Bias

[Hans Moleman]

This seems like the perfect place to ask about amp work. I've got a DRRI, which I bought second hand. It's had a million mods done, one of the things that is changed is the previous owner put NOS glass in. According to a tech it's 'worth more than the amp'. So obviously I really want to keep everything running well.

Anyways, I noticed yesterday that one of the power tubes was running much, much hotter than the others and there was some crackle in the amp. It's extremely bright compared to the other power tubes. Does it need to be re-biased? Is that something I can do myself? If so, how do I go about it? If anyone has done this kind of work before I'd love to hear some advice. Thanks.

[seriesnuns]

Anytime I get new power tubes I get them graded from eurotubes. If you read the manual it'll probably tell you.

If the tubes are worth more than the amp. I would take them to a tech.

[Buffered]

This seems like the perfect place to ask about amp work. I've got a DRRI, which I bought second hand. It's had a million mods done, one of the things that is changed is the previous owner put NOS glass in. According to a tech it's 'worth more than the amp'. So obviously I really want to keep everything running well.

Anyways, I noticed yesterday that one of the power tubes was running much, much hotter than the others and there was some crackle in the amp. It's extremely bright compared to the other power tubes. Does it need to be re-biased? Is that something I can do myself? If so, how do I go about it? If anyone has done this kind of work before I'd love to hear some advice. Thanks.

There's a few different ways to bias an amp. A what point on the dial does the amp start to distort? Does it sound shrill? Hotter in what way? Did you touch it? Did you use a DMM to measure it? I'm not sure for 6V6, but for a 6L6GC I like it between 34-36ma. I play a Fuchs ODS and Mesa 5:25+ (Fuchs takes a DMM reading from pin 8 and has a 10-turn pot to adjust bias, Mesa has their cathode type thing going on can't explain really) IIRC The DRRI like the originals have a pot on the chassis you turn to adjust bias for both tubes. That's why matched pairs are important in those older amps.

I have a compu-bias (or the equivalent the name escapes me now) works really well. Haven't used the Weber but that's a cheaper option. I've also used the bias king with good results. It's important to remember bias is an equation between a few different values, plate voltage among others. I like this article for explaining bias further:

http://proguitarshop.com/andyscorner/amp-bias-explained

Ask to have the tech balance the bias in front of you, so you'll know what to do next time. It'll save some cash and only takes about 10 minutes with all the tools. It's just regular upkeep like setting up your own guitars. Good Luck!