[Johnson] Johnson Viking II modulator

[C E]

Hi Bob:
In short, are you saying that with the Vik II in its
modded condition, you can't get it to modulate at 100%
with full power out?

Let me(us) know. Thanks. Cal. 
--- Robert Jefferis <jefferis at antelecom.net> wrote:

> 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
> 
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