Biasing my Fender amps

[Mike7]

From the little info I have been able to get off the net, I am going to try to bias my Dual Showman and my Bassman, my question is can I bias just by sound? The article said amp should idle quietly. Is this method OK to use?

[Gene]

Well, if you know what a properly biased amp sounds like, I would say yes. The "proper" bias has a range that you need to keep an eye on.

Maybe you are talking about the funny balance and hum pots on some later Fenders? They don't really bias the amp. For that, you can stick your head in the amp and adjust it until you get the least amount of hiss/noise.

[Mike7]

Both of them are BlackFace the Bassman a 66 anfd the Showman a 64

[Old Tele man]

...I am going to try to bias my Dual Showman and my Bassman, my question is can I bias just by sound? The article said amp should idle quietly. Is this method OK to use?...you can adjust "by sound" but ONLY if you've made the measurements of plate voltage and current and assured the idle plate dissipation does NOT exceed 70% of the tubes spec, for instance: 6L6GC = 30W so 70% would be 21W or less at idle.

...once, you KNOW the idle wattage isn't exceeding 21W, THEN you can "adjust by ear."

[yeahyeahyeah]

From the little info I have been able to get off the net...

If that's the only experience you have with biasing I would highly recommend this read: http://www.duncanamps.com/technical/lvbias.html

If that doesn't make perfect sense to you, get a qualified tube amp tech to do it with you. It would probably be cheaper than buying another set of output tubes if you fry them trying to do it yourself and most importantly its more SAFE. DC of 400 volts or more are very common in tube amps and will kill your ass dead if they pass through your heart.

[Mike T]

Layman's methods.... (work for me!)

1) To do it by ear keep an eye on the 6L6's and make sure the plates are not glowing red and listen as you turn the bias pot. You can usually hear a sort of "detente" area where the bias is right. Too cold sounds ratty (not good ratty) and too hot makes the tubes glow. Then let it sit at idle for 10 minutes and then put your hand on the PT. If you can hold it there for 15 seconds or so you're OK.

2) Or replace the braded wire to ground on the cathodes (pin 8) of the 6L6s with a 2 or 3 watt 1 ohm restister and you can read the bias there as millivolts on a DVM. When you are reading an exact figure you must know the DC voltage on the plates, or pin 3. Bias charts will tell you what the bias voltage should be at a particular plate voltage. Bias voltage means nothing if you you don't know what the plate voltage is.

[devbro]

Why not just go pay a few bucks a buy one of these. It's really quite simple to do.
http://cgi.ebay.com/Bias-Tool-probe-tester-for-tube-amp-amplifier-biasing_W0QQitemZ250166057357QQihZ015QQcategoryZ43 375QQssPageNameZWDVWQQrdZ1QQcmdZViewItem