"Tony Angerame" wrote in message
...
I've acquired a high power 50 watts plus tube type hifi output
transformer. I'd like to try modulating a pair of 6146's with a hifi
amp. The circuits I see use a modulating inductor in series with the B+
to the final to keep single ended dc off the transformer secondary.

Here's the rub. Where do you find a 30-50 henry inductor for 800+ volts?
I was thinking of checking out the secondary of a tv transformer? (Not
many left used to be common whatever happened to those tv's).

Anyone have any ideas on winding one? Seems to me taking an old
transformer apart and scramble winding as much wire as one can afford
might do the trick?

Ideas?


A blast from the past: Oftentimes the voltage rating of chokes was more
a limitation of breakdown from conductor/windings to the core or case rather
than interwinding breakdown. So........... a quick method of cheating on the
rating was to mount the choke on standoff insulators or at least insulating
the choke from any metal in the chassis or frame of the RF or modulator
deck. Only thing then is to be sure to treat that choke core or case as if
it might be at high voltage potential......yikes!
Although Choke modulation (or Heising as one respondent mentioned) is
"high level" modulation if applied to the RF final plate/s, it takes a lot
of tinkering to achieve 100% modulation (balanced, above and below zero).
Almost always a compromise in Amateur gear, and some broadcast gear came
close, but at great expense.
Many will throw rocks at me for this, but "low level" modulation is a
huge waste of RF power, and in the good (?) old days, was just another
weakly modulated hetrodyne the howling AM section of the bands. OH, OUCH,
OWEE, HEY!
Old Chief Lynn W7LTQ

Enjoyed reading your post Lynn...

www.telstar-electronics.com

Low level modulation is a huge waste of RF power?? Hey, _AM_ is a huge
waste of RF power, and of spectrum to boot! Except with unusual
modulating waveforms, 2/3 of the power is in a useless carrier. Now,
compared with plate modulation of the PA, low level AM modulation
followed by a linear amp may be somewhat less efficient (though a waste
of DC power, not RF power), but remember, the modulator, presumably
being a linear audio amplifier, isn't all that efficient either. In
addition, there ARE ways to generate AM with low level modulation and
efficient RF amplification--they have been used in AM broadcast
transmitters.

For the cost of a good high-level modulator, how many kilowatt-hours
can you buy?

Cheers,
Tom

K7ITM wrote:

Low level modulation is a huge waste of RF power?? Hey, _AM_ is a huge
waste of RF power, and of spectrum to boot! Except with unusual
modulating waveforms, 2/3 of the power is in a useless carrier. Now,
compared with plate modulation of the PA, low level AM modulation
followed by a linear amp may be somewhat less efficient (though a waste
of DC power, not RF power), but remember, the modulator, presumably
being a linear audio amplifier, isn't all that efficient either. In
addition, there ARE ways to generate AM with low level modulation and
efficient RF amplification--they have been used in AM broadcast
transmitters.

For the cost of a good high-level modulator, how many kilowatt-hours
can you buy?

Cheers,
Tom

The only modulation method that I've ever found to be of any real efficiency
in generating AM is "outphasing" or "ampliphase". You can use the /most/
effective PA designs - optimised FET-based Class E stages can be over 90%
efficient! This approach completely obviates all the problems with
modulation transformers or big chokes. Haven't tried it with valves
("tubes" - US) - perhaps that should be a future project.

Bob

--
Everything gets easier with practice, except getting up in the morning!