[Amps] Alpha 76PA Conversion - Drive Issue

[Steve London]

6 years ago, I converted a 3x8874 Alpha 76PA to a pair of 3CX800's. The
tubes were the Chinese ones that Alpha was selling for a while. Even after
removing the resistor pack in the input circuit, I had trouble getting more
than 1300 watts output without backing off on the loading and running with
the grid current near the trip out point. I always blamed it on the Chinese
tubes.

Today I did a science experiment. First, I put the input network back to
the standard Alpha 76PA "low drive" configuration with the 5x470 ohm
resistor pack. Then I borrowed the 3CPX800's from my Commander HF-2500M.
With those tubes, it's no better, and maybe even worse than the Chinese
3CX800's. These same 3CPX800's work wonderfully in the Commander with 1500
watts output at 35-45 watts drive. (Yes, the Commander runs them at a
higher plate voltage, and has a tuned input circuit on each band). Here are
the specifics with the 3CPX800's in the Alpha 76PA:

Drive: 95 watts
HV: 2000 volts
Ip: 880 mA
Ig: 65 mA
Po: 1260 watts
Efficiency: 71.5 %

Seems like I should be able to get higher Po with less drive.

73,
Steve, N2IC