[AMRadio] Mod Transformer Available

James M. Walker chejmw at acsu.buffalo.edu
Wed Sep 28 15:44:12 EDT 2005

Hi John,
Thanks for the clarification. I think I understand now where the problem
is coming from, it is the old apples and oranges thing with noise added!

When the announcement for the transformer came up, I commented that I
running the same transformer, in my modulator for a single 304TL class C
amplifier. I am running my system using 1500 VDC on both the RF stage
and the modulator stage. The RF stage plate current is around 250 MA
and the modulator plate current is 200 MA. This allows me to run the
transformer at the rating on the tag, and still get a respectable power
out of the RF stage and maintain the impedance ratio for the modulator
to RF stage at a respectable level IE; close to a match.

At 250 MA the RF stage is running around 375 watts input good enough
for me, if I want more power I would use the RCA BTA-1M rig or the
BC-610(I). My object was to resurrect an old HB rig not light up the
night sky! Thanks for the clarification again there John, better
understanding now. Maybe all around! Still waiting to do the audio sweep

"John E. Coleman (ARS WA5BXO)" wrote:
> Hi Jim:
>         Don't know what happened to Don's message it did get muddled up!
>         But here is another.
>         If the XFMR is to work into 5500 ohms and the final current is to be
> no more than 200 ma (0.2A) then the voltage on the final can't be more than
> 1100 volts.  (E = I * R). DC input power to the final would only be 220
> Watts.  It will obviously work well under these conditions but what a large
> XFMR for only 110 watts of audio.  You could double the voltage to 2200
> volts and unload the final to keep the current at 200 ma this would give you
> 440 watts DC input power.  But now the load is 11000 ohms.  The XFMR will
> probably work OK into this higher Z but how will it effect the frequency
> response.  Raising the voltage to 4400 volts and keeping the current down to
> 200 ma will give you 880 watts DC input to the final.  This XFMR is rated
> for over 1000 watts of audio so it would have no trouble modulating this 880
> watts DC but now the load Z is 22000 ohms.  Now were talking some serious
> changes in Z for what the XFMR is stated to work into.  I wonder what that
> does to the frequency response and coupling efficiency.   Plus the
> insulation would need to with stand at least 8800 Volts Peak for 100%
> modulation.
>         I think you can see that it is not possible to match all of the
> rating of the XFMR at once.  Something has to be worded incorrectly about
> this XFMR.  Perhaps it was never designed to have any current through it's
> secondary and was required to have a large choke to by pass the most of the
> current.  But you cannot design a 1000 WATT TRANSMITTER with only 200 ma max
> current through the secondary of this XFMR and maintain 5500 ohms load Z
> without a modulation reactor of some sort shunting it.
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of James M. Walker
> Sent: Tuesday, September 27, 2005 8:59 PM
> To: Discussion of AM Radio
> Subject: Re: [AMRadio] Mod Transformer Available
> Well,
> I guess I am still confused, Sigh but I am going to drop the request
> clarification, as the clarification seems to be muddled!
> Jim
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