[AMRadio] Building

Barrie Smith barrie at centric.net
Sat Dec 1 17:12:54 EST 2007

----- Original Message ----- 
From: "jeremy-ca" <km1h at jeremy.mv.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Saturday, December 01, 2007 2:45 PM
Subject: Re: [AMRadio] Building

> ----- Original Message ----- 
> From: "Barrie Smith" <barrie at centric.net>
> To: "Discussion of AM Radio" <amradio at mailman.qth.net>
> Sent: Saturday, December 01, 2007 2:27 PM
> Subject: [AMRadio] Building
> Well, I was building a new HV power supply, until I had my gallbladder 
> taken out last week!  Not supposed to lift anything over 20 lbs for a 
> month.
> It's a resonant-choke supply.  I was just to the point (with a bunch of 
> taking out and putting in of parts) of drilling the mounting holes and 
> finishing it up.
> Now (due to a wonderful fellow on this list, whom I'll name if I get 
> permission), I'm going to start working on an RF deck using a pair of 
> VT-127As.
> These tubes are a cross between a 100TH and the 250TH in a small, 
> strangely-shaped envelope.  Should be just right for 300 easy watts out.
> They are a 100TH repackaged for radar use in WW2 and use a 250TH filament 
> for sustained peak emission. It was also designated the 100TS by Eimac.
> An OT ham down the street from me in the 50's ran a pair at 1KW input on 
> 20 CW, it was fun watching those plates glow!
> Now I'm thinking about a modulator.
> Why not 2 more 127A's? With a window in the rack panel you can watch the 
> plate color with modulation.
> Carl
> KM1H

Carl:  Thanks for the response.

Well, I did think of that, but I'm kind of at loose-ends here as to what to 
do about a moulation transformer.

I have four modulation transformers in the junk box.  One is a very 
conservative KW PWD, which is large and heavy (I used that in a PP pair of 
450THs, modulated by a pair of 304TLs, a long time ago.  That transmitter 
made a heap-of-smoke . . . and RFI).  Another is a National that says 70 
watts.  It came out of an FAA transmitter, built by National, that used a 
pair of 4-65As in the final.

I also have a modulation trasformer that came out of a 250 watt Gates BC 
transmitter.  And, a Thermadore, that I can't seem to find any information 

The KW PWD is rated for 800 watts audio 40 to 7500 hz.  It has no air-gap, 
and is to be used with a matching reactor and cap, which I have.

The primary is 12,000 ohms, center-tapped.  The secondary is 4545 ohms.

I'm mentioning these figures because it's been at least 15 years since I've 
calculated various tubes and voltages for matching up modulation tubes and 

I do remember that on the 450THs I was running 2500 volts @ 600 ma.  Don't 
remember what the modulation voltage was.

73, Barrie, W7ALW 

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