Measuring anode dissipation in AC coupled cathode follower

[Gaz]

I'm trying to figure out how many watts my 12AU7 driver is dissipating, but I'm not sure if I measure the plate voltage relative to ground or cathode. In this case, there's a big difference in voltage. Here is the circuit I'm using:



Thanks.

[Enzo]

What if it were a resistor? WOuld you still be confused? As much as I like to point out the circuit is what matters rather than the parts, when we want to look at a part, we need to focus on it and not the rest of the circuit.

So ask yourself: do you want to know what the circuit is dissipating or the tube?

First off, is there a typo? Is that 4.2v supposed to be a 42v? You said there is a big difference in voltage, but to me 350v and 345.8v is not a big difference.


In any case, the voltage ACROSS the tube, not the voltage ON the tube, is what to use. So 345.8v times current, will give you the tube dissipation.

By your report, we have a cathode circuit resistance of 15k + 4.7k + 470 ohm. 15k+4.7k+0.47k=20.2k Normally I'd just call that 20k, but we can be precise.

So 4.2v across 20.2k = V/R = 4.2/20200 = 0.0002A = 2/10 of a milliamp.

345.8x0.0002 = 72mw

[eschertron]

Is the 4.2V cathode-to-ground, or is it across the 470R cathode resistor? If it's the former, there's a negligible amount of power being dissipated. If the latter, there's not enough info to calculate the dissipation, but you have almost 9ma going through the tube. Then, the tube power dissipation = 9ma (8.94ma to 3 digits) times (350V - voltage at the cathode).

edit: simulpost

[Gaz]

Thanks guys, both of those posts answered my question! The 4.2V noted on the schematic was in fact across the the cathode resistor. I measured the voltage from cathode to ground, which is 180V. So, to figure out dissipation, it would be 4.2V/470R = 8.9mA, then 8.9mA (350V - 180) = 1.5133W. Is that correct?

That is half about what I was calculating before when I was using the full 350V B+!

Thanks again.

[Gaz]

Or Enzo's method with the whole cathode resistance and voltage: 180V/20,170 = 8.9mA (350-180) = 1.5.17.

Of course, Ohms Law doesn't lie, so it's a good way for me to check my work. Thanks!