On Thu, 18 May 2006 19:56:21 -0700, Tony Angerame
wrote:

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?


Thanks,


Tony,

WA6LZH

It looks like AM is really becoming a "lost art". Of all the responses
here, only one or two guy knows what you are talking about.

The purpose of the choke inductor like you are describing is to keep
the DC off the secondary of the modulation transformer. This allows
the use of a modulation transformer with much less iron, which will
give better low frequency response. If the finals DC current is run
through the modulation transformer, as is commonly done, there needs
to be an air gap in the transformer to keep the core from saturating
from the DC current. The bigger the air gap the less inductance, the
greater amount of DC current it can handle before saturation and the
poorer low frequency response. To increase the inductance with a
larger air gap requires more iron (larger core) to bring the
inductance back up.

By using a choke and capacitivly coupling the audio from the
modulation transformer to the final plate there is no DC on the
secondary of the modulation transformer to cause saturation. The
modulation transformer then can be any audio transformer. It doesn't
need an air gap. The transformer can have much less iron in it for the
same amount of inductance as one with an air gap. Very good low
frequency response can be had this way.

By the way the screens of the final tubes need to be modulated along
with the plates too. The easy way is to supply the screens through a
dropping resistor from the top of the choke, the same point that feeds
the plates.

THIS IS NOT HEISING MODULATION. It is regular old plate modulation
just like you would do with a conventional modulation transformer.

Yes power transformers can be used quite successfully as a modulation
transformer with this type of setup even though there is no air gap in
the transformer. There is no worry about core saturation because there
is no DC current on the transformer.

A large enough choke for this application can be had by putting
several chokes in series to obtain the required inductance. Regular
power supply chokes work fine. Do not use a swinging choke! It has no
air gap and will saturate.

A TV transformer will not work well as a choke because it has no air
gap and the core will saturate quickly with DC going through it.

Cathode modulation is very similar to grid modulation in performance.
After all audio is placed between the grid and cathode just the same
as it is with conventional grid modulation. The amount of cathode to
plate modulation is minimal.

Screen modulation is similar in performance to grid modulation also.
Efficiency is around 35% carrier efficiency. Very tricky to tune up
properly.

Low level modulation with a linear amplifier behind the modulated
driver also has an efficiency of around 35%. Excellent AM can be
generated this way as is done with some SSB transmitters in the AM
mode. If properly set up one can not tell the difference between it
and a high level plate modulated transmitter except that the plate
modulated transmitter will probably have higher distortion.

73
Gary K4FMX