[Johnson] Johnson Viking II modulator

[Robert Jefferis]

Hello to Johnson list followers:

 

Back in March I posted a request for information on the external relay
control connector. It spawned a thread on the subject of  "fried ham".
Safety tips are always prudent and, trust me, my Viking 2 has a properly
installed 3-wire line cord and, I found a good solution to the connector
problem thanks to Mr. Zook. 

 

After many component replacements, repair of bad chassis ground connections,
and general cleaning,  the VII is up and running at normal power levels. I
am at square 1. The previous owner made a half hearted attempt to follow the
"level 1" mods from the ER article by Mr. Bono. The T3 inter-stage
transformer is a Hammond 124E wired in a 3:1 step-down configuration. This
transformer has adequate inductance for good fidelity, but winding
resistances are very high (3.8k Ohm on the primary as wired). Grid drive to
the 807s is ample and undistorted up to the point of getting into A2 drive
mode. Between 150 Hz and 2kHz I could initially get about 50% downward
modulation accompanied by only about 25% upward modulation before serious
distortion set in at 1kHz. I removed C55 on the primary of the mod
transformer (T4) and modulation symmetry improved dramatically, but still
not perfectly symmetrical. I know that V2 (6AU6) in this configuration,
i.e., wired in pentode mode (pseudo-triode mode does not have enough V2
stage gain for this T3 setup) presents a source impedance far too high to
expect A2 operation. 

 

I have carefully redone the meter shunts for PA and modulator current. When
I back the 40M PA loading off to 180 ma I can get about 75% downward
modulation undistorted out to about 3.5 kHz. Upward modulation almost tracks
but not quite. This is with ~300VDC on the 807 screens (I can get better
modulation at higher screen voltages, but.)

 

Now to the questions: I have seen a couple of one-line references to
substitution of 6AK6s for V2. It has lower transconductance than the 6AU6 by
about a factor of 2, but a far lower plate resistance (about 200k Ohm vice
about 1M Ohm). Is this worth trying? Does my description sound like sound
like something else is broken? Or, am I simply flogging a dead horse?

 

All comments welcome. Thank you.

 

Bob, KF6BC