Western Electric 1126/A vari mu limiter info needed

http://www.freeinfosociety.com/electronics/schemview.php?id=986

http://www.audiosharing.com/archive/western/we_amp/we_amp.htm (1126A Is last PDF in Amp sectio)

Was looking at these schematics thinking of DIYing for fun & am wondering about L1 coil/ coils in the 1126A schematic .......what are they ? what do they do ? on so on ....prob hard to DIY ?

The 1126 (first link) doesnt seem to have L1 coil or whatever.....why ? I take it the L1 is a later addition and is an improvement ?

The 1126 is hand drawn Im wondering if the drawer has shown coils as resistors ?

also worries me a little that the 1126 hand drawn is all correct if Im gonna DIY you see.....

One more question, why are there .03 MFD & .02 MFD capacitors wired in parallel connecting the 1st & second stages ?..........Ive seen in other circuits where Im told smaller caps pass round higher frequencies or something ?

Thanks for reading & any responses ...Happy New Year

Looks to me like both schemos shows the twin coil. It's the square box labeled 214c on the first schematic, and "D159807" on the second one...

Axel

DIY an happy new year!

Aha I was told that that box was a switch thanks Axel

The "box" is a repeat-coil, what we now call a transformer. They use it as a center-tapped choke. It needs a fabulously high inductance, over 200H per side (>800H end to end). I don't know if you can buy such a thing today.

The parallel caps are because W.E. hand-made their capacitors in the sizes that the telephone system needed. If they didn't make a 0.05u, they used a 0.02u and a 0.03u together.

The actual optimum value for these caps depends -critically- on the values of the repeat-coil/CT-choke, especially inductance. This was another hand-made part and I am sure you can not afford to buy the genuine W.E. item.

There's nothing really special going on here, except the VERY high quality. I think the best way to DIY it is to first get rich on telephone patents, and use some of that money to pay a building full of people to make custom parts, cost no object.

Thanks PRR interesting stuff thought the capcitor thing was gonna be something special ha but just down to part available & due to the interesting repeat coil transformer being a bugger to find I think I wont be DIY ing....so mabe taking a look at a Presto 41 A....cant see any unuaual parts in that one......

Would these grid chokes be good to replace the repeat coil in the 1126 circuit??

http://cgi.ebay.com/640H-grid-choke-6dj8-12ax7-5687-5842-6sn7-utc-2a3-12au7-/290472754975?pt=Vintage_Electronics_R2&hash=item43a1867f1f

Wow, no takers.  Maybe fine.  Fit the basic premise, if they are good enough in practice.

Grid chokes there to help soak up any potential thump pass through?

Been thinking about trying a clone. Any ideas on those transformers? Specs? Recommendations?
I saw those same grid chokes on Ebay above. Gonna give those a shot.

These chokes do not have a center tap.

Do they have to be center tapped? or can I just use 2 in the place of one choke?
Does the phase of the winding invert?

or??


Dual Plate Choke
600 µH / 3 amp

Enlarge Image
(ICH) D7825
Two individual 600 µH, 3 amp windings that may be used seperately, in series (1.2 mH @ 2 amps) or in parallel (300 µH @ 4 amps). Suitable for RF or power filter circuits. 2" high x 3-1/4" long.

It's essentially acting as a glorified resistor to ground, right?  So the tube can reference ground

gary o said:

Was looking at these schematics thinking of DIYing for fun & am wondering about L1 coil/ coils in the 1126A schematic .......what are they ? what do they do ? on so on ....prob hard to DIY ?

The 1126 (first link) doesnt seem to have L1 coil or whatever.....why ? I take it the L1 is a later addition and is an improvement ?

Click to expand...

It's a center-tapped inductor, or as PRR put it, an auto-transformer.
The reason is that it creates a 2nd-order filter that is useful to reject the low-frequency thump that occurs when the control voltage slams the grids negative, which drops the operating current, thus reducing the gain, which is expected, but also creates a large increase in the plates DC voltage.
Not only the 2nd-order filter has a better LF attenuation than the previous 1st-order RC, but, the coils being built anti-phase on a common magnetic core, the thumps from both halves of the input variable-mu stage cancel each other.

dustbro said:

It's essentially acting as a glorified resistor to ground, right?  So the tube can reference ground

Click to expand...

That would be a very narrow view.

dustbro said:

Do they have to be center tapped?

Click to expand...

Yes

or can I just use 2 in the place of one choke?

Click to expand...

No

Does the phase of the winding invert?

Click to expand...

Yes
See previous post for explanation.

I'm sure Sowter would be willing to build one, if you're ready to surrender a kidney...

[quote author=abbey road d enfer
[/quote] It's a center-tapped inductor, or as PRR put it, an auto-transformer.
The reason is that it creates a 2nd-order filter that is useful to reject the low-frequency thump that occurs when the control voltage slams the grids negative, which drops the operating current, thus reducing the gain, which is expected, but also creates a large increase in the plates DC voltage.
Not only the 2nd-order filter has a better LF attenuation than the previous 1st-order RC, but, the coils being built anti-phase on a common magnetic core, the thumps from both halves of the input variable-mu stage cancel each other.
[/quote]
.
Thanks so much for that detailed explanation!
it looks like sowter has something close, but they claim that it can't take any DC.
9585 Phase Splitter Choke 200H Center Tapped - low power
Suitable for phase splitting at the input of a push-pull amplifier this choke has a 100% Mumetal core for low distortion and high dynamic range. 5V at 20 Hz. 600H. No dc permitted.

Anything else out there? Not really sure where else to look.

dustbro said:

[quote author=abbey road d enfer

Click to expand...

It's a center-tapped inductor, or as PRR put it, an auto-transformer.
The reason is that it creates a 2nd-order filter that is useful to reject the low-frequency thump that occurs when the control voltage slams the grids negative, which drops the operating current, thus reducing the gain, which is expected, but also creates a large increase in the plates DC voltage.
Not only the 2nd-order filter has a better LF attenuation than the previous 1st-order RC, but, the coils being built anti-phase on a common magnetic core, the thumps from both halves of the input variable-mu stage cancel each other.
[/quote]
.
Thanks so much for that detailed explanation!
it looks like sowter has something close, but they claim that it can't take any DC.
9585 Phase Splitter Choke 200H Center Tapped - low power
Suitable for phase splitting at the input of a push-pull amplifier this choke has a 100% Mumetal core for low distortion and high dynamic range. 5V at 20 Hz. 600H. No dc permitted.

Anything else out there? Not really sure where else to look.
[/quote] Looks like it has everything to work correctly.
No DC permitted means that it should not be used with permanent DC current through it. In the 1126A, there is no permanent DC current there.

Thanks for the help! Game on