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Author Topic: Inca N-30A Modulation transformer  (Read 5907 times)
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KI4YAN
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« on: January 20, 2016, 05:44:02 PM »

So I've got one of these and it's time to put it to use. (finally)

It's a multi-match unit, rated at 40W. The case is sized physically similar to the UTC S-21 but it's not quite as heavy. I can get a good match primary-secondary for the 6M transmitter it now belongs in, but I am wondering if there is any preventative maintenance that you guys would suggest.

It appears to have been kept clean and dry, and it's never been used, as evidenced by no solder having been applied to the terminals.

I have absolutely no idea what the voltage and current ratings are, as the only information I have been able to find is that it's a 1940's model multi-match unit rated at 40W, with a push-pull primary and single-ended secondary.

Total turns ratio (using all the windings and taps) is 1.35:1. Weighs 8lbs.

Old black-wrinkle, with bakelite terminal plate.
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kb3ouk
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« Reply #1 on: January 20, 2016, 07:51:20 PM »

Seems like a lot of Inca transformers have surfaced lately (I have one in the homebrew modulator I finished rebuilding, an N-21). As far as care, etc., I would at the very least put a negative peak limiter in the transmitter just to make sure there is a constant load on it at all times when in operation, you can also insulate it from the chassis, add spark gaps on the windings, use a modulation choke and cap to keep the current off of the secondary, but at the very least I would make arrangements for it to have a constant load on it.
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KI4YAN
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« Reply #2 on: June 27, 2016, 02:56:50 AM »

So I'm back around to this. I'm waiting on some panel nuts that are an odd size for the 6 meter reciever, and an order of carbon comp resistors.

In the mean time I'm looking at the companion transmitter for this receiver. First place to look is the modulation transformer and the final amplifier-what do I have that fits together?

I've got this Inca "40 watt" unit, and a UTC S-19 "30 watt" unit. Grossly different size/weight...so I uncased the Inca to take some actual measurements. Popped the terminal plate end of the case off, and wouldn't you know it, it's fully potted in tar. A little past experience taught me that warming the whole deal on a hotplate will bring the tar up to a temperature where it will melt, from the outside in. Once it's liquid just enough to slide the whole block of tar, iron, and copper out of the can, I let it cool to just handle-able and peeled the tar off. Comes off pretty easy at that just-right temperature, and doesn't make a mess.

Anyway, the iron core measured out at 3.25"x2.75"x1.75", making it approximately 2.1 square inches of core area. The UTC S-19 transformer is considerably smaller...less than half the outer dimensions and that's including the case the S-19 is potted in.

I am thinking the old Inca can handle a bit more power than it is letting on. Just for giggles, I drove it with a pair of 6BQ6GTB sweep tubes configured for a 4000 ohm load, into a large 2000 ohm dummy load made up of big ceramic wirewounds I pulled from old Sharp microwaves. The sweeps are driven by the screens and grids, in full class AB-2, as zero bias triodes. In this mode, most brands of this tube will handle serious abuse without breaking a sweat...I knew I could crank up the juice and see what the transformer can handle.

At 40 watts of sine, distortion measured up at just over 7% from 55hz to 10khz and frequency response is pretty good over that same range. So I pushed harder-I wanted to see core saturation-and at ~62 watts I could just see the core saturating on the oscilloscope at 55hz. So, backed it off till I couldn't detect it in the scope anymore, and let it cook for 20 minutes. The transformer case got moderately warm to the touch, but I could hold my hand against the case for about a minute without undue discomfort.

The core area points to it being able to carry 140VA if it were wound as a 60hz power transformer, but the core saturation points to about half that. Granted, at 62 watts I was pushing a LOT of peak current, over 180mA through the primary.

Is this transformer just seriously conservatively rated, or have I got something a little better than what I thought? It's sized about half-way between the S-19 30-watter and the "SNC" brand 120-watter that I've got, although the UTC S-series...I've read here they're not so great in the audio department.
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w4bfs
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« Reply #3 on: June 27, 2016, 10:07:13 AM »

It all depends on what is important to you .... with transformers you have low, mid, and hi freq models and a savy designer will check the proposed implementation against them ... when push comes to shove and you want more power then low distortion hi and low freq response has to go bye bye
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KI4YAN
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« Reply #4 on: June 27, 2016, 02:53:50 PM »

This is true, and core saturation effects will mean the low frequencies will leave the building first. The high frequencies will hang on pretty well as power increases, as they tend to be limited by interwinding capacitance and coupling more than core saturation. That's why when I was testing last night/this morning, I did the measurements at 55hz. I assume that 55hz should be a low enough low for me, I'd like to have good audio but I don't need to be hi-fi on the air, I think.
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Steve - K4HX
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« Reply #5 on: June 28, 2016, 07:19:18 PM »

Did you have DC current flowing through the secondary during your tests?
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KI4YAN
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« Reply #6 on: June 28, 2016, 09:23:30 PM »

I did, 100mA DC in the secondary. The final amp in the transmitter I plan to build is a Russian GU-50 pentode, at 750V and 100mA for 80W input. I have two tubes, so I am trying to figure out if I can run them in push-pull for 120W input and have longer final amplifier life and still modulate fully. The plate supply voltage shouldn't exceed 800V for 50mc with this tube, so but the datasheet is pretty darn vague, and gives "recommended operating points" well below the "Typical Operation points" listed in the same sheet...such as screen voltage is specified as not to exceed 250 volts, but the "typical operation point" calls screen voltage out at 300 volts.

My plate supply will also comfortably do 750V at 350mA and 300V at 150mA, and stays cool to the touch doing it...not what I expected from an old "CB" amp power transformer!
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Steve - K4HX
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« Reply #7 on: June 29, 2016, 10:24:11 PM »

Very nice! Good luck with it.
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