[Amps] Question about GS-35b efficiency

[Bill, W6WRT]

I have a question about the plate efficiency of a GS-35b amplifier.

I have built two of them for 160-10 meters, and while they worked ok, the
efficiency was not as good as I expected. In both cases I ran them at 3 kVDC and
in order to get a full 1500 watts out, had to drive them to 1000 mA plate
current. That equates to about 50% efficiency, which seemed a bit low. 

In both cases, I removed the GS-35b and replaced it with an 8877. With no other
changes except bias diode and heater transformer, I was able to get 1500 watts
out with only 830 mA plate current, an efficiency of 60%. I swapped two
different GS-35b's with the same result. Both were brand new tubes. Results were
essentially the same on all bands, with a slight drop off on 15 and 10 meters
with all three tubes. 

With all of the different tubes, I set the bias for a ZSAC of about 200 mA.

I have spoken to two European hams and one US ham and they have reported similar
results.I wonder if the GS-35b might achieve higher efficiency at higher plate
voltages?

 I would appreciate hearing from anyone who has made similar measurements.

73, Bill W6WRT