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Tiny mod transformer, suitable for arc-5

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Author Topic: Tiny mod transformer, suitable for arc-5  (Read 1174 times)
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Posts: 49

« on: July 13, 2020, 04:49:01 AM »

Hey guys. Bought this little mod transformer for cheap from ebay, thinking I might be able to use it to build a universal modulator/power supply to use with my arc-5 transmitters or my german ww2 FUG10 transmitter and perhaps other PW transmitters.
Googling the marks on the trafo, I find it to belong to an AN/URT7c, which had 807's in the modulator and two 4CX150 as finals.
Attached a screenshot from the URT7 manual showing the modulator section. Voltages seems to be about what I'm planning on using, around 5-600 volts (although some claim the fug10 wants 800v).
And around 200v regulated screen voltage
Triode connected 807's feels simpler to build, uses less different voltages, is it s good idea?
Appreciate inputs Smiley
//Tommy, SA2CLC

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more inpoot often yields more outpoot

« Reply #1 on: July 13, 2020, 08:17:26 AM »

a lot of possibilities ... which 807 triode conx are you using .... the screen tied to plate is only good to 400V plate V and only with certain tube vendors .... screen tied to control grid requires about 5W of drive power but no grid bias or screen dc supply .... what is shown looks good and a driver transformer may be hard to find but can be contrived .... good luck with your project  Smiley


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to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
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CW is just a narrower version of AM

« Reply #2 on: July 13, 2020, 09:12:07 AM »

I like it. That is a fine circuit as is - 6SN7 and a pair of 807s in straight AB conventional with the feedback. Plenty good for an ARC-5. The low turns ratio is good too. If you do not get quite enough swing, and you have a power supply that supports it, run the modulator on higher voltage than the ARC-5.

These are the good old days of AM

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Posts: 205

« Reply #3 on: July 14, 2020, 08:43:10 AM »

Hej Tommy!

Nice to hear about your projects!
Wish I had a FuG10 S10K transmitter, if your happen to know where to find one, let me know!

Both the S10K as well as ARC-5 / SCR274-N series transmitters responds well to screen grid modulation.
My own experience how ever is limited to to the command transmitters.
Have you seen what I wrote in the military section?

Later experiments results in 40 W worth of carrier, or sometimes more, at somewhat above 85% modulation.
Transmitter adjustments are critical to get really good modulation.
And, yes, the finals gets hot.


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Posts: 49

« Reply #4 on: July 16, 2020, 01:14:41 AM »

Thanks for inputs guys.

Morgan, yes I was going to screen modulate them, but when I found this mod transformer I changed my mind.

Thought I had an interstage transformer hidden away but not.
I have seen someone driving the grids with a tube set up as phase splitter followed by cathode followers, might be worth checking into. Or use a backwards connected small audio transformer an use a small SS amp to drive it...
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