[Boatanchors] Tetroded as Triodes

Wilson Lamb infomet at embarqmail.com
Sun Feb 19 13:26:38 EST 2012

I acquired an amp with a pair of 4-125As in grounded grid.  It was a mess, 
never finished, and took a LOT of work, but is now on the air.

Does anyone have experience with this setup?  There's plenty of plate 
voltage, 2700 and off resonance the tubes will go bright red.  Max output is 
300W, however, which I think is low.  There's no plate current meter yet, 
but cathode current at max output is about 300mA, much of which is grid 
current, I think.  Key down CW at 300W out shows almost no color in the 
plates and increasing loading with the pi net output produces less output.

It sounds great on SSB and has a lot of gain (15W drive for 300W out).

So what's happening that keeps it from producing more output when the drive 
is raised?

I've seen some articles on 4-400s in GG, but never the little 4-125A.


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