot, so I'm not hitting the expected current of around 44mA to reach 70% plate dissipation. With the bias pot in the middle I'm reading about 21mA so something seems off.

The pot is good and I'm seeing around -52V out of it. Schematic is attached, though it shows four 6L6's in the power amp but this amp only has two 6L6's. Any ideas on what I should troubleshoot would be greatly appreciated. Thanks!

Carvin X-Amp Series III.pdf

[sluckey]

With standby switch open monitor the voltage on pin 5 of a 6L6. What voltage do you read with the bias pot turned to one extreme? What voltage do you read with the bias pot turned to the other extreme?

[xtian]

Also, 70% target is largely "old wisdom" and we (I) more sanely bias in the 50-60% range. Sounds good, tubes last longer, less prone to burning things.

[JD0x0]

70% is MAX dissipation in class A/B fixed bias. It is not the general recommendation. Some people, may find in some circuits it sounds best there, but I find that rarely to be the case. Many channel switching amps are Biased under 60% dissipation. Blackface Fender spec was around 50-60% dissipation. Many 'High Gain' channel switchers run even cooler than that. It's not uncommon to find ~30% dissipation in 'Metal amps' I'd say your 'middle' setting of 31.5% sounds about right. If you want a bit more range, you can adjust values in the bias supply. I suspect the relatively cool setting at max is to prevent dummies from cranking the bias up to max and over dissipating the power tubes. I suspect, much like 5150's and other similar high gain amps, they biased these cool on purpose, and essentially are only set hot enough to avoid noticeable crossover distortion.

[BungleSim]

With standby switch open monitor the voltage on pin 5 of a 6L6. What voltage do you read with the bias pot turned to one extreme? What voltage do you read with the bias pot turned to the other extreme?

Pin 5 voltage measurement ranges from -54.3V to -46.8V at the bias pot extremes.

70% is MAX dissipation in class A/B fixed bias. It is not the general recommendation. Some people, may find in some circuits it sounds best there, but I find that rarely to be the case. Many channel switching amps are Biased under 60% dissipation. Blackface Fender spec was around 50-60% dissipation. Many 'High Gain' channel switchers run even cooler than that. It's not uncommon to find ~30% dissipation in 'Metal amps' I'd say your 'middle' setting of 31.5% sounds about right. If you want a bit more range, you can adjust values in the bias supply. I suspect the relatively cool setting at max is to prevent dummies from cranking the bias up to max and over dissipating the power tubes. I suspect, much like 5150's and other similar high gain amps, they biased these cool on purpose, and essentially are only set hot enough to avoid noticeable crossover distortion.

Also, 70% target is largely "old wisdom" and we (I) more sanely bias in the 50-60% range. Sounds good, tubes last longer, less prone to burning things.

All of this is great information. Thanks a lot, guys! So if I throw out the general recommendations and tables that I've been using according to sites like this, how do I go about reliably setting the bias on this amp? Guess and check?

[xtian]

Keep using the tables! The current calculations are correct, it's just that you don't want to set to 70% by default.

[wpaulvogel]

Use your ear and because it’s never going to bias into dangerous dissipation levels turn it and play. Find the sweet spot.

[BungleSim]

OK, cool. Thanks, guys!