[AMRadio] Johnson Valiant Audio Driver Xfmr - Wake Up Call

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Tue Oct 23 08:42:21 EDT 2007

In my current 32v3, I used KT90 modulator tubes in AB1,
these are high voltage KT88's.
I used a phase splitter to drive the grids, a 6SN7 I think.

Removing the driver transformer has benefits, no phase shift,
no loss of frequency response, less space needed.
You can also unbalance the phase splitter a bit and get more positive 

I never had luck getting 6146's to sound good in AB1, even when regulating
the screen voltage.
The KT88/KT90 tubes are hi fi tubes, low distortion and designed
for AB1 service.

You can often use a wide range of transformers to drive the modulator
grids, all you need in many cases is he voltage swing, if you get into
AB2 or class B, you want as low as an impedance as you can get and still
allowing the voltage swing you need.
I would say most 'ham' gear was not designed or built to give best
results out of the driver transformer, some were designed to clip,
run out of steam below 100% modulation, were way to small for the job,
were low fi, etc.
I would not hesitate to eliminate or replace one with a quality unit.

As far as voltage goes, a lot of people seem to worry about high voltage.
What do you think is going to be a problem?
The filter caps? Replace them, they likely need it anyway, put in
higher voltage caps with more capacitance.
The other parts don't care much about voltage, I have run the 32v3 at
950 volts on everything for years and years with no problems at all.

Tubes don't care, as long as you don't exceed their plate dissipation
ratings, my 3X4D32 rig runs 1200 volts on the plates of the 4D32
tubes designed to run at 600 volts.


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