When are bias adjustments needed?
- Thread starter Biu
- Start date
aleclee
Major General GAS
Re: When are bias adjustments needed?
You're confusing fixed bias with adjustable bias.
Fixed bias doesn't mean that the bias is not adjustable, it's a more technical concept related to fluctuations in bias current during operation. In fact, most (all?) amps with bias adjustment are actually fixed bias. It's cathode biased amps that are not technically fixed bias. Cathode biased amps never need bias adjustments.
In answer to your original question, if your amp has adjustable bias, you should check it whenever you change tubes unless the new ones are electrically matched with the old ones.
You're confusing fixed bias with adjustable bias.
Fixed bias doesn't mean that the bias is not adjustable, it's a more technical concept related to fluctuations in bias current during operation. In fact, most (all?) amps with bias adjustment are actually fixed bias. It's cathode biased amps that are not technically fixed bias. Cathode biased amps never need bias adjustments.
In answer to your original question, if your amp has adjustable bias, you should check it whenever you change tubes unless the new ones are electrically matched with the old ones.
L
Lewguitar
Guest
Re: When are bias adjustments needed?
Whenever you change output tubes, have the bias checked and adjusted. You never adjust the bias when you change pre-amp tubes....just output tubes. You should learn to do it yourself...it's not that hard. Lew
Whenever you change output tubes, have the bias checked and adjusted. You never adjust the bias when you change pre-amp tubes....just output tubes. You should learn to do it yourself...it's not that hard. Lew
L
Lewguitar
Guest
Re: When are bias adjustments needed?
It's very difficult to explain and dangerous too! I believe Ted Weber makes a device that would allow you to adjust your bias safely. Go to the Weber VST website....you'll find the product. Might be called a "bias-rite" or "bias-probe" or something like that. I'd start with one of those. Much easier (and SAFER) for a beginner than using a multimeter and/or oscilliscope. Lew
It's very difficult to explain and dangerous too! I believe Ted Weber makes a device that would allow you to adjust your bias safely. Go to the Weber VST website....you'll find the product. Might be called a "bias-rite" or "bias-probe" or something like that. I'd start with one of those. Much easier (and SAFER) for a beginner than using a multimeter and/or oscilliscope. Lew
Gearjoneser
Gear Ho
Re: When are bias adjustments needed?
I think I got my mA bias tool on Ebay for $15. It's just a two sided socket that fits between a tube and the amp, with two leads that go to my mA meter/volt meter.
I would suggest that anyone with NO electronic knowledge NOT put their hands into a running chassis or even one that's shut off. The big filter caps can shock you if your hand or a metal tool touches the leads. I bias the amp with it running and plugged into a guitar and speaker cab. I keep the numbers on my meter close to where they should be (32mA for 6L6's/36mA for EL-34's), then very carefully listen to the tone while I tweak it. Just like adjusting the idle on a car, you know when it sounds weak or too pushed. You just find the point where the amp sounds punchy and strong, and the notes have perfect definition. Then, doublecheck the number to make sure it's close to where it should be, then do the same on the rest of the tubes, to make sure all of them are close to the same reading. When I'm satisfied that the amp is sounding it's best, I put it all back together.
I think I got my mA bias tool on Ebay for $15. It's just a two sided socket that fits between a tube and the amp, with two leads that go to my mA meter/volt meter.
I would suggest that anyone with NO electronic knowledge NOT put their hands into a running chassis or even one that's shut off. The big filter caps can shock you if your hand or a metal tool touches the leads. I bias the amp with it running and plugged into a guitar and speaker cab. I keep the numbers on my meter close to where they should be (32mA for 6L6's/36mA for EL-34's), then very carefully listen to the tone while I tweak it. Just like adjusting the idle on a car, you know when it sounds weak or too pushed. You just find the point where the amp sounds punchy and strong, and the notes have perfect definition. Then, doublecheck the number to make sure it's close to where it should be, then do the same on the rest of the tubes, to make sure all of them are close to the same reading. When I'm satisfied that the amp is sounding it's best, I put it all back together.
