[Amps] Alpha 76PA Conversion - Drive Issue

[Robert W5AJ]

Steve,

Have MLA2800 (converted MLA2500), it's still in the garage/tool room, it
can do 1500watts out, CW (the power supply isn't good to maintain that!)
standard 2xEmiac 3cx800

At one time, had Alpha 76 converted to 3CXP800 with the higher voltage HV
Dahl transformer, one day I wasn't thinking and sold it.
It was a nice amp.  Did install a matching transformer on input, bought
from Alpha.

I don't recall Drive being up at 95 watts on either.   Saw comment that
2000VDC was a bit anemic, agreed but one uses what one has. The MLA2800
plate voltage dips down in that range too.

73 W5AJ Robert





On Tue, Nov 30, 2021 at 3:09 PM Steve London <n2icarrl at 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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