Resistor value range for ground lift

[Guitarist]

I see that 10Ω is standard but how low or high can we take that? I'm out of those

[Mick Bailey]

For an amp?

15 and 22 Ohms are pretty common, often with a pair of back-to-back diodes in parallel.

[Guitarist]

Thanks Mick,

It's for a tube preamp. I added an independent output and was preemptively avoiding a ground loop. A 33Ω didn't work to well.

[Mick Bailey]

The value doesn't matter too much - you can play around with the the value to get the best result. But from a safety perspective a resistor will usually fail open and could disconnect the ground in a fault situation, so that's where the diodes come in and also gives a lot more flexibility in resistor choice. I use a pair of 6A 400v types the same as the Fender '63 reverb. Here's Fender's reasoning;

"CR5 & 6 (across R23) provide an important safety feature. IF the guitar amp chassis becomes electrified, current will
flow through the coax cable to the power supply ground of the Fender Reverb unit. The current will seek earth ground
through R23. When R23 opens, the earth ground connection is broken. This will electrify the reverb unit’s ground and
thus the guitar (ouch!). CR5 & 6 provide an alternate path to earth ground if R23 opens."

Apologies if you already new that.

[Guitarist]

Thanks so much, that's a clear explanation. I'd like to built a stereo reverb rack unit soon too sometime as I miss my Ampegs'.