Signal Corps AM-186 (WE 1126C) Power TX replacement

[Dmichel123]

I've got a Signal Corps AM-186 limiter here that came in with a dead PT. We've been using a spare PSU to power it, but are hoping to get it going with a new/used PT in it's built-in PSU.

I am finding many PTs with two 6.3V windings, not as many with three. This thing has a dedicated heater winding for the 6L6 series regulator, another dedicated for the remainder of PSU regulation tubes, and a third that heats the amp tubes. Do these three heater windings need to stay separated? 6L6 (300B in WE schem) winding has CT going to 6L6 cathode. Something to do with max heater/cathode voltage?  Can I do an artificial CT here? Something similar going on with the rest of regulator tubes, or just keeping hum off signal tube heaters?

Here's the WE PSU schematic. Almost identical, except the Signal Corps uses a 6L6 as the series regulator tube.

 

[emrr]

You see multiple filament taps in use in old equipment as a means to reduce noise.  It may or may not be more theory than practice, and have mattered more to them then than it does to us now. 

 

[PRR]

How can telephone company and government make such a mussed-up job?

OK. You are not going to find a single PT with that small HV allowance and the 6VAC 5Amps heater supply. So find a 6V 5A heater transformer.

The 60mA DC demand is a Fender DeLuxe PT, near-enough and cheap. This will give, even vacuum-rectified, 400V DC at a large first cap. The 6L6 can handle the 400V-275V 60mA drop comfortably. It can not handle its heater at 275V while 6J7 heater is at 60V, if on the same winding. That means yet another heater winding.

This is not 1959 anymore. Lose the tubes. 1N4007 rectifiers. A MOSFET on a large heatsink will do the 300/6L6's job. So well, that 280V of Zeners at its gate will nail the output more solid than a limiter needs. Only save the 313C, idling on a resistor, for "gas glow".

 

[emrr]

Found this in my pictures.

If you want to keep it original, it might not be too crazy $$$ to get it rewound. 

 

[Tubetec]

Why not wind your own Lt transformer ,
Im just trying a rejigged microwave oven transformer , primary kept intact ,2kv secondary stripped out and rewound,  Lt with short turns counts is easy to do , taps can be placed anywhere you like too , works out around 1 secondary turn per volt , I tried with 4mmsq high temp stranded insulated wire I was just about able to do 6and a half turns vertically  but at the inside of the bobbin the 4mm was a bit to hard to work , I'm thinking 2.5 mm sq on the inner turns closest  to the bobbin , then maybe a larger 4mm winding on the outer layers , the tapping points/start/finish of seperate windings  can simply be left as a longer loop while you put your turns on then later snipped and put on to a terminal block or soldered to the relevant point in the circuit . Im unsure if I should add interwinding screens , I can see a centre tap to ground on one or more of the windings would in itself give  some shielding between the windings  . If say we were using a tube rectifier would a screen between its heater winding and adjacent 6.3volt windings be needed ? obviouly we need to pay attention to insulation of a rectifier heater winding relative other nearby  LT supplies close to ground potential , I can wind either concentric layers the full height of the window available or make each winding on its own side by side ,any ideas or suggestions most welcome

 

[Dmichel123]

Found this in my pictures, I'm not seeing cathode tied to filament here, but plate to filament CT.  It's still part of the B+ path, won't work with an artificial CT.

If you want to keep it original, it might not be too crazy $$$ to get it rewound.

I still see heater CT tied to 6L6 cathode. This is just a reference for elevated heaters, no? B+ path is flowing other way into 6X5 ?

 

[Dmichel123]

Yep, I'm completely trippin', looking at the wrong thing.  Sorry!

Could this filament CT be artificial?

 

[emrr]

I really can't see that picture well enough to tell, I'm taking your word that it is connected.  With the 6L6 being indirect versus direct heated, the filament and cathode are tied probably as you say, for elevation so they match in value.  There may be some B+ current flowing through the filament, I'm not certain, can't say if it's critical or not.  Very low resistance path through that winding.  At first glance it appears to have no reason to be a CT for hum, though being to one side would superimpose AC on the B+ line. 

How much of the PT is bad?  Is it shorted and overheating, or just have an open winding?  If any is usable, is it possible to scab in a replacement section?  Probably not much room there. 

