Question on switchable bias

[dude]

I set up one of my 6v6 Plexi’s to run either cathode or fixed biased. I tried a pair of EL34s with Hammond AO43 PT and 35 watt Allen OT. My plate voltage with a 5ar4 is 382v, 250 ohm bias R, in fixed mode l set bias at about 70%, 42mA. Sounds good. The bias is switched by lifting the cathode or ground on the each bias mode.  In cathode mode with meter probes in the bias jacks, the meter reads 55mA which is close to 100% dissipation, changing to fixed bias drops to the set 42mA.
My question is, running in cathode bias the bias voltage is 55mA which l assume is about 90 to 100% dissipation? The PT with 5AR4 and EL34s doesn’t seem to get more then warm after an hour so l assume that AO43 PT can handle the draw.

[PRR]

> running in cathode bias the bias voltage is 55mA which l assume is about 90 to 100% dissipation?

I figure more like 72%. Why do you care? How does it sound? Smell?

[dude]

Don’t care, sounds good. Was just curious if when in cathode mode if putting meter probes in the fixed bias jacks the mA’s are readings the bias the tubes are running at in cathode mode. I did the math to get dissipation, voltage across cathode R, divided by value, (250ohms), X plate voltage, minus voltage across cathode resistor. That’s about 20 watts dissipation for a single EL34, not exactly 90%, but maybe not as low as 72% dissipation..? Maybe l for got the screens..?

[HotBluePlates]

... Was just curious if when in cathode mode if putting meter probes in the fixed bias jacks the mA’s are readings the bias the tubes are running at in cathode mode. ...

It probably is, but we'd have to know how those jacks are set up.

There's another tell-tale test you can do:

     -  In fixed-bias mode, measure the voltage at pin 5

     -  In cathode bias mode, measure the voltage across the cathode resistor.

If the (absolute value of the) voltage is smaller in cathode-bias, you will know why they tube is passing more current.

You mention 55mA and a 250Ω cathode resistor.  55mA x 2 = 110mA, and 110mA x 250Ω = 27.5v.  If your numbers are actual plate current (not cathode current) there is some additional screen current we're not accounting for (making the voltage across the cathode resistor a little larger).

Either way, I'm guessing your fixed-bias is more than -28v (and maybe closer to -35v).  If so, that's why there's a difference in idle dissipation.  It will also account for the difference in apparent sensitivity.