Generically speaking, I don't think the 2nd tube changes the tank tune
settings - assuming the same total plate current and the HV is unchanged.
What does change is the stray anode capacitance (Cout) which roughly doubles
in a 2-tube arrangement. The increase Cout shows up across the tune cap
which is why you can run against the min stops. Of course with that 2nd
tube, the temptation to push more total Ip results in a lower tank input Z
and that does change the tune settings.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: MU 4CX250B
Sent: Monday, February 13, 2017 1:34 PM
To: Paul Christensen
Cc: Amps Amps
Subject: Re: [Amps] Alpha 77Dx two hole
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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