The AM Forum

I have a modulation transformer which I believe is out of a Wilcox 99a transmitter.

It has a primary impedance of 14.75K and a secondary impedance of 6.6K.

It's an autotransformer, so you would need to run the modulator and final off the same B+ supply.

I believe it was used with a pair of 813s (probably triode connected) modulating a single 250tl.

The output of the transmitter was around 400 watts so presumably this transformer must be capable of 250-300 watts or so of audio.

Has anyone built a rig with this type of mod transformer or have any experience with it?

How does it work? Space shuttle audio, or better, or worse?

I  have what may be a larger version.  It came out of a Wilcox modulator that used a pair of 450TLs in class AB2.  There are two secondary taps, IIRC one for 5K and the other for 7.5K.  Primary is about 15K. Based on the amount of inductance I have measured, it would probably sound pretty good, although I have never used it as a mod xfmr.  I used it as a mod reactor years ago before I ever came across a real broadcast reactor.

You can run separate power supplies by using a reactor and blocking capacitor with it, and putting the blocking cap between the high side of the autoformer and the mod reactor.

Is it a true autotransformer? (just 3 terminals) or does it just have the primary centertap and one leg of the secondary tied together internally? (4 terminals)

I have one of the 4 terminal versions and IIRC it is rated at 350W. It has some weight to it. I dont remember what the impedances or T/R were when I tested. I picked it up at a hamfest some years back.

IIRC it was used in a Tx with a pair of 811As as modders, but I dont remember what the finals were, but I think it was an aeronautical ground station transmitter.

It is a true autotransformer and works just fine. The audio is good. I ran one of these rigs on 80M for several years. The modulator on the Wilcox had decent audio.

Skip

<Is it a true autotransformer? (just 3 terminals) or does it just have the primary centertap and one leg of the secondary tied together internally? (4 terminals)>

Actually, it has 5 terminals.

The taps are one for B+, two for the plates of the mod tubes, and two taps for the RF tube. One at 6.6K ohms and another at 5.5K.

When I used  mine as a mod reactor, I just connected to the two terminals that gave the most inductance for the current I was drawing through it, as determined by trial and error.  That didn't occur with the entire winding.  Using the full winding saturated the core, and the inductance actually went down.  As I recall I used the +HV tap and the 7500 ohm tap.

I measured the relative inductance by placing a 10H filter choke in series with the reactor, loading the transmitter to full power, injecting a 300~ or so tone from a signal generator to about 30% modulation, and comparing the readings when an a.c. voltmeter was (very carefully!) placed across each choke.

The 5K and 7.5K secondary taps are extensions of one side of the winding, with additional turns added beyond the modulator tube plate tap, relative to the +HV tap which would be the "midtap".  A good autoformer is probably superior to separate primary and secondary windings, due to tighter coupling between the virtual "primary" and virtual "secondary".

If you look at the Wilcox schematic they show the transformer as a continuous winding with taps. It is not shown like some of the b'cast transmitter schematics with a dashed line to connect the pri and sec. (denoting an internal jumper between windings). It really doesn't matter either way--just what I saw when running one on the air. The Wilcox was a neat transmitter with 4 RF decks, room for 2 modulators and a power supply with enough snot to run 2 AM and one RTTY channel all at once. Mine came from the WI hwy patrol. The original papers show it purchased new in the early 60's, well after the design was obsolete. Typical red tape--the request for the transmitter was probably put in around 1951 and it took 10 years to get it. By that time it was useless.
Skip

Skip, if you have a schematic for the Wilcox 99, could you post it or email me a copy of the modulator schmatic?

I have a hand drawn schematic of the mod transformer showing how to hook it up, but a full schematic of the whole transmitter (or even just the mod deck) would be useful.

Dave

hello pablo LU1MEK IN SEARCH OF INFORMATION REGARDING ONE THAT I HAVE AUTOTRANSFORMERS EQUIPMENT INSTALLED IN A VALVE WITH 2 AM 813 in RF PLATE WITH 2KV and the module is 813 VALVE WITH 2 IN TRIODE CONFIGURATION, WILL ALSO INSTALL SPARK BETWEEN PLATES The two valves use the same source, BUT THE ONLY PROBLEM IS THAT I HAVE TO OVERCOME THE CONSUMPTION IN THE MODULATOR 300 MA TEND TO SPARKLING AND I HAVE TO LOSE THE LEVEL OF AUDIO TO PREVENT THAT HAS BEEN MY EXPERIENCE WITH THAT AUTOTRANSFORMERS WILCOX.

Thanks. That Wilcox transformer was originally used with 813's modulating a 250tl. It should work just fine with 813s x 813s.

Not sure what caused your sparking problem.

Running at 2KV should be OK as I believe the transformer is rated at 7500v.

Dave

hi dave if it works pretty well, the audio quality is very good I use it with a microphone samson C01, BUT AS I SAID IN THE PREVIOUS POST I HAVE PROBLEMS AS SPARKS DEBI PLACE A SPARK TO PREVENT CLICK THE WOUND,IT  BEEN MY EXPERIENCE WITH THAT Autotrans, using the circuit TESLA 300 OF TOM K1JJ to variability in the excitement of RF IP TO USE A TANK AND SCREEN THE FOOD THROUGH A 22KOHMS RESISTANCE TO 200W AND POSITION IN AN RF POWER Of 400mA. GREETINGS DAVE
 PAUL LU1MEK

Anyone have the modulator from a wilcox 99 still laying around in your stash? I have a good home for one.

John