Steve,
I had N4UQ make the conversion several years ago on my 78. I used original new
Eimac 3CX800 s. I found the same results as you……1300 w output with 80w drive.
My alpha 76PA gets full output with 3-8874 with the same input.
I replaced the transformers in each with hyper-sil ones and had a vacuum relay
put in the 78 for full QSK. Now it is no tune from 80-10m.
Due to my age and physical limitations. They are both for sale if you know of
anyone. Each is a fraction of the cost of today’s amps.
Carol
Sent from my iPhone
> On Nov 30, 2021, at 4:09 PM, Steve London <n2icarrl@gmail.com> wrote:
>
> 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
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