[Amps] Alpha 77Dx two hole

[Paul Christensen]

>"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