[AMRadio] Building

jeremy-ca km1h at jeremy.mv.com
Sat Dec 1 19:05:54 EST 2007

----- Original Message ----- 
From: "Barrie Smith" <barrie at centric.net>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Saturday, December 01, 2007 5:12 PM
Subject: Re: [AMRadio] Building

> ----- 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 
> on.
> 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 transformers.
> 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

Well, you certainly have a nice stash there.

There are probably enough Gates owners on here that can tell you the specs. 
However you can determine the turns ratio with a bit of LVAC on the primary 
and then calculate the turns ratio.

Old Handbooks, especially the "Radio" version have extensive charts of the 
PP load of most tubes used a modulators and you already know that the 
VT-127A's are basically 100TH's so you can easily determine their load 
impedance. If you loaf them at 500W input using say 1700V @ 300ma for 
instance (rounded off) the mod xfmr secondary must be close to 5666 Ohms.

Then knowing the PP load of the modulators and the turns ratio of the xfmr 
you can determine what works.

If you want to run some numbers by me by all means do so.


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