Re: "Old Ham Radio Equipment" on e-Pay

[k4kyv]

From the photo, it  looks like a pair of 810's in the modulator.  The modulation transformer looks like one of those RCA 5500 ohm to 5500 ohm 1:1 turns ratio modulation tranformers designed for MCW, not voice operation.  They were used in morse code transmitters on Liberty Ships during the War.

They are definitely not broadcast quality iron, but are at least as good as the typical ham radio quality mod transformers of the "AM era".  The primary plate to plate load is probably way too low for the 810's, but nevertheless a lot Post-WW2 AM modulators used that combination.

Link

[Opcom]

looks almost like the iron from a RCA BTA-250L.

[k4kyv]

Same physical construction, but the core stack is thinner, it has a gapped core, and spark gaps on both primary and secondary sides.

[Opcom]

Ah yes. I'd have to agree now after handling one again. I'd bid on that thing for the parts, but so heavy and co$tly to ship to TX.

I see a filament tranny with unused terminals as well as a goodly sized driver transformer. I'd be willing to give the mod iron a try, maybe with a pair of 572's at 900VDC to pump it and the 810's in parallel as the RF PA.

That should fill it up power-wise but as you said the MCW frequency response would be in effect.

I bet it would sing nice at 100W audio level with a pair of 6550's on the primary and a quartet of 807's or some other tube that would run on 600V on the sec.

Another thing that comes to mind is a driver for a big modulator. If the secondary has no CT but a pair of resistors were put across it and the center grounded, that might do well.

I need to build something new because it has been a long time since I built something from junkbox-scratch, but I am a 'fraidy cat to build with what I have saved up, which is a quad of 3-1000Z's, sockets, blowers, etc, a gates BC-5 mod iron by Dahl, three identical 3.7KV/700mA power transformers, and four 2uF/7KV oil caps. I have no power supply iron yet or any pi-net RF stuff in that class, but I could make the coilage. Is this a crazy-size thing to think about doing?

so many parts, so little time.

[N3DRB The Derb]

900 vdc is a waste of perfectly good 572B's. Try 2 KV.

[Opcom]

Wonder what the original toobs were?

No arguments about the 2KV, except that iron might not be made for it? I was just throwing out 572's because they could do the high current / low voltage to use a 5K CT transformer. The cheaper 811A's could be used instead maybe at 750-900V as well and do 175 watts audio. I was trying to find a way to load the transformer less and get a better low frequency response. It's silly anyway, the shipping is going to be too much on that item for me.

[k4kyv]

Another thing that comes to mind is a driver for a big modulator. If the secondary has no CT but a pair of resistors were put across it and the center grounded, that might do well.

Only if the modulator was running class A or AB1.  That midtapped resistor setup would not deliver grid current to a class B stage without generating severe distortion.

[k7yoo]

here's a crazy idea--I have 2 of those RCA MCW transformers--why not hook the primaries in series (instant CT) and parallel the secondaries. At least the ratio is more usable.
I have a small RCA shipboard transmitter that used a smaller version of this transformer and what they did was feed 400 cycle AC from a motor generator into the primary to modulate a pair of 813's with about 1 KV on the plates. Pehaps this is how the bigger iron we see on Epay was originally utilized. I would like to figure out a use for these transformers, other than as a doorstop.
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[k4kyv]

I think a better bet would be to use pushpull-parallel modulators, something like 805's or 811A's.  Maybe even a half dozen 211's or 838's in p-p parallel, 3 tubes to a leg.  You should be able to get some real decent positive peaks with one of those transformers if you had a low enough impedance driver source.

I think they were designed to be fed by modulator tubes, since the primary is midtapped @ 5500 ohms.  I just wonder what tubes were used in those MCW modulators.  I have heard that the finals were 813's or 4-250's, but abundant as those transformers are from WW2, I  have never yet laid eyes on a copy of the transmitter they went into.

The fidelity wouldn't be anything to write home about, but I have heard some pretty decent sounding audio from homebrew rigs that used those transformers.