Working with unknown OPT

[92Volts]

I got a "vintage tube transformer" off Ebay.

It has cloth wire insulation, closed endbells, lay-down style (one endbell sits in the chassis). Weighs maybe 3lb.

There are 3 primary taps, the middle is 50% so I assume PP use (not SE ultralinear or SE multiple taps)

There are FIVE secondary taps... Black, brown, green, yellow, orange. I did AC voltage testing to find impedance ratios, and assumed black was common/ground.

If I assign each tap a common ohm value to reflect a similar primary impedance, I get:
Blk: Common
Brn: 1 ohm to 3.9k
Grn: 4 ohm to 4.6k
Yel: 8 ohm to 5.3k
Org: 16 ohm to 4.3k

All "close enough" loads to arrive at 4.5k primary impedance, considering I'm measuring with a cheap DMM.

But a 1 ohm tap is unusual. Maybe it's for feedback or something?

Let's look at it "backwards"...
Org: Common
Yel: 2 ohm to 4k
Grn: 4 ohm to 3.9k
Brn: 8 ohm to 3.9k
Blk: 16 ohm to 4.3k

Huh, those are VERY close to 4k primary impedance! Even better, a 2 ohm tap is something I was expecting.

But black usually is common/ground. And if yellow/orange are supposed to be the end of the winding for 8 to 16 ohm use, maybe they can't handle all that power (current) between them in 2 ohm use.

Not that it matters. I breadboarded it with 6V6s and it sounded great playing audio, I plan to make it a music amp with EL84s. I'll either "mismatch" the speaker to boost impedance or accept reduced power/increase plate dissipation. I won't be playing it full-blast anyways. Based on my assumption that the primary is 4.5k and speakers only go up to 16 ohms, this OPT shouldn't mind any of this, since it's probably designed for EL34s, 6L6s, etc, which provide double or more power output.

I'm just curious if anybody knows how this was expected to be used. Or if you approach unknown transformers differently from how I've done it.

Thanks in advance!

[tubeswell]

Assuming you have determined which winding ends are which, the next test is to determine if there are any shorted windings, and following that do a Vac test to determine the winding ratios.

[92Volts]

I did the AC tests, and I think I know the ratios. My question's less technical, I wonder what the intended use or origin might have been since it seems to have a "1 ohm" speaker tap, which I haven't seen.

I guess I haven't figured out which winding ends are which. I know which are taps on the same winding, but I'm not sure which end acts as the common connection for all middle windings, or what the reasoning/problems might be of using it one way or another

[DummyLoad]

I guess I haven't figured out which winding ends are which. I know which are taps on the same winding, but I'm not sure which end acts as the common connection for all middle windings, or what the reasoning/problems might be of using it one way or another

why does it matter? as long as the reflected Z based on a given load is what you expect, then wire it as measured.

and yes, it could have been a FB winding, although they were generally a much higher Z, e.g., 500-600ohm.


--pete

[92Volts]

why does it matter? as long as the reflected Z based on a given load is what you expect, then wire it as measured.

and yes, it could have been a FB winding, although they were generally a much higher Z, e.g., 500-600ohm.


--pete

That's pretty much what I'll do.

To increase reflected impedance while driving 2 "stereo" speakers (as mono, in parallel), I'll likely connect a 4 ohm load to the "2 ohm" section. I was concerned this much power wasn't designed to come out of a winding section that's between the 8 and 16 ohm "end" taps in normal use... but in reality this seems spec'd for tubes capable of higher power output, and I won't exactly be pushing this build's limits either.

[tubeswell]

I did the AC tests, and I think I know the ratios. My question's less technical, I wonder what the intended use or origin might have been since it seems to have a "1 ohm" speaker tap, which I haven't seen.

I guess I haven't figured out which winding ends are which. I know which are taps on the same winding, but I'm not sure which end acts as the common connection for all middle windings, or what the reasoning/problems might be of using it one way or another

For winding taps on a shared winding, the tap that gives equal VAC readings to each of the other taps will be the CT, if its a primary winding.

However, if its a secondary winding with two taps and a common, then you'll have to just pick any old wire to be your starting point and supply the source VAC from that to each of the different winding ends in turn, and measure each of the other windings as you go, noting the reading between each pair of ends. The best way to do this is with multiple VAC meters with insulated test clips attached simultaneously between each winding end
You will know which windings are which by reverse inference when you have ratios that approximate what a known (say 8k plate-to-plate) OT would produce. e.g. 22.3:1 would give 500:1, 31.6:1 would give 1000:1, 44.7:1 would give 2000:1 etc.