Question about a 784-139 output transformer

Custom 100 · Post by **Custom 100** » Mon Feb 18, 2008 5:53 pm

I have a '68 JMP 50 watt Trem head that I'm cleaning up and retubing. The output transformer has the usual white/brown/red on the primary side, and orange/yellow/green/grey on the secondary. There is also a second orange and a second grey wire that are taped off and not soldered to each other or anything else. Any thoughts as to what these mystery wires would be used for? Everything seems to be working as it should. The amp looks 100% original except for a replaces output tube socket.

Thanks for the help

novosibir · Post by **novosibir** » Tue Feb 19, 2008 11:12 am

That's really mysterious!

I've meanwhile had far more than 100 amps with the 784-139 OT on my bench, but I've never seen only one with more that the usual 7 leads coming out!

Do you have a photo of the OT?

Larry

Custom 100 · Post by **Custom 100** » Wed Feb 20, 2008 10:56 am

I'll see if I can get some clear photos together. The orange that gets grounded to the speaker jack is a heavier solid core wire. The grey 16 ohm tap that goes to the output selector is the same solid core heavy wire. The mystery orange and grey wires are the same stranded wire as the green and yellow that go to the output selector. The transformer looks to be original.

More info soon...

Maschinenmann · Post by **Maschinenmann** » Wed Feb 20, 2008 12:14 pm

Possible extra NFB taps?

thousandshirts · Post by **thousandshirts** » Sun Feb 24, 2008 7:49 pm

Hard to believe this is happening twice--I just posted this in another thread in this same subforum:

http://forum.metroamp.com/viewtopic.php?p=168646#168646

I just finished restoring a 1971 50W Marshall that had the same situation - - duplicate 16 Ohm taps from the OT, and duplicate common wires. Again, one of each was stranded, and one of each was solid core.

I ran this information by Brian Wallace of Marstran, because I couldn't think of anyone more knowledgeable about 50w Marshall OT's, and provided him with some photos and details. He was indeed very helpful!
It turns out the transformer in the 50W I had in was from the OT50-M maker, from the 1970's, but with a different cover.

According to Brian, it seems as though the solid core wires may be part of the actual coil, and not just wires attached to the leads that are brought out from the coil. This may or may not be the case with your amp! Hopefully Brian can chime in here and fix us up!

I've got around 150 photos of this amp at various stages here:

http://s2.photobucket.com/albums/y21/th ... 50%20Watt/

Here's a shot under the bell housing; you can see the dual orange taps:

Another shot inside the bell (you can see a bit more of the label on the paper here):

OT Secondaries/Commons:

OT Primaries:

Outside, PT/OT/Choke etc:

Again:

Also:

What I ended up doing was slicing both the orange and grey sets of wires quite short, near the OT; I spliced them, added fresh wire, shrink tubed, and ran them out to the impedance selector and output jacks as follows:

This amp was not quite stock in components when it came in. It was very dark, and the top/high end was being filtered off. We restored this, and found that it was masking a lot of strange high end noises. The noise problem was around the OT wires. To give you an idea of what the output section looked like before:

And after:

The amp sounds great!

Will

novosibir · Post by **novosibir** » Sun Feb 24, 2008 10:19 pm

To me it seems, that the OT and the PT are US made, although they're wearing the Drake numbers - but end bells might be swapped to other iron!

My suspiction is based on the fact, that the sheet metal isn't U.K. like attached into the bobbin. The E/I sheets on the PT are always two from one side, then two from the other, aso.

On the OT even three or four from one side, then three or also four from the other side. The UK's never did this with the only exception of the RS OT, but at US transformer companies it's usual practise.

But the most apparent fact is, that the sheet metal isn't slotted, to may mount the end bells just with two bolts on each transformer. This sheet metal shape often is used in the USA, but I've never seen UK made trannies with this sheet metal shape, where you almost always find the 'British Empirial', which is slotted.

Back to the leads:

Have you measured, whether the orange solid & the orange stranded are connected together inside? And of course also the grey solid & stranded?

You also may attach heater AC across the primaries and may measure, whether between the solid orange & grey the same voltage (AC) appears as between the stranded orange & grey, to discover the 'secret' of the OT!

I'm curious!

Larry

Kevin · Post by **Kevin** » Mon Feb 25, 2008 12:23 pm

That Hammond choke looks pretty cool! I want one!

thousandshirts · Post by **thousandshirts** » Mon Feb 25, 2008 11:51 pm

novosibir wrote: But the most apparent fact is, that the sheet metal isn't slotted, to may mount the end bells just with two bolts on each transformer. This sheet metal shape often is used in the USA, but I've never seen UK made trannies with this sheet metal shape, where you almost always find the 'British Empirial', which is slotted.

