5150 Bias question.

[TheRiverFlowsFrozen]

I'm getting my 5150 modded for adjustable bias and I've noticed that there isn't really a step by step guide on biasing a modded 5150. I just ordered a bias probe from amp-head.com. There were two types available, an mA version and an mV version. I went ahead and ordered the mA version. My friend has a multimeter to use with it.

I emailed Bob from eurotubes and he said:

Ohms law states that one volt equals one amp so there is no difference.

And I also emailed Jerry from FJA mods and he said:

I'm not familiar with those. I don't use a bias probe. Most bias probes
measure mv. Measuring ma is very dangerous if you're not familiar with
electronics.

So...I'm a little confused. Can anyone give me some detailed directions on biasing my 5150 with the mA bias probe? Any feedback would be a huge help.

 

[kelch]

to do this correctly you need to know two things:

1) Plate Voltage

2) Plate Current

Looking at the Amp-Head bias tool it looks like you can only read the Current. Get whom ever mods your amp to tell you the Plate Voltage. It'll be good enough for this.

Now, multiply the Plate Voltage and Current to get Power. P = V * I

Now adjust the bias for the idle power you want. For 30W 6l6s a good power level is about 60% of max power. .6 * 30 = 18W

Assume for discussion the plate voltage is 480VDC

I = P / V => I = 18 / 480 = 37.5 ma

 

[Ravenous Enemy]

errr, just have to point out that Ohm's Law states that Voltage (p.d.) is in direct proportion to current (for constant conditions), NOT what 'Bob' said.
the eqn is V = IR where R is resistance measured in Ohms.

 

[kelch]

I think Bob assumes a 1 ohm Cathode resistor. therefore, the Current and Voltage do equal.

 

[Ravenous Enemy]

numerically only yes.