OT wire gauge mystery

[maxkracht]

I was given a few massive organ amps. Way too big for guitar stuff as is but decent looking iron. I believe these were out of a Baldwin? One primary wire is 20awg, the center tap and other primary is around 24awg. Same on all 3 transformers. Curious why they would do this. Strange way to indicate polarity? I didn't take pictures beforehand and only noted what tubes were going to which transformers. Everything tests fine and they are useful values, but the different size wires are weirding me out. Any ideas?

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[Phil_S]

I've dabbled in organ transformers. I never gave this a second thought. My gut reaction is, these are the wire sizes of the tails they solder onto the windings. I doubt it means very much. If you are able to peak under the paper at the actual windings, I think you'll find fine gauge wire. Looks like you've got a pair of the 6BQ5 trannies -- good for a hifi stereo!

[TUBEDUDE]

I've dabbled in organ transformers. I never gave this a second thought. My gut reaction is, these are the wire sizes of the tails they solder onto the windings. I doubt it means very much. If you are able to peak under the paper at the actual windings, I think you'll find fine gauge wire. Looks like you've got a pair of the 6BQ5 trannies -- good for a hifi stereo!

Or a Gibby 79RVT clone.

[Phil_S]

Or a Gibby 79RVT clone.

Excellent suggestion, but what to do about the unobtainable opto-coupler for the "T"? Has anyone figured out how to roll your own? I think the LDR side wouldn't be so hard to guess. I tried to identify an appropriate incandescent bulb but was never satisfied with what I could find. A bit of flimsy plastic that could be rolled up and covered with head shrink would probably be ideal. Maybe a dab of silicone to make parts stay put long enough to roll the roach.

[maxkracht]

I know the wire gauge difference shouldn't make its way to the windings, that would make no sense. I just don't understand why they would waste the cash on 20 awg for one leg when 24 would do. At least make the CT the big wire, to appease the obsessive in me that demands symmetry. Having the ct and one outer leg the same color but different gauge just feels wrong...

No plans for these guys yet, but definitely something in stereo. I've never tried to do stereo vibrato or tremolo, but I have also been meaning to build a hifi for a while... A few projects in line before I have to make decisions.

[TUBEDUDE]

I know the wire gauge difference shouldn't make its way to the windings, that would make no sense. I just don't understand why they would waste the cash on 20 awg for one leg when 24 would do. At least make the CT the big wire, to appease the obsessive in me that demands symmetry. Having the ct and one outer leg the same color but different gauge just feels wrong...

No plans for these guys yet, but definitely something in stereo. I've never tried to do stereo vibrato or tremolo, but I have also been meaning to build a hifi for a while... A few projects in line before I have to make decisions.

Or a Magnatone.

[TUBEDUDE]

Or a Gibby 79RVT clone.

Excellent suggestion, but what to do about the unobtainable opto-coupler for the "T"? Has anyone figured out how to roll your own? I think the LDR side wouldn't be so hard to guess. I tried to identify an appropriate incandescent bulb but was never satisfied with what I could find. A bit of flimsy plastic that could be rolled up and covered with head shrink would probably be ideal. Maybe a dab of silicone to make parts stay put long enough to roll the roach.

I fooled with these back in the day. The best result I got was a CdS cell and a brake lamp. I think it was a 157. LED's were a fail.

[Phil_S]

I just don't understand why they would waste the cash on 20 awg for one leg when 24 would do.

I dunno. It's what they had on-hand in the color they wanted to use or what they got for lowest cost? There's no saying #20 was actually more expensive even though we think that ought to be the case. Logic doesn't always attach.

[Phil_S]

I fooled with these back in the day. The best result I got was a CdS cell and a brake lamp. I think it was a 157. LED's were a fail.

There were Vactrol incandescent/LDR optoisolators (Perkin Elmer?) made that looked like they'd work perfectly, but they were replaced by LED models that looked wrong to me. Not surprised at all the LEDs didn't work. Your info caused me to do yet another search and I found that Weber is selling a roll your own kit with one of those tiny lamps in choice of 3, 6, or 12v bulbs. According to the schematic pasted into my GA20-RVT, the voltage difference is 136-124, making the 12V the appropriate choice. But there is a note that says "voltages vary widely with R38 assemblies." R38 references the CdS cell.

[pjd3]

Hi,
This what you see below is an excerpt from a thread I created just last week. I was curious how to determine the current max on a secondary coil of a mystery power transformer I have. The thread developed into a fairly deep education of viable approaches to testing and determining power transformer specifications regarding current maximums. And much more. Below is only one of the posts in that thread and appears to be a guide to at least getting a picture of the PT's power capabilites based on Primary DCR.
The thread is called "Transformer secondary max currents - techniques for determining" in Technical Discussions>

Those trannies sure look like a great find. Yeah, some nice hi -fi amps in there I bet.


"From the Primary Winding
For a 120 volt AC supply the VA rating and primary resistance is as
follows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply.
Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings."

Best,
PJD3

[maxkracht]

Thanks pjd3, I appreciate the info and have been following that thread. I usually estimate what a PT can handle based on what I am removing it from but I will definitely use that info on the occasional mystery transformer that finds its way into my shop/ when someone really needs to mod their amp for higher powered output tubes. The PT from this organ amp weighs about 20 pounds, powered 2 5u4's, 8 el84's, 3 12ax7's and possibly a satellite preamp with who knows what. Not going to do any math/ don't think I could overrun that thing if I tried...

[Phil_S]

... I was curious how to determine the current max on a secondary coil of a mystery power transformer I have...
From the Primary Winding
For a 120 volt AC supply the VA rating and primary resistance is as
follows.
...
160 VA = 2.5 to 3.5 ohms
...
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply.
Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings."

From the abbreviated chart, let's assume yours is the 160VA. 160-15 for the filaments = 145VA. (VA is same as Watts, well not exactly, but who cares?) Derate to 65% of 145 = 94. Round up to 100 because I'm lazy. You will need to test for unloaded voltage on the HT secondary. Yes, plug the primary into 120VAc source or some lower voltage and do the conversion as if it was 120. Let's say it is 400V. You have 100VA. You know V = 400. A = 250mA. This is a rough estimate. If in the lower part of the DCR range, you might feel a little more comfortable with it.

I suggest this is a starting point. You can probably safely apply a load equivalent across the HT secondary based on Ohms law. V=I*R. You now have 400 = .025 * R. Solve for R = 1600Ω. Put that between the two secondary leads and find the voltage drop. Calc again, voltage drop over 1600Ω. Now you have one fact, not an estimate. Meanwhile, is the PT too hot? Use an infrared gun if you have one and if not, a temperature probe. Keep going until you have the mA you need (reduce the load gradually) or you let the smoke out. In the former instance, you have pretty well proven it will stand up to your intended circuit. In the latter, you've proven it won't stand up, though it is too bad that you've ruined it. So, keep your eye on temperature. I wouldn't allow it go go above 100C, which is too hot to touch but probably won't melt the insulating varnish. Remember, when loaded with tubes, the filament windings will contribute to the heat, too.

Please be careful. Don't get electrocuted when you do this. Please don't blame me if this doesn't work. Someone else passed this on to me. I have no information on how this chart was created. If not comfortable with these ideas, don't do it!