wrote in message
...
On Fri, 19 May 2006 15:12:53 -0400, "Pete Bertini"
wrote:


"John Ferrell" wrote in message
. ..
I assume you are talking AM modulation and that you really want plate
modulation.

Otherwise you are discovering why screen modulation was so popular
with 6146 finals!

de W8CCW John


I suspect he is talking Heising modulation, which normally is used
with a single-ended Class A modulator whose plate is directly tied
to the PA plate (the choke is used to provide a high impedance to
the filter chokes. But, what I wonder, a 50 watt tube audio amp is
very substantial, and is probably running Class B P-P. I don't see
how Heising modulation would work in that application.

Pete

Thats one form. The other is to use a huge plate inductor and couple
with a cap (usually 8 to 20uf) to the plate side of the inductor.
The modulator can be either single ended or PP and the plate side of
the Mod amp couples through the cap to the mod inductor.

Reason for doing this. At 6146 power level you seeing 600-800V
DC power for plates and modulation will take this to over 1200.
Those numbers are manageable. For bigger tubes (say a pair of
3-500s) the plate voltage can be 2000V or higher and with modulation
peaks hit 4000. By using an inductor the PP transformer is isolated
from the really high volts. Choke with required insulation are/were
common in big transmitters for broadcast and HF. However mod
transformers that can stand 1000V on one side (say PP plates) and
2-5000V on the other side can be hard to come by. Gets the big DC off
stuff even though there are some impressive AC voltages with
modulation.

The trick for 6146 size amps is a big choke on the final plates DC and
a good 8uF 1000V cap and you can then use a tube amp of suitable
power if you couple the cap to the plate side of the output
transformer. You can even use a solid state amp with a 8ohm to few
thousand ohm transformer that doesnt have to stand 600-900V dc.
Cheap 8ohm to plate(say 2000ohm) transformer? Try a 6V to 220
control transformer backward or maybe a transformer out of an old
(tube before transformerless) TV.


Allison

Agreed, but the choke must be able to stand the high DC standing
current without going into saturation. Also, the choke method
somewhat limits the maximum modulation unless a few
additional steps are taken. I'd be leary about some of the
suggestions regarding the use of pwr. transformer windings
to accomplish the task.

Pete