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Grid (A-73C) transformers, whare are people getting them these days??

1000EE-Monarch

Aluminum
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Location
CA Bay Area
So I was working on a friend's WIAD 10EE (1950's tube type) and sure enough, it's got a bad grid transformer (Stancor interstage A-73C). Big surprise!

Those have always been the #1 most irritating electrical components in these machines as far as I'm concerned. Because of how often they fail, how hard they are to test accurately, and how hard it it to find a good replacement.

Last time I needed one (about 7 years ago) I called Monarch Lathes LP and they had it. Forget the price but it was reasonable. Called them again, and they said no stock, unsure of lead time and price. Meaning they have to get them made.

The other thing that really had me concerned is they said it's not an item people call asking for too often. I'm really surprised.... there's not too many other places to get that transformer and they go bad all the time. And there's still a lot of WIAD machines out there. So are people just not fixing them much any more and going AC motor and VFD?

My questions are:

Is there any other source for a replacement that I'm not aware of?

If Monarch Lathes LP does come up with a reasonable price and lead time, how many should I buy? Anyone interested in a group buy? They are likely to get harder and harder to find. I'm thinking it would be wise to own at least 5 or 10 if I plan on seeing glowing tubes 20 or 30 years down the road. That is without redoing the grid circuit which might not be a bad idea.

Are the new manufacture replacements from Monarch Lathes LP any good? Anyone put a lot of run time on one see how they hold up?

Anyone have specs (as in # of turns, wire AWG, length of wire and arrangement of winding) on that sucker? Trying to unwrap one to get that info has been a waste of time I lost my patience! Since both the turns ratio and the winding resistance are important in the circuit, that info would be important if one was to make a copy. And I have no idea if "cheating" by using the correct turns ratio but lower winding resistance than adding the resistance externally would work or not.

Anyone try an A-53C (from a module drive machine)? How did it work?

Any input on those pesky little transformers will be appreciated!
 
I have a bunch of transformers in the parts I recently picked up. I know a bunch of them are the multiple tap ones for the tubes in a modular drive but there are others as well. I can take a look tonight.
Obviously not new.

paul
 
I remember that Stancor catalog from a while back. It's amazingly vague, inluding talking about "impedence" but not specifying at what frequency that's measured at.

Got a quote from Monarch, and as I feared the price is much higher than what I paid last time and pretty much prohibitive. Also there's mention of "need info off transformer".... hmmm

Will contact some of the audio transformer people.

As far as used A-73C's, I used to avoid them (good chance of getting a bad one) but in this case just to get the machine running I'd try it. Still does not provide a long-term solution to the problem.

I'm really curious how the circuit would behave with a transformer of correct turns ratio but much less winding resistance and impedence, and adding resistors externally.

If I remember correct, the A73c is 10Kohm primary, 90K secondary, but is that DC resistance or impedance measured at 60Hz?

Alex.
 
ARS ...

Transformers & Chokes | Antique Electronic Supply

... lists a Stancor A-53C equivalent for $15 and change.

The details are here ...

Transformer, Audio Interstage, 10 mA | Antique Electronic Supply

The function of the A-53C in the Modular is identical to the function of the A-73C in the WiaD.

Incidentally, ARS is very occasionally a source of thyratrons, NL-C6J/Ks, and sometimes some others.

At one point, ARS was selling socket-mounted thyratrons which theoretically could be used in a 3 HP WiaD (but not a 5 HP WiaD or a Modular) at exceptionally reasonable prices.

Alas, the "jumbo" tube socket required a slight modification to effectively contact one of the two filament pins.

An NL-C6J/K has the same thermionic emission (33 amps, filament), but a somewhat lower anode dissipation rating as a C16J and, of course, base mount, not clip mount with wire pigtails.
 
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Thanks for all the audio related links.

It seems an A-53C sized transformer is pretty easy to find. 10K/90K (pimary/secondary) impedence is correct, DC resistance 400ohm/1500ohm. Wish Stancor would have published DC resistance.... I have not seen it yet for an A-73C.

