[R-390] R392 help

[Meir WF2U]

I seem to remember that on some manufacturing runs (or maybe even at maintenance depots) the audio amplifier circuit was modified to accommodate the solid-state replacement for the 12A7. If the receiver looks stock around the audio section, it may have been one of these units, except someone replaced the solid-state plug-in module with the tube. 
Receivers with original 12A7 circuits sound awful when the solid-state module is plugged into the 12A7 socket without the circuit modifications. 
I think I may have some written information on this, but not sure - I may have just read it somewhere, without saving a copy.

73, Meir WF2U
Landrum, SC

On November 30, 2016 8:17:40 PM EST, Dave and Sharon Maples <dsmaples at comcast.net> wrote:
>All: I know this is off-topic, but I have an R-392 that appears to be
>unmodified that I am restoring.  The IF stages are not clipping even on
>100%
>modulation, and the audio is recovered as well as can be from the diode
>load
>point (just a bit of distortion, but nothing huge) but the audio chain
>is
>really ugly.  The first AF amp (which uses contact bias) appears to be
>underbiased and drawing too much plate current, but all the resistances
>are
>just fine (and the contact bias cap is only 12 PF, which seems really
>strange, but that's what is in the schematic).  Finally, the final
>amplifier
>(26A7) is boiling hot; from all I can find that is the way it is
>expected to
>work (more contact bias)!
>
>I am highly inclined to do something with the audio.  I have thought
>about a
>couple of things:
>
>a. Standard cathode bias on the first AF stage.
>
>b. Standard cathode bias on the final amplifier to bring it back into
>its
>specified current draw and reduce the heat.
>
>c. Chuck the whole mess and put in a good solid-state amplifier.
>
>(c) makes it "non-stock" but it will sound a lot better.  I am not sure
>I
>want to do that; I'd really rather leave it as it is if possible.  
>
>It occurs to me that I could do (c) and simply pull the tubes, and tell
>the
>new owner that if he wants it stock he can remove the amp and plug in
>the
>tubes.
>
>What do you folks think?
>
>Thanks,
>Dave Maples, WB4FUR 
>
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