You are 100% correct, sir. Now that you mention it I have not seen many trannies that are missing slots. This OT, for sure, has no slots. This PT in these pictures does have the slots; it is possible that the OT was replaced. There was a LOT of reddish, hardened shellac/waxy type stuff on the chassis and board when I removed them for cleaning. Also this amp was purchased by the previous owner as, I quote "parts in a box" so it seems possible that this was one of the many 1970's Marshalls which cooked an OT and had it replaced with a like model.

novosibir wrote: Back to the leads:

Have you measured, whether the orange solid & the orange stranded are connected together inside? And of course also the grey solid & stranded?

You also may attach heater AC across the primaries and may measure, whether between the solid orange & grey the same voltage (AC) appears as between the stranded orange & grey, to discover the 'secret' of the OT!

I'm curious!

Larry

Yes, the orange/orange and grey/grey were connected. I did some OT testing, made some estimations on the number of winds, figured out the ohms for each tap (although I knew which was which already

), and as for the readings between solid and stranded, it was so close, that it was a tie left up to my multimeter. . .

Still I am not without much surprise, to hear your experience, 100+ of the 139 OT's and all or most with your basic 7-wire situation. Things that make you go hmmm.

thousandshirts · Post by **thousandshirts** » Mon Feb 25, 2008 11:53 pm

Kevin wrote:That Hammond choke looks pretty cool! I want one!

The Hammond had the following information on it:

Top of choke: "Hammond Canada"
Side of choke: 158M 10H 100MA

Your basic open frame 10 Henry choke as far as I can see. Might or might not be available new, but being Canadian, you gotta like our chances to get these without international shipping (UGH!) or duty at the border (Charlie Brown AAUGH!)

novosibir · Post by **novosibir** » Tue Feb 26, 2008 12:23 pm

I assume, that this OT is wound the way, the 100W OT's are wound, which aren't interleaved, but do have a second parallel 16 ohms secondary (w/o taps for 4 & 8 ohms), which should be attached in parallel to the entire main secondary (0-16 ohms), to reduce stray capacitance and to increase the coupling factor and efficiency.

On the 100W OT's (i.e. Dagnall C2668) this already is internally connected, to avoid mistakes by connecting the OT.

Larry

bluze81 · Post by **bluze81** » Tue Feb 26, 2008 7:04 pm

thousandshirts wrote:
Kevin wrote:That Hammond choke looks pretty cool! I want one!
The Hammond had the following information on it:

Top of choke: "Hammond Canada"
Side of choke: 158M 10H 100MA

Your basic open frame 10 Henry choke as far as I can see. Might or might not be available new, but being Canadian, you gotta like our chances to get these without international shipping (UGH!) or duty at the border (Charlie Brown AAUGH!)

Nice amp will be a great restore! wish I had that ohms selector for my 71 Superbass,

Mine broke, no drop in replacements available, good luck with the amp, bluze

demonufo · Post by **demonufo** » Wed Feb 27, 2008 10:45 am

Sorry to Hi-jack, but I have the same OT in an early '76 2204 (stoopid original non-cascaded circuit) and the laminations on one end of the OT go way out of line (and slightly out at the other end).
Would this cause nasties in the sound, as it's really hard to tell with this crappy circuit.
Currently I'm not convinced the OT isn't in a bad way anyway as it sounds radically brighter on a 16 Ohm tap than on the 8 Ohm (which is so dull even with everything on 10 that the amp is rendered useless)

Somebody's already replaced the original PT with a new Dagnall so I'm going to junk the OT anyway, I just wanted an opinion on whether it's worth me trying to sell it, or just just it.

Cheers,
Daymz

Kevin · Post by **Kevin** » Fri Feb 29, 2008 12:34 pm

The Hammond had the following information on it:

Top of choke: "Hammond Canada"
Side of choke: 158M 10H 100MA

Your basic open frame 10 Henry choke as far as I can see. Might or might not be available new, but being Canadian, you gotta like our chances to get these without international shipping (UGH!) or duty at the border (Charlie Brown AAUGH!)

It IS nice to be able to get trannies and chokes without ridiculous shipping and duty costs. Looks like that 158M is still available. It won't have the nice vintage look and logo on top like yours does, though. I used their open frame 5H and Marshall transformer copies in a recent built. Nice trannies.

kevin

razelfrax · Post by **razelfrax** » Fri May 02, 2008 8:07 pm

Kevin have you compared the new Hammond trannies to metro or marstranies? I built a clone from an old Girard pa head with 7591a's in it but would like a PT with a 5v tap and enough heater current to run kt66s'. Was thinking bout a Hammond. Also do you have any Idea what henri value those huge ham chokes are that come in a voicemaster or yba-1?

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 11:38 pm

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Question about a 784-139 output transformer

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