[Amps] Alpha 77Dx two hole

[Jeff AC0C]

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 at 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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