old buzzard modulation transformer replacement or for new rig

[w4bfs]

I've been thinking about this for a while .... Stu did yeoman work about using toroidal power transformers as modulation transformers and several questions were raised about construction methods concerning winding breakdown high voltages (ref tech discussions 2010 thru 2012 )

I have a conservative method in mind that should yield a high level modulator starting with a high power solid state audio amp driving a two step transformation process.

   1. use a dual 115V to 40V, 400 to 800 VA stepdown transformer hooked up backwards and the now secondary 115v windings in series.  The 40V primary windings are in series. This gives a 1:2.875 turns ratio and a 1:8.27 impedance ratio.  Antek model  AN8440 suitable, $84.00 ea.,  many others usable

   2. Use 3 or 4 or even more dual 115V to 350V, 200VA power transformers hooked up normally, that is series the 115V primaries in each transformer and place the resultant group of 230V primaries in parallel driven by the transformer in statement 1. above .... connect the 350V secondaries in series which should not risk arcing between the bifilar wound secondaries if decent arc supression techniques are used in the transmitter ... then series the groups of secondaries. Each transformer will have a 1:3.04 turns ratio and a resultant 1:12.17 for a group of 4 ....With 4 transformers in series this will develop up to 2770V peak swing so use good insulating mounting techniques ...more swing is available as this 2770V figure is with 56V rms drive.... the filament windings can be used but only in the secondary circuit due to insulation concerns but will only make 1% differences so are not of much use.  Antek AS-2T350 suitable $38.50 ea.

 ...this isn't rocket science but allows the best use of the primary to secondary 3500V insulating material specifiied by the vendor.  $238 + shipping ain't cheap but is competetive.  oh yes, I have no ownership of Antek but like their stuff    

[AB2EZ]

Beefus

I ran my legal limit plate modulated rig (a pair of Russian triodes in parallel to obtain sufficient plate current headroom) at 1700 DC on the plates. Therefore, my Heising modulator, with Antek step up transformers and a Peter Dahl Heising reactor, was producing around 2000V on positive voice peaks. It worked fine (no problems) for several years. I sold the transmitter and the modulator when I moved last year.

I understand your approach. The HV would have to punch through two transformers: the secondary of one HV transformer to the primary of that HV transformer... and then through the primary of a 2nd HV transformer to the secondary of that 2nd HV transformer [or, depending upon the Heising configuration you are using, i.e. whether one side of the audio step up transformer is grounded, from the secondary of one of the LV input transformers to the primary of that LV input transformer... and from there to ground via the 240V AC house wiring in combination with the transmitter's own RF/lightning/safety grounds]

Still, I think that there will always be "surprises" with voltages above 2000V... so be very careful! As you know, the high electric fields can lead to long term failures as insulating materials break down over time.

Stu

[AB2EZ]

Note the suggestion from Jim, WD5JKO (see the T-368 thread started by W9BHI, below), that one might want to take steps to keep DC out of the primary of the audio step up transformer. Putting a 1 ohm resistor (of suitable wattage) in series with the primary of the audio step up transformer seems like a good solution... if the audio amplifier already has circuitry in it to keep the DC on its output reasonably close to 0V. With voice modulation, the average audio power producing heating if this 1 ohm resistor will be much less than the peak audio power producing heating of this resistor.

Stu

[w4bfs]

Still, I think that there will always be "surprises" with voltages above 2000V... so be very careful! As you know, the high electric fields can lead to long term failures as insulating materials break down over time.

Stu

thanks for responding, Stu .... I think your combination of theoretical and practical knowledge is approaching (asymptocicaly ?) unique

perhaps you are thinking of the externally dangerous common mode insulation failure ...I was considering as well the harder to define internal differential mode failure (i.e. a bifilar winding insulated by coat per wire formvar or whatever is used nowadays) which is why I was suggesting multiple transformers in series .... my concern may be unfounded IF the xfmr vendors do single winding at a time .... I have asked John Ango with no response .... I removed a secondary winding from an Antek toroid to investigate methods ... all windings were bifilar and the 3500 V insulation appeared to be about 5 layers of heavy mylar tape

[w4bfs]

Note the suggestion from Jim, WD5JKO (see the T-368 thread started by W9BHI, below), that one might want to take steps to keep DC out of the primary of the audio step up transformer. Putting a 1 ohm resistor (of suitable wattage) in series with the primary of the audio step up transformer seems like a good solution... if the audio amplifier already has circuitry in it to keep the DC on its output reasonably close to 0V. With voice modulation, the average audio power producing heating if this 1 ohm resistor will be much less than the peak audio power producing heating of this resistor.

Stu

YES .... a good idea

[AB2EZ]

Beefus

Thanks for the compliment!

I think I now understand the failure mechanism (the "rainy day scenario") you are concerned with...

I.e. in a multilayer winding, arcing from a turn in one layer to a turn the layer above or below it. Spreading the total voltage across two separate transformer secondary windings will reduce the layer-to-layer voltage by a factor of 2 for any pairs of turns across which an arc could form.

Thanks again

Stu