Screen to cathode resistor in guitar amp?

I have seen a lot of 6l6 amps, but never with a resistor between screen and cathode.
This is a Gibson GA 90 amp from the 50s. Im restoring one at the moment, and this resistor puzzels me..
The center tap from the heater fillament connects to the same point in the drawing, but has been removed
at some point. Should I also remove this resistor?

I agree with you, look strange but can work.
I think it is dual purpose circuit, screen voltage voltage divider and heater reference.
The 2k5 and 25k fix the screen voltage at 90% of anode voltage, 10uF reduce ripple.
The series of 2k5 and 25k (27k5) with the 150R set the heater reference and the bias point for the power tubes.

I found sever schematic for the GA90 with the same resistor, i think it can work also without the 25K, but you change the Screen voltage and also the bias point for the whole power amp.

Thaks a lot! That make sense. I wonder why this solution to get the screens to sit at 90% is not used in any other designs.
Saveing one resistor cant be the reason... Can you see any drawbacks? 
The GA90 is a very clean amp, can this clamping of the screens
be one of the reasons, besides the feedback and low gain structure in the amp?
J

That circuit does provide some bias voltage to the output valves during turn-on - that may alleviate some stress on the 6L6's, and/or hum.

That amp has a few 'nods' to hi-fi, so heater elevation to that point was a common technique for alleviating hum.

> Should I also remove this resistor?

Why? What harm can it do?

It is only 15mA added load. It stabilizes cold-start voltages and running bias. It bleeds the main cap at power off.

Yes, for MAXimum efficiency and power, we would avoid that. But you don't own a 1950 Gibson for ecological stadium blasting, you own it because it is NOT just another 1960s Fender/Marshal/Ampeg.

Leave it as original. I'd put back the heater CT also.

No plans to make it into a stadium blaster I will keep it as original as possible.
My new mains transformer dont have a fillament center tap, but I will try the two resistors trick
and attach center to the cathodes.  Thanks for all input!
Power up tomorrow...

J