Like other similar radios, I gather, the 6U5 ``tuning eye'' never closes
completely on BC and hardly twitches on the SW bands. What's bothering me is
this: (1) It seems this is a feature of the design; (2) I found a repair that
revives the tuning eye to some extent and substantially improves the
performance of the radio on SW bands; (3) the designer went to lengths to
*not* do this; and (4) he knew better what he was doing than I do.

Here is how I believe it is intended to work: Current flows from
the diode section of the detector (6Q7G) through the secondary of the output IF
transformer. The IF carrier passes to ground through a small capacitor; the
AF signal couples to the audio preamplifier section through a larger
capacitor. The DC current completes to ground through a 250K ohm resistor and
to the AVC line through a 1meg resistor. The AVC line goes to the grids of IF
amplifier (6U7G), translator (6A8G), and RF amplifier (6U7G), as well as the
tuning eye.

On the BC band the cathodes of the IF amp and translator and a screen grid in
the IF amp are together biased by a 600 ohm resistor. (That's what the
schematic shows; the radio had an OEM 220 ohm instead, which I changed to 600
ohm.) On BC, everything works as I expect: the detector section determines the
AVC voltage, tracking the strength of the station received. The tuning eye
closes, if not completely, in proportion to the strength of stations.

On the SW bands the RF amplifier is bypassed, and the cathodes of the IF amp
and translator, as well as the IF screen grid, are connected directly to
ground, intentionally bypassing the biasing resistor. Consequently the grids
on these tubes accumulate a negative charge on the AVC line. It leaks to
ground through the 1meg resistor into the detector section, and continues to
ground through the 250K resistor there.

The voltage accumulating on the AVC line in SW operation is three to five
times as high as the AVC voltage in BC operation. It substantially suppresses
the gain in the translator and IF amp, and freezes the tuning eye, since the
detector section voltage almost never equals it.

If I bias the cathodes of the IF amp and translator (as well as the IF screen
grid) with a 220 ohm resistor on the SW bands, the radio performs
substantially better. Stations come in much stronger, the AVC evens out the
volume, to an extent, and the tuning eye shows some life.
But if this is such a good idea, why did Mr Silvertone go out of his way to
ground those cathodes?