Biasing an Allen Amp

[coyoteblue]

Just got an Allen Sweet Spot. It has built in jacks to check and change bias on the back panel. I've never used a feature like this before. What kind of Voltemeter should I get and how would I go about adjusting bias with one? Thanks.

[nella]

You use a digital multimeter and you may want to email David Allen for the suggested dc voltage. Usually there is a bias pot you access through a hole with a flat blade screwdriver. David can tell you exactly, I have never seen schematics/layouts for his amps on his site, but they are definitely Fender-based.
Scott

[phsyconoodler]

the Sweet spot can use 6v6's and 6L6s,so you need to determine which you have first.Then you need to know what the bias current should be set at for either of those tubes.Then you need a multi meter that can register in millivolts.You plug in the positive lead to the red bias port and touch the ground lead to the cahssis.Turn the amp on and warm it up for about five minutes.Then read the bias in millivolts on your meter.If it is in the range it is supposed to be in,leave it alone.If it's off,there is a bias pot on the underside of the chassis that is ascessible with a skinny flat blade screwdriver.Turn the bias pot until the bias is at the suggested level and you are done.

[coyoteblue]

That's great info...thanks. If I'm changing from 6L6's to 6V6 is there any problem switching tubes while the readings are high?

[GearHeadFred]

If you want to be 100% safe, twist the bias pot down... that way the tubes will be overbiased (safe), and you can crank it up to the proper value after they are warmed up.

That said, a few seconds of over dissipation probably won't do any harm. (may take flame for this one!)

The other variable you should consider in this equation is the plate voltage.. It will vary with rectifier type and power tube type. You need this to calculate the idle dissipation (voltage X current = watts). Even though your multimeter is measuring (milli)volts, it's actually telling you current (milliamps) because it's across a 1 ohm resistor inside the amp.

I've heard David Allen is a great guy and would probably give you the recommended settings if you email him.

[bbarnard]

I do believe that David would recommend that you switch rectifier tubes if you switch power tubes so you should definitely ask him.

[coyoteblue]

Why switch both?

[GearHeadFred]

6V6s have a lower maximum plate voltage.. You would switch the rectifier to lower the voltage for the 6V6s. Except for JJ 6V6s (500V), which are closer to 6L6s in most regards.

[coyoteblue]

Can't see anywhere to adjust the bias. Anyone know where it is on the Sweet Spot?

[coyoteblue]

I just got a Digital Multimeter and find that when I take readings for ma the figure varies constantly. When I take a reading for mv I get a proper reading. I've been told that this amp should run 6V6's at 20-25ma...how does this correspond to mv? Or is it likely the ma reading that I was advised should be mv? Thanks.

[GearHeadFred]

My previous post explains this. Set the meter for mv... but it's telling you ma.

[coyoteblue]

Thanks for that clarification. For some reason the DMM won't settle on a figure in the DMA section. The only place I get a proper reading is in the DMV section when I dial in under the 200mv setting. The power tubes in there read about 21mv and then the older RCA's I was substituting initially read about 10mv. I raised them to 21mv with the bias trim pot. If this is equivalent to 21ma, then I should be set. Thanks for your help.