[AMRadio] Mod Transformer Available


James M. Walker chejmw at acsu.buffalo.edu
Mon Sep 26 22:35:19 EDT 2005


Yep,
that is identical to the one I have. I am using 805S driven by 2A3s, I
chose not to exceed the ratings for the 805s or the transformer, so I
kept the voltage at 1500 volts @ 200 MA, as I said I haven't done an
audio sweep into a load for the modulator and monitored it with a scope
yet, but when I do I will post the results on my web page. I am not sure
the transformer windings will handle 500 watts. I know that at 1500 
volts the 805s can safely handle the 300 watt input level, per the tube
specs available on my web pages under home-brew and 304TL/TH then 805
specs. It is a close match to the plate impedance called out on the
sheet.
Jim
WB2FCN
http://eshop1.chem.buffalo.edu 


Paul Sokoloff wrote:
> 
> The model number is RCA 901769-501 and it has the original paperwork with
> it.  It says that the secondary current is .198 amperes, so I guess you need
> 2,500 volts to get 500 watts??  There was a page on the internet that said
> max current for this beast is 450 ma., so I don't know if that is a
> practical and not theoretical conclusion.  Thanks for looking.
> 
> Paul
> 
> ----- Original Message -----
> From: "James M. Walker" <chejmw at acsu.buffalo.edu>
> To: "Discussion of AM Radio" <amradio at mailman.qth.net>
> Sent: Monday, September 26, 2005 3:40 PM
> Subject: Re: [AMRadio] Mod Transformer Available
> 
> > I have that transformer as well, it is in the pair of 805
> > modulator stage for my HB 304TL rig. The system runs at 1500 VDC
> > For the plate supply to the modulator and also the RF deck.
> > I rewired the speech amp to a pair of 2A3s to drive the 805s.
> > I haven't done an audio sweep to check the range yet however.
> >
> > The ratings are 5500 primary 1.14 KVA, and 5500 secondary
> > .198 Amp DC.
> > Jim
> > WB2FCN
> >
> > ----- Original Message -----
> > From: "Donald Chester" <k4kyv at hotmail.com>
> > To: <amradio at mailman.qth.net>
> > Sent: Monday, September 26, 2005 3:22 PM
> > Subject: RE: [AMRadio] Mod Transformer Available
> >
> >
> > >
> > >
> > > >     I have a 1940s vintage 500 watt modulation transformer made by RCA
> > for
> > > >sale.  It has 5700 (approx.) ohm P-P primary and a 5700 ohm secondary.
> I
> > > >would like $250 plus shipping.  Local  pickup near Philadelphia
> > preferred.
> > > >I checked the windings and they have continuity.  There is also another
> > > >winding of a higher impedance maybe for screens.  If interested please
> > > >contact me directly at dogfaces at comcast.net
> > >
> > > After WW2 those transformers flooded the market as surplus.  You could
> > > hardly attend a hamfest without seeing at least one.
> > >
> > > They are not broadcast quality, but the quality is probably at least as
> > good
> > > as typical trarsformers sold for ham radio use.
> > >
> > > For years I wondered what they came from or what they were manufactured
> to
> > > go into.  A couple of years ago I found out that they were not used in
> > phone
> > > transmitters at all, but in tone modulated CW rigs on board Liberty
> Ships
> > > during the war.  So they were desgined to pass only a single tone
> > modulating
> > > frequency, but they work passibly for voice.  I would still prefer a
> > > transformer designed for voice use, preferably something from a
> broadcast
> > > transmitter.
> > >
> > > Don k4kyv
> > >
> > >
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