[Boatanchors] Tetroded as Triodes

Robert Groh rgroh at swbell.net
Sun Feb 19 19:12:02 EST 2012

A few no-practical-experience-but-just-educated-guesses:
1.   You have 250W of available plate dissipation.  Allowing 60% efficiency, 
300W watts out, 500W in, 200w for plates.  Not far off the mark.

2.   Grid current should be pretty much zip so the cathode current pretty much 
equals plate current so plate current is approximately 300 mA (the data sheet 
lists the grid at 5W maximum).

3.   Assuming no sag in plate voltage: Pin = 2700 * 0.3A = 810 watts.  That 
would imply 500W plate dissipation (with 300 W output). That is NOT consistent 
with no plate color so I suspect a large drop in plate voltage under load.  OR a 
lot of grid current.  But even if you allow 50 mA of grid current (quite a bit), 
that would put the dc input at 600+ watts and that should have a ton of plate 
color.  So my suspicion is lots of plate voltage drop.

What load impedance are you shooting for and what band are you on?  All sounds 
interesting and I (for one) would certainly like to hear more!


From: Wilson Lamb <infomet at embarqmail.com>
To: BOATANCHORS <BOATANCHORS at puck.nether.net>
Sent: Sun, February 19, 2012 12:26:38 PM
Subject: [Boatanchors] Tetroded as Triodes

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 

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

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