Adjustable bias mod

[Ace_F4i]

I own a 5150 that i would like to mod with an adjustable bias. I've found basically everything i need including bias test plugs and pots etc but i'm not sure what values to get for the potentiometer. All i can find is that it has to be 10k value coupled with a 5.1k resistor to replace the 15k resistor but it doesn't mention anything about what KIND i need. Do i need ½ watt, 1 watt.. i'm not sure all of this is confusing to me.

[FortinAmps]

Bias voltage with hardly any current, 1/2 watt will plenty.

Are you going for 4 individual bias adjustments or 2 ?

Cheers,
Mike

[Ace_F4i]

actually i think it's only a one for the all type deal.

[Ace_F4i]

also, do you have any recommendation for a bias test probe? Weber makes a "plug into the tube socket" type that appears simple enough but if it's easy to install my own into the amp i'd just as soon do that.

[skipm45]

There are a couple of methods.
1.) Use a Weber Bias Rite or similar tool. Simply plug the output tubes into the Bias-Rite sockets and plug the sockets into the amp. Plate voltage and cathode current can be read directly. Use the Weber bias calculator to determine the correct bias point.
http://www.webervst.com/tubes1/calcbias.htm

2.) Install a 1 ohm 1% resistor between the cathodes and ground, in series with any existing cathode resistor. Measure the voltage drop across the resistor (in mV ), which is the same reading in mA of current
ie: 40mV across 1 ohm = 40mA. Again measure the plate voltage, and crank both values into the bias calculator to determine the bias point.


HTH

Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)