The AM Forum

I am about to start laying out the design for my modulator, (2 x 811's modulating a 4-125). My question is, the modulator chassis will have the mod transformer, but will also have the extra chokes to act as mod reactors.  My question is: should I mount the chokes on the chassis with the power supply, or the modulator? All the equipment will be housed in the same 6 foot rack, with a good ground buss. I would like to put the chokes on the power supply down at the bottom, to keep the modulator chassis from being crowded and heavy.  I am trying to build it like a Collins 30K design.
Tell me what you think.
Thanks
Bob

How about a shelf above the Power supply for the audio iron so you can try other transformers as you find better parts. Mod deck just heater and bias supply

Hi Bob,

In the stuff I've built, I've never experienced problems when mixing AC and audio iron on the same chassis. I've had chokes and mod transformers remoted and even once had my mod transformer mounted 30' away from the transmitter due to talk back. No instability problems or high frequency attenuation that I could see.  This is probably cuz we are working with high level signals at that point. Low level would be a different story.

Bottom line, unless someone has other reasons, I would go ahead and mount your chokes at the bottom of the rack and let gravity work for you... :-)

Tom, K1JJ

I have some tx's with all the audio iron on the same chassis as the mod tubes and some tx's with the iron bolted to the sides of the rack cabinet, and some with the iron on the bottom.....Most of the time I prefer to keep the audio iron grouped up close to the mod tubes as it takes less hi voltage wire that way..Since you are using a 6 ft rack, you have a lot of room....you can put it anywhere you like...Picture shows PP 250th's mod by 813's with 20v mod tx and gates bc1 choke bolted to cabinet sides.

Since I like to experiment, I put a rack mount shelf in the modulator rack, put a piece of plastic on it, and put the mod iron and 50 henery choke on it.
I had a cvm5 at first, and sold that when I found the big RCA mod trans sounded so much better.
I also sold the reactor, no improvement over the RCA with current through it that I could hear.

I tend to not mount big iron at all, the power transformers go in the bottom of the cabinets, mod iron on the floor also, or on a shelf.

I did put a fair radio (collins) power transformer on a chassis for the 3x4d32 rig as its solder sealed and chassis mount.
Nice little iron, gives 1200 volts at 600 ma choke input center tap.

Brett

With the 813 rig I've been wrestling with I have the mod transformer on a rack mounted chassis. The reactors are at the bottom of the rack, only a couple of feet away.  As far as I can tell from my preliminary testing the displaced components do not present an issue with performance, RF wise. The few RF issues I had were mainly in the low level arena, the onboard audio amp. As Tom suggested, the high levels in this area should be OK fine.

I have had several qso's with Steve using this rig and it is excellent well rounded audio. Nice craftsmanship also.

Thats a real fine 250TH amp.

Rather than go to 24" wide racks I mounted my HV xfmr (6000/7500V CT @ 1.5A CCS 1946 vintage) and mod xfmr (7500W 285# BC iron in oil with 304TL's; dont laugh it was cheap and doesnt saturate ;)) in wooden boxes on casters and run 20KV wire inside armored flexible water supply tubing to the rack where the 25A variac and PS filter hold down the floor. A 45' run of #8 and a pair of 5kVA 240/120V xfmrs take care of the AC.

Carl
KM1H

Many larger transmitters, including broadcast rigs,  have all the iron, both power and audio, mounted on the bottom shelf, with wire leads to the tubes where ever they may be located.

Just try to avoid  locating a power or filament transformer or power supply filter choke next to a low level audio input or interstage transformer.

What's in the other rack!! ;D ;D

813's x 813's   in the other rack  "TESLA 360"