[R-390] Modded my R-390A...

[Philip Atchley]

Hello all.
Today I did a couple things that I've been wanting to do to the R-390A.
First I put solid state rectifiers in it (3 "fast" 400 piv series diodes in
each leg as I had em) for two reasons.  Less heat and I suspect more
reliability (though I had two "pulled" spares that I've had for a couple
years).   Yes, I put a 220 Ohm 25 Watt (I had the 25W) resistor in the power
supply to "compensate" for the diodes smaller Voltage drop.

Secondly, I wanted to change the "Local" audio amplifier to a circuit that
would provide more drive to my speaker (which is not overly efficient).
Sometime back Roy Morgan had sent me an email converting the local audio to
a 6AQ5 which was wired as a Triode.  This same "mod" changed several other
items on the audio deck for better biasing of the 12AU7 tubes, removal of
feedback etc.  ALL being aimed towards lower distortion and wider bandwidth.
This was GOOD as I had a small audio output transformer scrounged from a set
that used a 6AQ5 as the output section.  This transformer had BOTH an 8 Ohm
and a 500 Ohm tap on the secondary.  I chose to use the 8 Ohm and ignor the
500 Ohm winding (I have the "line audio" output if I need 600 Ohms.  For
this reason, rather than wire the 6AQ5 as a Triode I opted to use it as a
Pentode so it'd match the transformer well.  And rather than use the 15 Volt
Zener for Cathode bias of the 6AQ5 (as the mod sheet showed) I chose to use
a 390 Ohm 2 Watt wirewound resistor.  I DON't like Zeners in cathode biased
circuits as they 'usually' fail in the SHORTED mode and this would zero bias
the output tube, drawing excessive current and possibly (likely) doing other
damage!!!

I removed the original "Local Audio" output transformer and the new
transformer fit and mounted well in the original space, though only with one
mounting leg screwed down.  A terminal strip was mounted below chassis to
use as tie lugs for the leads that connected to the original transformer and
the new transformer.

THIS MOD IS 100% REVERSABLE AS I CEKPT THE TRANSFORMER ETC AND NO HOLES WERE
DRILLED!

The results??  Fidelity is much improved and the audio drive is more than
sufficient for the speaker I'm using.  YES I know this is a communications
receiver but that doesn't mean it has to have lousy audio 8^)

73 de Phil  KO6BB

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