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

Where did you get the 30-50 henry inductor figure?

www.telstar-electronics.com

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

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
John Ferrell W8CCW

Yes... I think he should seriously consider low-level modulation
instead of high-level modulation.

www.telstar-electronics.com

"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

Tony

What you need to do is to find a universal modulation transformer and
replace the audio output transformer used on the audio amplifier. That is
the most practical solution to your quest.

Pete

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

"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

Tony,

I did exactly what you are describing. I used a solid state audio amp,
and drove a tube type audio output transformer backwards to step up the
impedance from 8 ohms to several thousand ohms. I used a 4 uf oil capacitor
to couple that across a HV choke that carried the RF final amp plate
current. For a choke I used a 125 watt Stancor poly-pedance modulation
transformer where I used as much winding on the primary as available. It
worked very well.

It might be easier to consider cathode modulation, and in this case the
transformer turns ratio will be much less, and the HV problem will also be
less. Maybe a 110/220 vac to 12vac 5 amp filament transformer will work out
here. This is just a guess, but it might work. Cathode modulation is a
combination of plate and grid 1 modulation, so the % eff is somewhere
between 66% (plate) down to 33% (grid) , and might end up at about 50%.
Consider the 220 vct winding where the grid return goes to the CT, and the
cathode goes to the top of the 220 v winding, and the bottom of the winding
goes to ground. Your turns ratio need may vary, but the concept proposed is
workable.

Jim
WD5JKO


"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?


Thanks,


Tony,

WA6LZH