Unknown OT?

[Phil_S]

An eBait auction got the better of me. Here is my $19 acquisition (including s/h.) It weighs 2 lb 1 oz. The seller claims it is from a Wurlitzer organ running a pair of 6L6. That's all the info I've got. EIA 549626 tells us it is Midwest Coil & Transformer, 1966 or 1956?

I reasoned, perhaps erroneously, that 4 wires on the secondary must mean there is at least one useful combination and hopefully two. Testing for turns ratio, here's what I found using a filament transformer input to the secondary:
R/Y + Y 24.22
R/B + G 37.41
R/B + Y 37.46
R-Y + G 66.1
Y + G 73.71
R/B + R/Y 73.95
I was careful to measure the input voltage and output voltage on each combination, as the load caused variations in the input voltage.

Secondary Ohms range from about .4 to 1.0. I think these readings are too low to be useful here. The primary is 180 Ohms between the outer legs, split about 95/85.

It looks to me like I've got some 4 and 8 ohm secondary possibilities.

I'm having some difficulty deciding which wire was meant to be the "0" wire on the secondary and the sequence of outputs on the secondary coil.

Yellow or Green with Red-Black has virtually the same turns ratio, 37.4, and Yellow+Green has ~74 turns. 37.4+37.4= ~74, suggesting that the layout is Yellow-Red/Black-Green.

Yellow-Green or Red/Black-Red/Yellow both have about 74:1 turns. This throws me. Where does the Red/Yellow wire fit into this OT?

I can't seem to figure where the Red/Yellow fits.

I'd be very appreciative of any thoughts you'd be willing to share, even if it is just a guess.

Thanks.

[katopan]

I can't make head or tail of your measured numbers either. You said the secondary resistance measurements were too low to be meaningful, but they will still confirm that all secondary wires are all taps off the same winding or not, and tell you which order at least that they are connected which will give you the phasing of each tap. But I agree your Yellow-Red/Black-Green layout is the only combination of numbers that make sense.

[Phil_S]

I think, if there are 4 wires, there are six possible pairs. There is continuity (ohms and voltage) on all six pairs! This suggests there is a linear solution to the problem.

Ordinarily, I'd look to the plain wires for regular direct to speaker ohms in the 4-16 range, and the stripped to be 250z, 500z, 25V or 50V. That's not making sense.

Is there some other test I can do? I've got a scope and several meters, two of which are old Flukes, not calibrated but believed to be reasonably accurate. I do not have an inductance bridge or meter. I really want this OT to be good for something. If not, I guess it goes back on eBait for the next guy to figure out.

Taking this in a bit of another direction, does anyone care to speculate what the primary might have been for an old organ amplifier with a pair of 6L6?

[katopan]

What are the ohms readings out of interest? Also just as a reference what's you meter read as the test lead resistance (just touching the two together)?

[Phil_S]

R/Y - Y 0.8
R/B - G 0.7
R/B - Y 0.4
R/Y - G .04
Y - G 1.0
R/B - R/Y 0.5
Fluke 8025B, leads together 0.2

The last two pairs have the same turn ratio. Input on the secondary is 6.2VAC, output at the primary is 457 and 460VAC. The filament supply is 7.1VAC unloaded, so I'm actually quite surprised there is so much of a drop. The OT remains cool, though it is a brief test, and there is no noise or any obvious signs of a problem.

The 2nd and 3rd pairs also appear the same. VAC in is 7.0 and 6.9. VAC out is 260VAC on both.

[Espjr215]

I dont believe this but I actually have a chassis with an output transformer that looks the same. Its connected to a 5 position switch. Yellow is to chassis ground and dark green is for one speaker. Position 2 is green and red/black. 3 is yellow/red. 4 is red/black. 5 is green only.

[Phil_S]

Wow, this is amazing!!!

LOL, actually, I don't generally rely on hope as a means of problem solving, but this is one case where I admit to using it. It is bad, because you have reinforced the use of hope as a problem solving method and now I'll want to use it again.

I am very grateful to you for posting the switch layout as it should prove to be very helpful in understanding this transformer. Thank you very much!

The single most important bit of information is that the Yellow wire is the "0" and that alone is most helpful to know and suggests new tests to be run.

In looking at the switch config, this is really quite curious. If pos #1 shorts G+R/B, I think that puts to two 37:1 turns windings in parallel and I am thinking that is probably about 4.5K:3.2; this is the same as Pos #3 with R/B only(?). Pos #2, R/Y only, is probably 4.5K:8. Pos#4 gives turns ratios that don't make sense to me, so I'm assuming it is a line out at 250 or 500 ohms. I'm going to do a bit more testing now that I have new information. I'll also look at a Hammond I've got that has 250 and 500 ohm secondaries to see what sort of turns ratios I find there, as it is also meant for 6L6's.

Is there any other useful info to be had from your chassis? For example do you know if it uses 6L6's? Does it have a PT and if so, what is the probably plate voltage? The latter might suggest a load line and could be very useful.

[Espjr215]

I'm headed off to work now but I'll get you some info tonight on the PT. The model number is 7020 and it does use 6L6's.

[Phil_S]

Thanks. I'll look forward to seeing what you can find.

Meanwhile, I work at home some of the time and played hookey at lunch time

Here's what I found in a bit of additional testing (results rounded). I think my original read on the Green-Yellow was just wrong.
Yellow-Green is probably 4600:8
Yellow-Yellow/Red is probably 5000:4
The other combinations will not work with 4-8-16 ohms speakers.

Roughly averaging the two useful combinations, I am going to assume a primary of about 4800. Let's see if there is additional information that might modify this assumption. Then maybe I can draw a load line and see what sort of plate voltage is right for a pair of 6L6's.

Of course, I could also use it with a pair of 6V6's, probably doubling the primary and secondary assumptions. Given the lower output of the 6V6's, I'd expect the OT to be rather forgiving.

I'm starting the think this one's a keeper!

[Espjr215]

Ok, for the power supply I put 11v in and got 61.2v out. Hopefully that makes sense and I haven't moved back to the 6L6 sockets so let me know if you want pics of those. I was basically going to strip it down with hopes of building a Rockster.

Thanks!

[Phil_S]

That would equate to 110:612. This was unloaded, no tubes in sockets? Assuming it is unloaded, I'm going to guess the PT is about 300-0-300 and that is close enough. It is not hard to figure things from there. Thanks for the helpful information.

I'm beginning to think this PT is going to be a great match for a pair of 6V6 or 6L6.

Fun with tubes. That's what we're here for!

Phil

[Jered]

Try this:
http://www.diyguitarist.com/GuitarAmps/ ... edLoad.htm