Leslie 122 Bias Monitoring Resistors

[patlaw]

Is there any reason NOT to add a 1Ω 1/2 watt resistor between Pin 8 on each 6550 and the 150Ω cathode resistor to make it easy to reliably check the plate currents on the tubes?

[patlaw]

Sorry, this post should have been in "Other Amps". I don't see how to delete this post.

[Helmholtz]

Is there any reason NOT to add a 1Ω 1/2 watt resistor between Pin 8 on each 6550 and the 150Ω cathode resistor to make it easy to reliably check the plate currents on the tubes?

No problem.
But you'll need to measure voltage drop directly across the 1R resistors and not from cathodes to ground.

[Enzo]

Or simply measure voltage across cathode resistor, calculate current from that, then divide by two to get current per tube. I don't recall ever having a bias issue on a Leslie amp.

[patlaw]

Or simply measure voltage across cathode resistor, calculate current from that, then divide by two to get current per tube. I don't recall ever having a bias issue on a Leslie amp.

Me, either. However, the tube shortage and prices have some people hypersensitive to tube bias. Before the pandemic, most of them probably didn't know what it is.

[Enzo]

Most of them still don't know, it is just a fancy word to throw around.

I bet if you just mount a test point on the chassis wired to the cathode, and point it out to them, they will be impressed.

[Helmholtz]

. I don't recall ever having a bias issue on a Leslie amp.

While that's a strong argument, I understand the wish to read each tube individually - especially if there's some residual 120Hz ripple hum.
I think that a common cathode resistor tends to increase imbalance of unmatched tubes.

Voltage drop across the cathode resistor is produced by the sum of the cathode currents, but doesn't mean that each tube contributes 50%.