JS Transformers ( Jørgen Schou) 1:40, 20ohm-30kohm, recomended applications?

trobbins said:

That scan was fine 
Does the Bäckström catalog have a date?  If you don't mind, I will add it to a collation of Williamson details on http://dalmura.com.au/projects/Williamson.php

If you come across any late 1940 / 1950 articles in local tech mags, etc on the Williamson amp I would really appreciate knowing about them.

Tim

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Of course, feel free to add it to your collection.  The catalogue does have a date: February 1954.
Apparently Bäckström was the authorized dealer of JS in Sweden.

Speaking of Williamson - have you come across any  Williamson output transformers made by dutch Unitran? I have a schematic somewhere (with part no), but never seen the transformer IRL.

I've seen an original Goodsell Williamson amp, but I didn't have the chance to check out the transformers (probably Partridge?).

I'll be sure to inform you.

Conviction said:

As Jakob says, a 50 ohms primary might point to phono cartridge use. However, a 50 ohms primary was actually not that uncommon for microphone use in the mid 1950's. At least not at SR/Televerket. I've got scans of several preamp designs utilising JS transformers with a 50 ohms primary (only). The input valve would have been EF12 or EF14 and later on ECC40 and EF86.
Likewise, loads of (ribbon) microphones were wired for 50 ohms.

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Old topic i know, but your inbox was full so i couldn't PM you.
I just got my self a Sela tube mixer with JS trafos and 2x EF86 in the preamp modules. I'd love to have a look at that scan!

ELM said:

Old topic i know, but your inbox was full so i couldn't PM you.
I just got my self a Sela tube mixer with JS trafos and 2x EF86 in the preamp modules. I'd love to have a look at that scan!

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I'll PM you my email! I have a lot of scans that might be of interest.
Which SELA (or did you mean NEFA?) mixer is it?

Best regards,
Olle

Love this place! 
The knobs says Sela but thats  all i know, i found zero about this thing on the net so i guess its pretty rare.
I email you later, thanks!
The beauty:

ELM said:

Love this place! 
The knobs says Sela but thats  all i know, i found zero about this thing on the net so i guess its pretty rare.
I email you later, thanks!
The beauty:

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Ah, the "Frömixer"  It's not a SELA construction (only the attenuators), but a SR in-house design. The designer was called Fröberg (unfortunately I don't remember his first name). It was used as a location mixer until the end of the 1960's. I've seen photos of it being in use in the mid-to-late 1970's too.

The output's 150 ohms. A little on the low-side, but will work fine. The output transformer is a rather hefty JS, driven by a E80CC (I've seen 12DW7's in some). The inputs might be wired for 50 ohms (check the input transformers). I've changed mine to 600. It takes some work, but it's really worth the effort.

The mixer can in fact also work on battery. It has a vibrator transformer that ups the 6VDC input to 300VDC for the anodes. Typical for remote gear.

The only downside with the mixer is the on-the-verge-of under dimensioned power transformer. Does it vibrate? Hum? Buzz?

Yes, mejl! I have some product sheets, pictures and whatnot.

Wow, thanks a lot!
I tested it (2 inputs) yesterday and all i can say is that input z is low and output z is high. Using a transformerless dynamic and conecting the mixer to a DI it sounded good. The powertransformer is silent as a mouse.
Is it complicated to ad transformers on the micpres for direct out? I mean electronicly complex?

Btw, I have not recieved any PM from you yet. You can also find my mail adress in my profile/
Erik

Conviction said:

As Jakob says, a 50 ohms primary might point to phono cartridge use.

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A little dubious. MC cartridges are typically about 1-2 ohms.

However, a 50 ohms primary was actually not that uncommon for microphone use in the mid 1950's.

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Yes.

Likewise, loads of (ribbon) microphones were wired for 50 ohms.

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Not only ribbon. Many Neumann mics could be wired for 50 ohms.
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abbey road d enfer said:

A little dubious. MC cartridges are typically about 1-2 ohms.

Yes.

