I use a modified 75A-4. All mods are reversible.
Outboard mechanical filters plus the internal slots give the following
choices of mechanical filters: 300~, 3.1 kHz, 4, 6, 8, 9.7, 16.
Use outboard 10w hi-fi audio amplifier driving 16" coaxial speaker in G-E
broadcast studio monitor box. The audio is taken off the grid pin of the
1st audio tube, with a plug-in adaptor which contains a cathode follower to
an unbalanced line to the outboard amp. Audio coupling caps from the diode
detector to the 1st audio tube socket were changed from .01 mfd to 0.1 mfd.
Pulled out the unused 6AQ5 to cut down internal heat and to avoid wearing
out a good tube for nothing. Using a 7-pin plug, pull DC power to run the
cathode follower from the 6AQ5 socket.
The mechanical filters in the 75A series and in the R-390A (and 51J series)
put a rough edge on the audio. My R-390 with L-C i.f. selectivity is almost
as sharp as the mechanical filters in the 390A, but the 390 sounds a lot
better on AM.
Other receivers in the shack include SLRM, R-1000, HRO Sr.
Don k4kyv
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