I agree completely with Paul's comments. I had a 77SX for a few
months and didn't care for the amplifier. Best I could tell, it was
identical to the 77DX, except for the extra tube and changed
multiplier resistors in the metering circuit. The plate voltage sagged
considerably under load, as I recall, and the unmodified tank circuit
wouldn't tune 10m.
73,
Jim W8ZR
Sent from my iPhone
On Feb 13, 2017, at 1:00 PM, Paul Christensen <w9ac@arrl.net> wrote:
>> "With two tubes in parallel perhaps the output circuit was designed to
>> transform the combined impedance?"
>
> The output Pi-L was tapped at the factory for single tube operation. While
> I'm not 100% sure, I've never seen a Sx version amp tapped any differently
> than the Dx.
>
> If you're inquisitive, you can always check the plate Z and confirm tap
> positions by running an impedance analyzer injected into the amp's output
> jack, then create a R/C combination that substitutes the computed parallel
> plate Z. You'll need to activate the output vacuum relay with a +24V bench
> supply.
>
> I would check with Dick Byrd, N4UQ. He's probably seen, and worked on as
> many Sx models as anyone. He should be able to tell you where the Plate and
> Load controls normally end up after the second 8877 addition.
>
> I've never considered the 2nd 8877 for several reasons:
>
> 1) The 77Dx already performs in continuous duty service at the U.S. legal
> limit on all bands;
> 2) The power supply design is adequate for a single 8877, but marginal when a
> 2nd 8877 is added to run the amp at 1500W+;
> 3) Taking full advantage of the 2nd 8877 almost requires an upgrade to the AC
> line fuse shell size, and arguably, the RF output connector;
> 4) I'm not a fan of running large filament current on a chassis. With a 2nd
> 8877 added, that's 20A of AC chassis current. However, there's nothing
> strictly wrong with it;
> 5) Pi-L output should be re-optimized for the new plate Z. It's often just
> too much work for most owners to get the added efficiency;
> 6) For me, it's a helluva' lot easier to troubleshoot an amp with one tube
> versus two or more.
>
> Paul, W9AC
>
>
>
>
>
>
>
>
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