Not only ribbon. Many Neumann mics could be wired for 50 ohms.

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You're right - I just didn't want to entirely exclude the possibility. 50 ohm MC would likely be a modern-ish piece of gear, like the new Ortofon SPUs.

On a side note; what would one do with a bunch of 1:1 50 ohm JS trafos?

Conviction said:

On a side note; what would one do with a bunch of 1:1 50 ohm JS trafos?

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Just guessing here, but just like EMI had everything in 200 ohms at a time, it is not unconceivable that someone chose to have everything in 50 ohms. I can imagine that RF engineers commissioned to design a studio would chose 50 ohms as their standard. In that case 50ohms 1:1 would be as common as 600 ohms 1:1.
There's nothing sacred in 600 ohms, it's a just legacy from telephone lines.

> 600 Ohms.... legacy from telephone lines.

And open-wire lines at that.

Also practical sensitivity of VU meter rectifiers.

Cable telco lines ran (like all twist-pair) closer to 100 Ohms. But matching is not critical in audio. Over short-medium runs, 200 Ohms may be lower loss on thin copper.

ELM said:

Wow, thanks a lot!
I tested it (2 inputs) yesterday and all i can say is that input z is low and output z is high. Using a transformerless dynamic and conecting the mixer to a DI it sounded good. The powertransformer is silent as a mouse.
Is it complicated to ad transformers on the micpres for direct out? I mean electronicly complex?

Btw, I have not recieved any PM from you yet. You can also find my mail adress in my profile/
Erik

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Coming!

That's good news! A lot of them suffers from buzzing. The PT is tiny, feeding a serious number of tubes. It won't handle any excess.

Regarding direct outs the answer is yes, but it's not that easy to achieve a pro solution. I experimented with JFET diamond buffers, but was never 100% pleased. You might tap directly after the 0.47 output caps or post-attenuator, hi Z, but then you'll naturally load down each of the modules.

Op transformers is complex, yes, since the modules won't be able to drive it.

Conviction said:

Coming!

That's good news! A lot of them suffers from buzzing. The PT is tiny, feeding a serious number of tubes. It won't handle any excess.

Regarding direct outs the answer is yes, but it's not that easy to achieve a pro solution. I experimented with JFET diamond buffers, but was never 100% pleased. You might tap directly after the 0.47 output caps or post-attenuator, hi Z, but then you'll naturally load down each of the modules.

Op transformers is complex, yes, since the modules won't be able to drive it.

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Please contact me Conviction! I´d love to read all you have on the frömixer and the JS input/output transformers in it!
Come on!!! 
Thanks for your PM, Much appreciated!

Bump! Feels unnecessary to make a new one every time a JS related question pops up.

So. Yesterday I found a JS 6101/a input transformer in my storage. I'm pretty sure it's a microphone input (DCR compared to known JS inputs indicates 600 ohms). Ratio is 1:10.

For some reason, however, the primary is marked "80mH" (as in Millihenry) instead of, let's say, 250 or 600 ohms.
Why is this different? Could it be a tape head input?

I've never, ever seen that before. Have you?

Conviction said:

Yesterday I found a JS 6101/a input transformer in my storage. I'm pretty sure it's a microphone input (DCR compared to known JS inputs indicates 600 ohms). Ratio is 1:10.

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  Dubious, since 80mH suggests a rated impedance of about 20-30 ohms.

  For some reason, however, the primary is marked "80mH" (as in Millihenry) instead of, let's say, 250 or 600 ohms.
Why is this different? Could it be a tape head input?

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It's a plausible possibility.

abbey road d enfer said:

  Dubious, since 80mH suggests a rated impedance of about 20-30 ohms.
It's a plausible possibility.

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Thanks abby. It turns out I measured wrong: the primary measures approximately 6 ohms DCR.
The 600-ohm winding of the 5087 microphone inputs I compared with measures 37,7 ohms DCR.
I blame winter time . Or my notes ;D

So. This leaves me with a pretty transformer. I've no use for a 20-30 ohms primary.