Bias Monitoring Question

[Trout]

I have a buddy locally here that brought his amp over today and he showed me something I never noticed/tried. I in my limited experience wonder if this is a normal occurance, or if something might actually be wrong.

Anyway, here is the symptom.

He has a Clone/Homebrew Super 5F4 (http://www.ampwares.com/ffg/schem/super_5f4_schem.gif) that has a bias pot added. It has bias test points added as well as the 1 ohm resistors for using a simple multimeter for testing purposes.

He set the bias at about 60% dissapation 45ma w/400V plate(JJ 6L6GC's) No instrument attached.

It is stable and seems fine, but if you watch the meter when actually playing through the amp, the bias reading jumps REALLY High then falls back as signal fades.

I had never tried taking readings while actually using the amp.

Anyone have any further insight on this?

[Faustine Amps]

What you're reading at the test points is Idle Plate Current, not actual bias voltage. The plate current is going to jump all over the place when the tube is amplifying a signal. What you're seeing is normal. This is why you set the bias with no signal.

Your "bias" setting (idle plate current) of 45mA is good at that plate voltage.

Regards,
Tim Gregoire

[brad347]

Notive that bias recommendations will say something like "70% max dissipation at idle."

That's because the amount of power the tube is dissipating will change in proportion to how much signal is being fed in.

That's the reason we do 50-70%, by the way. Because 100% is the "ceiling" that you don't want to go too far above for too long. By biasing it between 50 and 70% dissipation, that gives you a margin when you are actually putting signal through the tube.

Remember that when checking bias, you're not measuring bias voltage... you're measuring plate current to determine power dissipation in the tube.

[brad347]

damn I was too slow on the draw, haha.

[Trout]

Thanks Guys!,

I suspected that was the case. I had just never actually looked at a meter connected while the amp was being played.

BTW, I can see why 60-70% is recommended, the jump is pretty significant.:AOK

Remember that when checking bias, you're not measuring bias voltage... you're measuring plate current to determine power dissipation in the tube.

Yep, that part I already knew, I forget the actual bias voltage exactly, but at idle 43V seems to be sticking in my head.