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Author Topic: Modulation transformer with odd winding configuration  (Read 2712 times)
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KI4YAN
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« on: July 03, 2020, 07:59:46 PM »

I was looking through a few transformers in a box at a yard sale today and bought a modulation transformer. It's marked (scratched up) -14, Modulation Transformer, 4200 ohms-CT-4200 ohms primary (is this an 8400 ohm primary, center tapped?) and 8000-9000-10000 ohms secondary 1, and 1000 ohms CT secondary 2. Yes, two secondaries, one a tapped winding for 8K, 9K, or 10K impedance, and a center-tapped 1000 ohm secondary.

Rated at 65W on secondary 1, and 20W on secondary 2. Only rated for use on one secondary at a given time.

There's another on Ebay, too.

What could that second winding be used for? Cathode feedback for the modulator? Screen grid?
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Patrick J. / KD5OEI
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« Reply #1 on: July 04, 2020, 01:37:51 AM »

Winding 2, for driving a larger audio stage over a balanced line?
Alternate it could be used for feedback but not an output at the same time, that is, not at 20W, but as voltage FB, 1-2 Watts maybe?
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Radio Candelstein - Flagship Station of the NRK Radio Network.
KI4YAN
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« Reply #2 on: July 04, 2020, 03:25:01 AM »

I'm wondering if it could be used to modulate the screen on a pentode/tetrode final.

I always have aspirations of finishing a little oddball transmitter, and have had the idea recently I wanted to do a pair of 2E26's in the modulator-they would wanna see around 8K plate to plate, at 450v or so. Would put out around 25W of audio, too, in class AB1. Seems to match up nicely so far.

Now, what pentode/tetrode final will put up 50W, and present an 8 to 10K plate impedance? I can't think of one off the top of my head. Most are lower impedance.
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Patrick J. / KD5OEI
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« Reply #3 on: July 05, 2020, 11:55:14 PM »

This is my opinion, I could be wrong.

Do you have a picture? It seems strange - the impedance ratios. For a few reasons, the circuit would need to run 600V to be an 8000 Ohm load at 50W DC input and 707V to be a 10K load.

For 35W, it could possibly work around 450V, but still there are some possible issues with the modulator side, like the 2E26 current-carrying ability.

If the RF stage is running 400VDC @ 50mA carrier cond., these are all data sheet values, that's 20W input, and a 13.5W carrier.

The only audio data I see for the 2E26 is for AB2 operation. @500V, the load is 8000 Ohms plate to plate for 54W output. If it's 4200 Ohms plate to plate, too much current might be demanded from the 2E26's in the modulator at 54W, but at 25W (or just enough to modulate the final), it might be OK. A bunch of calculations may need done because it is a wierdly mismatched application.

So, first could you please check the turns ratio? That would be one way to be sure of the impedance ratio.

If you can feed 120VAC to the whole 4200-CT-4200 primary winding, then measure the voltage on the secondary 8K tap, everything can be made sure of from that ratio. May as well check the voltage across the whole 1000 Ohm winding too.

120V should be safe for a 4200 Ohm 50 Watt winding, it would see 460V at 50 Watts. A variac can bring it up.

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KI4YAN
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« Reply #4 on: July 06, 2020, 03:45:53 AM »

Here's a photo of the top. It's pretty scratched up.



From the photo, I'm fairly certain the primary is 8400 ohms plate to plate, and is +/-0.5db over the range 100hz-15000hz.

The secondary is 10000 ohms tapped at 8000 and 9000 ohms. The secondary is rated for 65W power, 140mA current, and 2100 Volts peak voltage.

The 1000 ohm center-tapped secondary is rated for 300 volts.

I just won a second one on Ebay for a reasonable price so hopefully between the two I can make out all the markings.
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kg7bz
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« Reply #5 on: July 06, 2020, 08:03:05 PM »

I thought that transformer looked familiar! It's out of an AN/SRT-14 transmitter. Used two 5933 tubes to modulate a 4-400 with 1050 volts AM, 1300 volts CW,FAX for 100 watts RF out. The 8K tap is used. If the Booster unit (3000 volt PS and high power modulator) was added it became the AN/SRT-15, the 1000 ohm winding drove the grids of a pair of 4-125 tubes to modulate the same 4-400 for 500 watts out.

What caught my attention was the amazing frequency response of the transformer, mine was the same. I had one of these monsters 12 years ago, fully operational. Wish I'd never got rid of it. 1300 pounds, 100 tubes, synthesized in 10Hz steps from 300KHz to 26 MHz. Only 2 MHz to 26 MHz at the 500 watt level. AM, CW and FAX. Drew about 1300 watts making 100 watts out.

I still have the 4 inch thick manual.

See the 14 and 15 near the top of this page. The 16 is a 14 and a 15 together.

http://www.navy-radio.com/xmtr-ship.htm

August KG7BZ
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KI4YAN
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« Reply #6 on: July 07, 2020, 02:25:21 AM »

Well now that's an oddball. 807's modulating a 4-400!

Now, that *IS* an idea! Instead of a 4-400, a single 4-65 would need about 1300 volts and 130mA to make 130W of carrier-almost perfect for this transformer, and pretty well within the realm of 807's, 2E26's, 6L6's, to modulate.

I can see this coming together. I have a few capacitors that would easily handle 2600V for the plate network. Somewhere, I ought to have a transformer that can manage the high voltage, and I definitely have a transformer capable of powering 2 2E26's in class AB2 for a 54W modulator, and I also have one that will feed 750V for the 807's as well-it was a voltage doubling transformer, outputting 350V and being voltage-doubled to feed 3 8950's in an old Palomar 11 meter amp. (those 8950's draw 400ma-EACH!) I think if it was able to handle voltage doubler service at those kinds of power, it can feed a modulator.

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w4bfs
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« Reply #7 on: July 07, 2020, 08:13:03 AM »

Well now that's an oddball. 807's modulating a 4-400!

Now, that *IS* an idea! Instead of a 4-400, a single 4-65 would need about 1300 volts and 130mA to make 130W of carrier-almost perfect for this transformer, and pretty well within the realm of 807's, 2E26's, 6L6's, to modulate.

I can see this coming together. I have a few capacitors that would easily handle 2600V for the plate network. Somewhere, I ought to have a transformer that can manage the high voltage, and I definitely have a transformer capable of powering 2 2E26's in class AB2 for a 54W modulator, and I also have one that will feed 750V for the 807's as well-it was a voltage doubling transformer, outputting 350V and being voltage-doubled to feed 3 8950's in an old Palomar 11 meter amp. (those 8950's draw 400ma-EACH!) I think if it was able to handle voltage doubler service at those kinds of power, it can feed a modulator.



looks like fun .... just one thing .... if you go with 4-65 be sure you have a decent supply of known good tubes .... the tube base seals seem to not be up to long term stability and a lot of 4-65 I have tested have loss of vacuum
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Patrick J. / KD5OEI
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« Reply #8 on: July 07, 2020, 01:08:20 PM »

This explains the primary and secondary impedances. Grossly different plate supplies for primary and secondary tubes is unusual.

Perfect!



* srt-14_low-mod.png (60.96 KB, 622x428 - viewed 216 times.)
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KI4YAN
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« Reply #9 on: July 07, 2020, 03:17:17 PM »

Now the kicker-I wanna use that second winding on the secondary to modulate the screen. I could use half the winding, or the whole winding ignoring the center tap. Or use it as an audio feedback winding...or all three.

I guess I need to lay all the bits out on the table and see how big a chassis I need.
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