Re: [IRCA] WABV transmitter muddy audio

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

> Assuming a pair of  833s (e.g.) in a push-pull configuration: Now that
> you replaced one, you should replace the other as now they are
> probably unbalanced.
>
> Since that's probably not easy on the station budget, I'd suggest
> that you try to increase the negative feedback around the modulator.

Often in transmitters using a pair of 833A's in p-p, the driver section is a
problem.  Some years ago I had a late 1940's GE XT-1A to repair.  It used a
couple of 845's as audio drivers, and the driver transformer quit.  I
happened to have a fairly high power UTC transformer with around a 30 ohm
primary winding, and a split winding of about 8,000 ohms or so on the
secondary.  I put a Marantz solid state stereo amplifier on the primary,
using the two channels out of phase as a balanced push-pull output.  I had a
small transformer of about 600 ohm to maybe 10k ohm at the input to go from
balanced to unbalanced out of phase at the amp input.  The split winding of
the secondary allowed me to use the existing feedback ladder and modulator
bias adjustment pots.

When I got done, the transmitter sounded much better than original.  The
Marantz amp was mounted in a rack in another room.  Never had any feedback
issues, which was lucky.

That transmitter also had a very large 220v Variac that fed the main power
supply and RF tube bias circuits.  It could be dialed from over 1,000w down
to under 25w and still sound very good.  It was at WRIB and taken out of
service in the late 80's.  Anyone with a QSL from before roughly 1989 heard
that transmitter.

I might have a couple of used but still good 833A's around somewhere.  Great
old bottles, they were.

Craig Healy
Providence, RI

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