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Re: [Amps] Alpha 76PA Conversion - Drive Issue

To: "" <>
Subject: Re: [Amps] Alpha 76PA Conversion - Drive Issue
From: Steve London <>
Date: Wed, 1 Dec 2021 06:43:27 -0700
List-post: <>

I received this, and a number of similar responses from others.

Nobody ever seemed to mention the lower power output in the Alpha 76 3CX800
conversion articles, although AC0C came close. A little disappointing that
going from 1200 watts of plate dissipation (3 x 3CX400) to 1600 watts (2 x
3CX800) results in lower output.

3CX800's have become as unobtainium as 3CX400's. I may convert my amp back
to 3CX400's.

Steve, N2IC

On Tue, Nov 30, 2021 at 4:40 PM <> wrote:

> 3cx800 tubes like 2600-2700 on the plates for maximum gain.   2000 volts
> is a bit anemic for 3cx800 tubes.  That modification was good when 3cx800
> tubes were plentiful.  They are going in the same direction as 3cx400’s did
> now and the modification doesn’t make much sense anymore.  I’d settle for a
> used Commander these days..
> Sent from the all new AOL app for iOS
> <>
> On Tuesday, November 30, 2021, 4:09 PM, Steve London <>
> 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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