Hybrid fixed/cathode bias measurements?

[Carlsoti]

I've recently picked up a Magnatone M15 3x8, and have rebuilt the power supply after finding a shorted cap had taken out the 10W dropping resistor that feeds the reverb amp. A number of the other original electrolytics were leaking orange gunk like the shorted cap, so all of those were changed as well. Additionally, I upgraded to a IEC socket and 3-wire ground scheme and eliminated the death caps. I've gotten all my voltages in check and, save for a wonky stock speaker, it all seems to be working properly, though I want to verify the bias of each of the output sections of this amp.

   On the 7189A tubes, I've got 390 on the plates, 390-391 on the screens(UL config), -16.28 on the grids, and 2.95 on the cathode. In this amp, there is also a 100 ohm cathode resistor in addition to the fixed bias circuit. Both of them in this amp measure 107 ohms.

Does calculating cathode current by measuring voltage across the cathode resistor work the same in this configuration?

If so, does that mean this amp is designed to bias somewhere below 50%?

Attached is a schematic in which I arbitrarily assigned tube numbers to help with troubleshooting. I did not update the drawing with my voltages, the biggest deviation being at point D, where I have 223 instead of 240v. 

[PRR]

Look in the 7189 datasheets. Some conditions DO idle the tubes cold.
https://frank.pocnet.net/sheets/127/7/7189.pdf   page 2, Fixed Bias

Yes, the AB1 condition does run the tubes a lot hotter. 

[WimWalther]

Does calculating cathode current by measuring voltage across the cathode resistor work the same in this configuration?

Absolutely. Cathode current (Ik) is cathode current regardless of how it's being controlled. The only difference in this case is that you need to be sure to measure your plate voltage (Ea) between the plate & cathode pins on the tube socket, rather than between plate & ground.

You don't want the cathode resistor (Rk) voltage to be part of the power equation. That power is being dissipated in the Rk itself, not within the tube.

[pdf64]

If there's a very large change in cathode current from idle to full output, then full fixed bias tends to be preferable.
As otherwise, if some / all was self bias, then the bias would change significantly according to the signal level, ie get colder at high signal level, which would tend to promote less linear operation, perhaps leading to class B and crossover distortion.