Old RCA PA amp question

[hylaphone]

Hi gang,
Tinkering around with an old 30W RCA PA..
Can anyone shed some light on R25 in this little schematic?


I see it working in front of me but don't understand -- why is the screen supply tied to the cathode?

[Jazz P Bass]

As far as I can see, the output tube cathodes are 'elevated'.

[DRH1958]

And this resistor along with R26 form a voltage divider to elevate the heaters to 28.5 volts since this point is tied to the heater supply CT instead of the usual ground connection for this point.

[Gingertube]

It does 2 things - one important, one probably not:
1) It provides 35mA of bleed current down through the 175R cathode bias resistor (R26). That gurantees that the screens sit 105 to 110V below the anode voltage at all times (due to the voltage drop across R24). This is the important thing.
2) It develops approx +6V across that 175R cathode bias resistor which means that the heaters are elevated by this +6V before the output tubes start to conduct at power on (cold heaters). Once they are conducting then the voltage at the cathodes rises to that 28.5V which takes the heater elevation up to that voltage. It also means that there is 6V of bias on the output tubes at switch on. If hot plugging tubes or switching on with hot tubes (that is a fast switch OFF and back ON again) this will limit the current spike through the tubes while the cathode bypass cap charges up. This pulse of current through hot tubes in cathode biased amps (while the bypass cap charges) is why standby switches are a BAD idea in cathode biased amps.

I think the 1) is the important thing though.

Cheers,
Ian

[hylaphone]

Interesting!
Seems like a neat solution for big bottle screen supplies
Thanks for the explanation