Balance pot for common cathode resistor bias - protection resistor necessary?

[stephenl]

Kind of a dumb question...
When using a common cathode resistor and a balance pot to bias power tubes, is there any need to put a protection resistor across the wiper in case it opens? The tubes would just stop conducting right?

[Stevem]

Yes, no need for that!

If one leg of the Cathode bias balance pot should go open then that tube(s) will just drop down to fully below having any idle current on it.

Many old Cathode biased Gibson amps used a standby switch set up that placed a big ohmage resistor in series with the common Cathode ground to drop the output tubes down to like .015 ma of idle current.

[sluckey]

Many old Cathode biased Gibson amps used a standby switch set up that placed a big ohmage resistor in series with the common Cathode ground to drop the output tubes down to like .015 ma of idle current.

I can't find any Gibson amps that do that. Can you give a specific model?

[Stevem]

Well I may have misspoken, like a few Gibsons had that standby set up, like my 1954 GA75 model that took me 2 years to find the correct sehematic for due to my model using 12AX7 tubes and a 6J3 PI tube.

[sluckey]

Ah, this one? Looks like a rare combination of fixed and cathode bias. Lots of surprises with Gibson's amps.

https://el34world.com/charts/Schematics ... 7_1954.pdf

[SoulFetish]

What kind of voltage do you expect across the pot? Are you using a Rheostat for this?
You are going to have the equivalent of the full cathode current (screen + plate current) of half the output tube(s) going through the potentiometer. I would just caution to make sure that the potentiometer is rated to handle the full class B output current (if class A/B) for it's ohmic value.
For instance, in class B operation when the conducting tube is in it's highest current state, the power rating of the potentiometer needs to be derated to the percentage of the resistance that the particular tube is using.

[stephenl]

I'm going to use a 25ohm, 5 watt pot.
The power tubes are biased at 45mA each, limiting class A push pull.

[stephenl]

Ah, this one? Looks like a rare combination of fixed and cathode bias. Lots of surprises with Gibson's amps.

https://el34world.com/charts/Schematics ... 7_1954.pdf

I used the mixed bias approach on one of my amps. Gingertube (Ian) had suggested using a common cathode resistor(no bypass cap to allow a little feedback)that was about 15% of the typical cathode bias resistor - the rest of the bias is supplied by adjustable fixed bias. I used a pair of KT66's into 8K primary. I think I used a 50 ohm common cathode resistor

The rest of the amp was the same circuit as another amp I had built which had switchable cathode or fixed bias. In comparison, I liked the mixed bias the best out of the three setups...I really can't say why

[sluckey]

My Magnatone M10 uses fixed and cathode bias.

[SoulFetish]

My Magnatone M10 uses fixed and cathode bias.

My amp does as well.