Fisher 400 Output Transformers

[wrenchone]

I was in Halted this afternoon (famous surplus store in SF Bay area) looking for some odds and ends, and noticed a cardboard box in one of their display cases labeled "Fisher 400 Transformers", containing 3 likely-looking hunks of black iron. Needless to say, they were out of the case and on the way home about as fast as you can imagine. The price was $75, which is not flea-market prices, but less than the fleabay going rate, especially with no shipping. How does the iron for the 400 compare to the 500 series.sound-wise? The transformers are a tad bit rusty, but nothing a mild wirebrushing and some Rust-Oleum gloss black wouldn't cure - it's mostly the end bells that need touching up.

[wrenchone]

I took a 400 output transformer and a 500B output transformer in to work for some measurements on our nice LCR meter, and found out some interesting things

400C transformer:

Primary (BLU to BLU/WHT) - 14.5H

Sec1 (BLK-BRN) 14.8 mH

Sec2 (BLK-GRN) - 35.9 mH

Sec3 (BLK-YEL) - 64.2 mH

I'm assuming the BLK-GRN connection is for 8 ohms, so this gives me a primary impedance of (8 X (14.5/0.0359)) = 3231 ohms

500C transformer

Primary (BLU-BLU/WHT) - 27.2 H

Sec1 (BLK-BRN) - 15.5 mH

Sec2 (BLK-GRN) - 33.9 mH

sec3 (BLK-YEL) - 63.4 mH

Again, assuming that the BLK-GRN connection is for 8 ohms, the primary impedance works out to (8 X (27.2/0.0339)) = 6419 ohms

Quite a difference. I'm dragging the other two transformers of each pair to work tomorrow to see if they measure in similar fashion.

[Tom Bavis]

I measure transformers at 60 Hz - 100V from variac on the pimary, measure secondary voltage, and impedance ratio is voltage ratio squared. For low end, inductance at 60 Hz, 100V is probably relevant too - measure primary current and Z=E/I. I'll post the measurements on a 500B transformer when I get around to it...

[Francois G]

John Atkind's One electron website has his 1990's test data of output transformers. He does not have data for thr 400 though.

See page one of 4 here: http://www.one-electron.com/trans_p1.pdf

[wg_ski]

Originally posted by Tom Bavis
I measure transformers at 60 Hz - 100V from variac on the pimary, measure secondary voltage, and impedance ratio is voltage ratio squared.

I measure the other way (drive the secondary with 12VAC), but it's the same idea. I measured 10k plate to plate on a salvaged Fisher 400 tranny. I play with tubes occsionally for fun (nothing serious), so I don't know how it compares with others out there. No UL taps, so it's either triode (about 20 watts PP) or pentode.

[Bandersnatch]

hey-Hey!!!,
I prefer to drive the primary with AC. A variac does an excellent job. V_primary/V_secondary squared times the output tap loading gives the primary load.
cheers,
Douglas

[wrenchone]

Interesting - measuring the inductance gives credible-looking results applied to primary and secondary windings (each primary is 1/4 the inductance of the winding end-to-end, etc.), but not when trying to determine the turns ratio.

We are blessed at work with sine wave AC sources with variable frequency output. I cranked the output frrequency up to 300 Hz on my bench unit to make sure I was really, truly in the passband of the transformer (though the sine wave source is beefy enough to bull its way through that) and looked at an Edcor XSE-15-8-5k. It measured out as a 25:1 turns ratio, pretty much right on the money. This was my verification with a known transformer.

I looked at one each of my 500B and 400 output transformers. For 40V excitation primary-primary, I got the following secondary readings on both transformers: BLK-BRN - 1V, BLK-GRN - 1.5V, and for BLK-YEL, 2V. If you take the BLK-GRN pair as the 8 ohm output, you get an primary-primary input impedance of ((40/1.5)^2) X 8 = 711 X 8 = 5689 ohms, or about a 6k primary impedance.