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[Amps] Mystery Triad HV transformer

Subject: [Amps] Mystery Triad HV transformer
From: John Lyles <>
Date: Wed, 16 Nov 2022 02:28:54 -0700
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I have a 1955 and a 1971-72 Triad catalog. Also a 1952 Radio's Master buying guide from Leo Meyberg Co of Los Angeles. All list the same Triad transformers for plate power. Their commercial line of 1955 listed the P-1A, P-3A, P-5A, etc up to the P-20A. There is also a mil spec hermetic variety, the HS series, but I am assuming yours isn't that. Your transformer either is a special 750717 made custom for some OEM, or that is a date code like 1975, July 17. Look carefully for a P #.

Like K5UJ suggested, you can drive it with a variac at low voltage and measure the secondary voltage to estimate the actual HV RMS voltage it makes at 120/240 line. Thats another question, whether it had dual primaries or just one. As for the VA rating that will get you roughly the current, some hints come from the commercial line. The P-13A measures 5 3/8 H x 4 1/2 W x 4 3/4 D and weighs 12 lbs. It is rated for 1780 VCT RMS and 750 VDC from FW rectifier. Current is 310 mA ICAS. The P-15A is slightly bigger on the depth dimension, weighing 15 lbs. It is capable of 2340 VCT or 1 kV DC at same 310 mA. The P-17A is against deeper, measuring 6 1/2 inches, all other dimensions the same. It weighed 21 lbs. and would supply 2880 VCT, 1.25 kV DC at 310 mA. The largest in 1955 was the P-20A, measuring 6 1/2 H x 5 1/2 W x 7 D, 35 lbs. It was capable of 3300 VCT, for 1.5 kV DC at 425 mA.

My 1972 catalog extends the plate power transformer range to the P-18AL, 2880 VCT RMS with 600 mA ICAS current. It weighed 32 lbs. The biggest was the PR-21AL, with 3300 VCT at 600 mA ICAS rating. It measured 6 1/2 H x 5 1/2 W x 6 1/4 D and weighed 29 lbs.

You may be able to "bracket" the current rating of yours by weighing it and comparing dimensions to these.




Date: Mon, 14 Nov 2022 16:39:10 -0500
From: Steve Bookout <>
Subject: [Amps] transformer identification

Hello all,
I found a transformer which I had in storage. Quite sure it?s a plate
transformer, but that?s all I know. It?s marked ?TRIAD 75071?. It?s
about 6 in. cubed.
Does anyone know what this transformer might be?? Or, where to find a
catalog on-line.

73 de Steve, NR4M

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