One way to measure accurately the current through the output tube of an amplifier is to break the connection between the cathode and ground. It is often advised to put a 10 Ω resistor between the cathode and ground and measure the voltage, use Ohm's Law and figure the current.

My question is, should that resistor be shunted with a capacitor to keep the cathode at AC ground as they do in a lot of small radios? Or is that value so low that it doesn't matter.

I hope that question makes sense.

Thanks.

10 Ohms won't make much difference - it's tiny compared with the plate resistance of the tube. Yes, there's a little bit of negative feedback from it, reducing gain a tiny bit, but it's probably inside another feedback loop which will reduce gain by a tiny bit less than it would have without the resistor...

Thanks Tom, I figured as much. I appreciate your input.

A lot of the guitar amp guys insert 1 ohm resistors from the cathode to ground. It makes no discernible difference, and allows for pretty simple calculations...!

73...Jordan VE6ZT

A number of commercially designed amplifiers like the Eico ST-70 have unbypassed cathode resistors with jacks to do measurements while setting bias, and they work fine that way. I don't think you could possibly hear or measure any difference if they were bypassed.

Bias set with no signal? Amount of AC on cathode negligible?

Yes, the bias would be set with no signal. I guess there is minimal AC loss (with a signal) across the bias checking resistor.

At least one Heathkit power amplifier (W4M) has jacks in the o/p tube cathodes to allow a mA meter to be inserted to set bias. It scared me that the circuit could go open for some reason, sending the cathode voltage sky-high! As insurance, I bypassed each jack socket under the chassis with a precision 10 ohm resistor. Now a simple volt meter allows current balance to be set with no risks.
Cheers,
Roger