[Boatanchors] Tetroded as Triodes

Fuqua, Bill L wlfuqu00 at uky.edu
Mon Feb 20 15:06:45 EST 2012

 I have run 4-125A and 6155 tubes. A pair should produce 500 to 600 watts with glowing plates. 
The 6155s are much more durable and don't glow as brightly as the 4-125As
  I agree that the plate current or cathode current at almost 3kV does not match up with results of
low output power and no glow of the plates. 
  Something is amiss perhaps the cathode current meter.
Or possibly the power meter.
  The gain seems a  bit greater than I recall but I was not limited in drive so did not pay much attention to it.
Bill wa4lav

From: boatanchors-bounces at puck.nether.net [boatanchors-bounces at puck.nether.net] On Behalf Of Robert Groh [rgroh at swbell.net]
Sent: Sunday, February 19, 2012 7:12 PM
To: Wilson Lamb; Boatanchors List
Subject: Re: [Boatanchors] Tetroded as Triodes

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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