[Hammarlund] SP-600: Strong-signal distortion fix

[David Wise]

In the archive I saw a few complaints of distortion
at high signal.  After bringing up my first and only
SP-600 (recap, check resistors, test/sub tubes, align),
I noticed this problem too.  With AGC working normally,
many local stations were distorted unless I backed off
the RF gain.

By injecting signal at each stage to get the same
final output, I traced this problem to V11, the
Driver, and cured it completely with a simple change.

My SP-600 matches Issue 2 of the manual.  They
reduced V11 bias (from -10V to -7.5V) in Issue 3.
I tried this, and it helped slightly, so I kept going.

As I went from -7.5 to -5, it cleared up.  At -2
it was distorted again.  I guess the large grid
signal on this stage had been driving V11 near
cutoff on the negative swings.  This would clip
the negative peaks of the audio coming out of the
detector.  (This also fits with observations that
the S-meter kicks downward on strong signals.)

Changing the bias changes the gain, of course, so I
attenuated it (with a shunt resistor on the grid)
to get a fair comparison.  It was still clean when
I had the gain distribution back to normal.

I decided to try an unbypassed cathode resistor,
like V7 (Gate) in Issue 3.  With G1 returned to
ground instead of -10, I got lucky: 470 ohms
gave me the desired gain and bias (about 4V)
in a single stroke.  Audio is clean, even the
mile-away blowtorch that takes my meter to 100
and used to be unintelligible.
I tried several different 6BA6's; all were fine.

My V11 socket had a ground wire running from pin 2
(G3) through the center shield and pin 7 (K), to
one of the socket's ground terminals.  I cut pin 7
loose, wired the rest to the socket's other ground
terminal, and ran my new resistor from pin 7 to the
original ground.  I moved R56 from the hot end of
C127 to the cold end.

That's for Issues 1 and 2.  For Issue 3 and up, use
390 instead of 470 because V11's gain is initially
higher, and since R113 returns G1 to ground, simply
disconnect R56.

I can't figure this out.  Why did Hammarlund modify
V7, which wasn't the problem, and not V11, which was?
Contrariwise, why don't you others have trouble?

Regards,
Dave Wise
SWL in Portland Oregon