 

[Dmichel123]

I really can't see that picture well enough to tell, I'm taking your word that it is connected.  With the 6L6 being indirect versus direct heated, the filament and cathode are tied probably as you say, for elevation so they match in value.  There may be some B+ current flowing through the filament, I'm not certain, can't say if it's critical or not.  Very low resistance path through that winding.  At first glance it appears to have no reason to be a CT for hum, though being to one side would superimpose AC on the B+ line. 

How much of the PT is bad?  Is it shorted and overheating, or just have an open winding?  If any is usable, is it possible to scab in a replacement section?  Probably not much room there.

One of the heater windings is shorted, best I can tell. Overheating. Our original plan was to have it rewound, but we're having a hard time finding someone willing to do it. I'm thinking rewinders are more into resurrecting sexy audio transformers than stinky potted power transformers....

 

[emrr]

Crap, oh well.  Edcor would certainly do it, but it'd be a long wait.  Probably a long wait with almost anyone.  CJ? 

 

[PRR]

> I still see heater CT tied to 6L6 cathode

6L6 cathode is at +275V. Max heater-cathode voltage is historically rated 180V. Its heater can be elevated at +95V to +465V.

6J7 cathode is at +60V, 90V max H-K. Its heater can be elevated at -30V to +150V.

6X5 cathode is at +270V, 450V max H-K. Its heater can be elevated at almost any voltage we can find.

It *does* appear that all heaters could be elevated to say +120V to +140V.

That is if you believe the 180V number on old 6L6 data. It is extremely rare today to find even 40V on a 6L6 cathode; I have to wonder if modern 6L6 makers use the 180V stuff or the 100V stuff used in the majority of tubes.

Heater-cathode insulation breakdown can be a very annoying fault, because it isn't supposed to happen, and the readings "don't make sense".

Center-tapped is not important. The 300B version has slightly less buzz when CT. The separate cathode on 6L6 is unipotential and it does not matter which end of heater is tied to a reference, as long as it IS tied and can not float hundreds of Volts away from its cathode.

 

[Dreams]

If the problem is in fact just a bad heater winding, usually you can fix those without too much trouble. The heater winds are usually the last windings wound on to the coil, owing to the large size of the wire needed, so they're the first things to be unwound if you're going in. Just count how many turns as you unwind it, put them back on with fresh stuff. Maybe another layer of insulation between it and the HT winds if you have the room?

 

[CJ]

that thing probably needs a complete rewind, trouble is, those old school transformers are welded into a can full of toxic black tar, getting that thing out, cleaning it up to get the lams loose, then cleaning the coil so you can unwind it without getting lung cancer is a major PITA,  and then what do you fill it with?  re heat as much toxic tar as you can salvage,

plus, when you take the can apart, the paint gets screwed up so it looks kind of ratty,

now i know why Doc Hoyer charges $500 to do those old rewinds,

Hammond sells about three million power transformers, plus they do customs for like a 20 dollar set up fee, so i would scan their catalog,

 

[Dmichel123]

Would this Hammond work? https://www.ebay.com/itm/372JX-1-Piece-Hammond-Power-Transformer/273643267752

How much VDC must I have before the regulator for this thing to work?

 

[Dmichel123]

> I still see heater CT tied to 6L6 cathode

It *does* appear that all heaters could be elevated to say +120V to +140V.

Max heater to cathode for 6SN7 is 100V... Haven't found a number for 6L7, yet. 6SJ7, 90V. There appears to be a very small window where I can run all heaters on a single tap.

 

[Dmichel123]

Got the PT back from a very expensive rewind. The limiter now has a hum that was not present with the bench power supply we were using it with.  Hum is 60Hz, mostly, appears to be a heater thing. The bench supply has ~60V elevated heaters and the original supply simply grounds the center tap. I reckon I could elevate the heaters from somewhere in the circuit, or I could lift the CT and put a "hum balance" pot in there? Any tips/recommendations?

Edit: I elevated the heaters to ~100V with no change in hum.

 

[Dmichel123]

I tried connecting the heaters to the bench supply with the B+ powered from the internal PSU. I still have 60Hz hum. Could the rewinder have left out an internal shield/screen?  Is there something in the regulated supply that I should look at? I've tried different tubes, same problem. I am thinking if it was a filtering/regulation issue, I would be seeing 120hz hum rather than 60hz.