Idle Current, Plate Voltage and BIAS!

[MrBump]

Guys, I'm looking at the Weber BiasRite.

It has a switch that allows you to view idle current and plate voltage - my understanding is that you then multiply the two to get a "bias" figure, which you match against a list of appropriate values for the valve in question, and adjust using the bias current trim pot.

Does that sound right?

Looking at the weber site, it suggests matching the idle curent value to the desired level, rather than the idle current x plate voltage figure.

Am I missing something?

Mark.

[hamfist]

Guys, I'm looking at the Weber BiasRite.

It has a switch that allows you to view idle current and plate voltage - my understanding is that you then multiply the two to get a "bias" figure, which you match against a list of appropriate values for the valve in question, and adjust using the bias current trim pot.

Does that sound right?

Looking at the weber site, it suggests matching the idle curent value to the desired level, rather than the idle current x plate voltage figure.

Am I missing something?

Mark.

Mark,  I don't think you're missing anything !.    It's always essential to take into account your plate voltage when biasing.  It's a generally lazy and inaccurate way of biasing to only use the idle current, although I am aware that many folks (and even techs) do it like that.

The power dissipated by a tube is calculated by multiplying the idle current  (in Amps, not milliamps) by the plate voltage (in Volts).   That gives you the power dissipated in Watts.

So, if we biased an EL34 amp with a PV of 350V and an idle current of 50mA (0.050 Amps), then the power dissipated by each tube would be 350 x 0.050 = 17.5W. An El34's max dissipation is quoted as 25W, and 17.5W is 70% of that, so we are in the safe zone.
  However, if we just used idle current and went to another EL34 amp (with a PV of 500V) and simply biased it using the same idle current, we would get a power dissipated of 500V x 0.050A, which equals 25W, which is 100% if the max dissipation of an EL34. The tubes would only last a matter of minutes/hours under those conditions. So we can see how important it is to consider out plate voltage as well as the idle current.

each type of power tube will have a figure quoted for maximum power dissipation (in Watts). This is NOT the figure you should aim for, but about 60-70% of this figure. Go as low as you can, whilst still getting the amp to sound how you want it to.

Have fun !  This could be the first step into a whole world of amp-tweaking !

[MrBump]

Excellent!

Thanks very much for the explanation.

Is 70% of 25 watts the generally accepted norm?  Do you know of a source of that imformation? 

Cheers.

Mark.

[shaman]

Excellent!

Thanks very much for the explanation.

Is 70% of 25 watts the generally accepted norm?  Do you know of a source of that imformation? 

Cheers.

Mark.

it is the accepted norm for a class ab amp-did you see the "calculator" page on the weber site?very helpful !

[hamfist]

Excellent!

Thanks very much for the explanation.

Is 70% of 25 watts the generally accepted norm?  Do you know of a source of that imformation? 

Cheers.

Mark.

Somewhere between about 60 and 70% is where most people find the amp sounds best, without dissipating too much power. Thankfully, you can play the amp whilst biasing, therefore tweaking the bias and listening to what effect it has.

  Just make sure that you don't end up with the tubes dissipating greater than 70% of maximum, as tube life will suffer.

[HTH AMPS]

to expand on what Hamfist said, 70% is the accepted standard for fixed bias push-pull amps to get the best tradeoff of tone, crossover distortion and lifespan.

a quick and dirty way to bias is just to take the maximum plate dissipation of the valve (25w in the case of an EL34), multiply it by 0.70 and then divide that by the plate voltage (say 450v which is about average for an EL34)...

(0.7 x 25) / 450 = 38mA (per valve)

it's just ohms law...
P=IV
I=P/V