Low plate voltage and BIAS

[rh]

Can the bias for a tube be safely set higher than usual if the plate voltage is lower than usual?

I've got a 6V6 amp reported to have low plate voltage, and it's biased at 31mA. Sounds great, but I don't want to blow through tubes if I don't have to.

[justonwo]

Yes, it's the product of the two, plate voltage times current, that is the number you have to worry about. If the voltage of low, then the current should be higher to make up for it. You wouldn't happen to know the voltage on your plates?

[rh]

Originally posted by justonwo
Yes, it's the product of the two, plate voltage times current, that is the number you have to worry about. If the voltage of low, then the current should be higher to make up for it. You wouldn't happen to know the voltage on your plates?

No, but I'll see what I can find out. I'm reasonably competent (and careful) -- what would I hook a meter to, in order to measure plate voltage?

Alternately, we could use alegbra to estimate a range for the plate voltage, given that (I understand) the normal bias range for a 6V6 is 20-25mA and these are at 31mA.

[Old Tele man]

...you can estimate the idle current using the following equation:

Ip.q = G * (-Vg + Vs/mu1 + Vp/mu2 )^(3/2)

where:

Ip.q = Plate quiescent (idle) current, amps
G = Tube constant, Perveance, 0.00084A/V^(3/2)
Vg = Control grid bias (idle) potential, volts
Vs = Screen grid potential, volts
Vp = Plate potential, volts (at idle)
mu1 = Tube triode amplification factor (6V6 = 9.8)
mu2 = Tube plate amplification factor (6V6 = 205)

...for example, at Vg = -12.5V and Vp = Vs = 250V:

Ip.q = 0.00084*(-12.5V + 250V/9.8 + 250V/205)^(3/2) = 0.045A

...if you know the measured voltages in your amp, you can estimate what happens as you change: (A) control grid bias voltage; (B) screen voltage; or, (C) plate voltage.