Bias Ampeg V4BH

[Randall]

I need to bias an Ampeg V4BH. I am a little perplexed by the instruction on the schematic, "Adjust AP1 for 120w or 1.3A draw at 115v". I see the bias pot, but what exactly does that mean, and where should I measure? The cathodes come back seperately thru fuses to ground. 120W would be 30W each, do they really want to bias at 100%?. Confusing to say the least.

https://ampeg.com/support/files/Sche...20Series/V4BH/

[Helmholtz]

I need to bias an Ampeg V4BH. I am a little perplexed by the instruction on the schematic, "Adjust AP1 for 120w or 1.3A draw at 115v". I see the bias pot, but what exactly does that mean, and where should I measure? The cathodes come back seperately thru fuses to ground. 120W would be 30W each, do they really want to bias at 100%?. Confusing to say the least.

https://ampeg.com/support/files/Sche...20Series/V4BH/

Sounds like mains input values.

[Enzo]

The clue there is current draw of 1.3A@115v. You are to be measuring the mains current. Monitor mains current draw and set the bias so the whole amp draws 1.3A. Or just 120 watts. WHy 1.3A and 120 watts don't agree numerically I have no idea. (1.3A at 115v is more than 120 watts.)

Carvin used to do the same thing on some amps. They knew when the amp was biased up right, the mains draw would be X amount. So instead of making people go inside and bias electronically, they just had you use mains draw.

But the bottom line, Randall, is this is just a guitar amp. You know how to bias guitar amps. Just use whatever biasing method you usually use and are comfortable with.

[Helmholtz]

The clue there is current draw of 1.3A@115v. You are to be measuring the mains current. Monitor mains current draw and set the bias so the whole amp draws 1.3A. Or just 120 watts. WHy 1.3A and 120 watts don't agree numerically I have no idea. (1.3A at 115v is more than 120 watts.)

The increased mains current is due to the PT's exciting/magnetizing current which is inductive/reactive and thus does not contribute to real power.

https://en.wikipedia.org/wiki/Power_factor

[Randall]

Interesting, I'd never seen this method before. Seems to not be that accurate since my wall voltage is 124vac today, and as Enzo points out the math doesn't check out with 115v and 1.3A. But I did put the amp on my Kill A Watt meter, whilst also measuring cathode current and B+. At 1.3A and 133W mains draw, I am at about 60% max diss., and that works for me.

[Helmholtz]

Interesting, I'd never seen this method before. Seems to not be that accurate since my wall voltage is 124vac today, and as Enzo points out the math doesn't check out with 115v and 1.3A. But I did put the amp on my Kill A Watt meter, whilst also measuring cathode current and B+. At 1.3A and 133W mains draw, I am at about 60% max diss., and that works for me.

Seems your Kill a Watt takes care of the power factor (PF) of 0.8 in your case.