Using a toroidal transformer as a mod transformer Part II

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WA1GFZ:
Stu,
I think you may flash it over. Consider a homebrew primary over the stock windings fc

WA1HZK:
Just checking around.
What if you had a nice custom KW+ donut.
And you tied one of these amps to it:
http://www.bswusa.com/proditem.asp?item=FR2500
That could be a real cheap modulator when you consider the tubes you do not need to feed. Very efficient also. This X (2) Russian toobs? = SNOT!!
Keith

AB2EZ:
Keith

Sorry for the confusion.

The audio amp you pointed me to is very nice!

The Russian triodes are the RF tubes in my RF amplifier (currently configured as a linear amplfier, and destined to be converted to plate modulated, Class C operation)... they are not being used to build an audio amplfier.

Stu

WA1HZK:
I know. I'm thinking of easy ways to plate modulate a large final!
:)

WD5JKO:

  Stu,

    Years ago I did a similar solid state amp to a tube final. I used a Crown M600 to a Viking I 4D32 RF amplifier. I played with an Ultra Modulation circuit, and even went a step further. I had over 600 watts of audio to play with, so I used some to increase the 4D32 B+ (raise the carrier power) so that the carrier would pump up to 200 watts out prior to modulation. This was simple to do with a variac, a step up transformer (1:4) a FW bridge and a 100 uf capacitor. The DC made from audio was put in series with the unmodulated B+ for the 4D32. The B+ went from 750 to 1250 with a whistle into the mic. The 4D32 is so stout it modulates linearly upward to 800 watts PEP output this way with an unmodulated carrier at 100 watts. The rig was VERY effective and sounded good. Overmod in the negative direction was prevented with a 4-2-1 progressive negative cycle attenuation (not loading) using 3 bias supplies, diodes, and a bunch of power resistors. The system was transparent and did not kick in until 80% negative modulation of a 200 watt carrier.
     I will caution you on one thing. The waveform of your voice will likely be asymmetric and therefore have a DC offset to it. If you use a BIG direct coupled solid state amp into a MOD transformer directly, the DC component in your voice will result in a huge DC current into the mod transformer primary. This may cause the amplifier protection circuits to kick in. To get around this problem with my M600 amp, I added a capacitor in series with the amplifier output. In my case I used two 10,000 MFD 100V capacitors in series (+ to +) to essentially make an AC capacitor. I tried with and without diodes across each cap (cathode to + side) and the diodes made no difference, so I left the diodes off.

Regards and good luck,
Jim
WD5JKO

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