how to bias an amp

[Graffiti Yellow]

I am a complete amateur when it comes to tech issues on tube amps. Is biasing an amp something I can do myself. If so where do I lean how?

[Blue Strat]

What amp? The easiest and safest way is to use a bias probe. Bias King is one such product but there are several others.

[Graffiti Yellow]

66 Deluxe reverb and a silver face champ

[allynmey]

Check out the Compu-Bias from Mission amps. It is incredible. It gives you the plate voltage, Cathode Current, and plate dissipation in watts on a LED screen. You can view all three settings at once on two tubes. You can see the relationship between volts and Current flow as you turn the Bias knob. It's hard to tell what the Current flow should be without knowing the Plate voltage. That's dangerous to someone who in inexperienced with amps to go prodding around an open chassis. That tester checks it without opening the amp.

Allyn

[phsyconoodler]

The bias probe fits into the tube sockets and it has a voltmeter attached that reads the current.The average current for a silverface Deluxe is around 21 to 30ma with 6V6's.There is a bias pot access on the underside of the chassis between the power transformer and the output transformer that can be accessed with a small screwdriver.
You plug in the bias probe,put the tubes back in and bring the amp up to operating temperature and read the meter.Turn the bias adjust screw until the meter reads about 21 to 30ma.No more,no less.Most good meters also give the plate voltage reading so you can access a tube operating manual and get the right bias setting for the plate voltage and tube type.
Your champ is cathode biased or 'self biased' so it doesn't need to be biased.Just pop in the right tubes and play.

[Blue Strat]

What phchonoodler said, except, the correct range is 15 to 25mA. Anything higher and you're out of spec for the power tubes (even though it will work ok in the short term).

[StompBoxBlues]

Check out the Compu-Bias from Mission amps. It is incredible. It gives you the plate voltage, Cathode Current, and plate dissipation in watts on a LED screen. You can view all three settings at once on two tubes. You can see the relationship between volts and Current flow as you turn the Bias knob. It's hard to tell what the Current flow should be without knowing the Plate voltage. That's dangerous to someone who in inexperienced with amps to go prodding around an open chassis. That tester checks it without opening the amp.

Allyn

Hey...thanks for pointing that out! That looks really useful, I just sent in an order.

[ScottWolfe]

I have one, too. Very happy with it.

http://www.compu-bias.com/images/Meter/amp_2probe_cb1_s.JPG

[Occam]

I have one, too. Very happy with it.

http://www.compu-bias.com/images/Meter/amp_2probe_cb1_s.JPG

Everytime I see that it makes me wonder...why do the wattages seem so different from what I've always been told...why aren't EL34's for instance 25watts?
Which brings up another question....my amp is rated at 120-watts with 4xEL34's so shouldn't I be using 30-watts instead of 25?

[StompBoxBlues]

Everytime I see that it makes me wonder...why do the wattages seem so different from what I've always been told...why aren't EL34's for instance 25watts?
Which brings up another question....my amp is rated at 120-watts with 4xEL34's so shouldn't I be using 30-watts instead of 25?

Could it be because they are per tube? Two tubes at 12.5 watts, 25 watts. Push-pull.

[hasserl]

Everytime I see that it makes me wonder...why do the wattages seem so different from what I've always been told...why aren't EL34's for instance 25watts?
Which brings up another question....my amp is rated at 120-watts with 4xEL34's so shouldn't I be using 30-watts instead of 25?

It's showing the working ranges of the tubes, which is less than the max dissipation. You don't want to set the tubes for max dissipation or they will run too hot and have a short life.

With your 4 x EL34 amp rated at 120 watts, no, you should not use 30 watts instead of 25. The power rating is a rating of the power produced at the speaker, that is measured as AC voltage. The bias is set according to DC power dissipated by the tubes plates. The DC power dissipated by the plates and the AC power produced at the speaker are two different things, don't confuse them.

[doctorx]

Go Here, Read This. (http://www.regiscoyne.com/tech/bias/)

[Glynn Davies]

I was looking into this same issue, since I can't find a decent amp doctor (or guitar tech, come to that) near where I live.

I found a manual from Groove Tubes which apparently guides you through it, and can be purchased with the tools to do the job. Maybe something else to consider?

http://www.hotroxuk.com/store/erol.html

I haven't used it so can't vouch, but whatever method you wind up using it'd be interesting to hear how you get on.