[ARC5] SCR-183 Transmitter: Essential Knowledge and Tips for Revival, Part One

[David Stinson]

On 11/16/2021 9:25 AM, J Mcvey wrote:
 > Dave,
 >
 > After I looked at the SCR-183 TM, I had even more questions!
 > Such as the 10 and 45 tubes have 2.5V filaments in series,
 > that's good for a 5V input.
 > The TM says the radios take 14 to 24V input.
 > Where's the filament ballast? Is it in the control box?
 >

Good Morning, John.
This is a 1930s Military tube nomenclature "snafu"
which has caused the premature and tragic death of
many type 45 tubes.  The standard type 45 has a
2.5 Volt filament. The VT-52 Modulators are also
nomenclatured "45 SPECIAL"; an unfortunate decision.
The VT-52/45-Special has a 7.5 V filament, matching
the current draw of that in the VT-25/10 and
VT-25A/"10 Special."  Many 45s became BB-gun targets
and fishing floaters, the BC-230s blamed for the
wreck and left to fall to ruin in some crumbling barn.

 > That being one question, the other about inter stage cathode feedback 
concerns was addressed with your post.
 >
 > The next thing is the bias "seen" by the new tubes
 > is very different than the originals. I have no idea
 > how they designed circuits with the old direct heated
 > VT's,...

The bias development is a major subject I mean to cover in detail.
When one changes away from the original tubes (which would cost you
many hundreds to re-tube a 10-W transmitter), bias changes are needed.
One of the reasons people give-up (too easily) on using the 230
on phone is, while you can get RF out, the modulator is stone dead.
There's good reason for this:  with the "stock" bias circuit, the
modulator tubes are seeing many times cut-off bias.  With that much
negative juju on the modulator grids, you couldn wake those tubes up
with a fire hose.

Also to your next point:  The single-end connection of the cathode
to the "proper" end of the filaments: I intend to cover these in
in detail, but most of the info I would need is in the missing
note book.  I need to put a couple of the transmitters on my
bench and develope the data again, so I don't end-up "talking
through my hat."  I do that enough without actively asking for it.
Fortunately, I'm familiar with procedures developed after lots
of "hit and miss," so it shouldn't take as long.
Work, family and illness throw "monkey wretches," but I'll
get it done.

73 Dave AB5S



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