The 400/1500 ohm resistance seems a but lower than what I'm used to seeing measuring used A-73C's, but then again chances are every single one is partially defective.

Trying to get a hold of the interstage transformer guru at Hammond Mfg., he was out today. Hoping he will be able to help.
 
I'm really curious how the circuit would behave with a transformer of correct turns ratio but much less winding resistance and impedance, and adding resistors externally.

In a word, poorly. Adding resistance as you describe would reduce the grid drive signal.

If I remember correct, the A73c is 10Kohm primary, 90K secondary, but is that DC resistance or impedance measured at 60Hz?

Neither. Usually impedance would be specified at 1 kHz for an audio transformer. When it comes to DC resistance, lower is better (less lossy) generally. The 10k/90k impedance is consistent with Monarch's description (In the manual for a modular unit) as 9 VAC primary, 27 VAC secondary. The 10k number also seems reasonable given the source impedance of the 40k/1uF network in the primary. If you happened to have a 240V(dual 120V primary) to 80V transformer, that might work, if hooked up backwards. It's a 1:3 step-up, and the impedance might be high enough to work.
 
Usually impedance would be specified at 1 kHz for an audio transformer. When it comes to DC resistance, lower is better (less lossy) generally.

OK, so was it standard practice in 50's audio transformer terminology to specify "impedence" at 1kHz? If so, that's useful info.

As far as impedence vs. DC resistance, having a high impedence while a low DC resistance means more turns, of bigger wire on a larger core. Why would having more inductance with less winding resistance be better (in this case)? Jim McIntyre, I'm not trying to argue what you said, I'm trying to really understand how this works! Please don't take offense to my questions....

There's a number of things I have a hard time understanding in this circuit:

Are they controlling the thyratron firing point by a fixed (90deg ? ) phase shift with the capacitor/resistor network. And controlling level using the winding resistance vs. the effective tube resistance.

Or are they changing phase shift using the combination of inductance/resistance/capacitance in the transformer, the fixed resistor/capacitor network, and the variable tube "effective resistance".

I bet a little of both are going on. Plus there's the negative feedback factor of the thyratron's grid being affected by the current flowing though the tube.

This circuit could be cleaned up and modernized by firing the thyratron's grid using an accurately times pulse. And we can use anything from a mechanical (imagine car distributor) contraption, to a DSP or microcontroller.

But that circuit is really elegant and forgiving, and made good use of readily available components of the day.

I want an easily obtainable drop-in replacement for the A-73C in this case. Knowing and fully understanding the specs of the original A-73c and understanding how those affect the circuit are something I'm having a hard time with!
 
These transformers are wound with very small wire. Over time the insulating materials turn corrosive and eat away the wire.

In high tension magnetos, most that are 60 years or older usually have bad high voltage windings. Even NOS parts that old are bad too.

Same failure cause. The measured resistance is higher than it should be, up to, and including open circuit.

Bill
 
Bill,

I agree, anything wound using really small gauge wire upon layers of paper is likely to be no good after 50 or 60 years. Running non-stop or sitting on a shelf or anything in between doesn't seem to make a big difference.

Unless it was hermetically sealed in oil.

Alex.
 
"Are they controlling the thyratron firing point by a fixed (90deg ? ) phase shift with the capacitor/resistor network."

The R-C networks are for pulse-shaping.

The trigger point changes, both with spindle load (I-R compensation), and spindle speed (the command input from the speed pot).

Clearly, each thyratron turns off at the zero crossing of the ac waveform.

The firing point is roughly 97 degrees into the 180 degree half-cycle, but varies earlier (corresponding to higher armature/field voltage) or later (corresponding to lower armature/field voltage).
 
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The A-73-C issue has been covered on this forum in the past. Here's a copy of a post concerning a "new" replacement.

~TW~
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08-17-2006, 04:22 AM #1
Jim H


Join Date
Oct 2005
Location
Texas
Posts
21

Post

I recently has the unpleasant experience of finding that one of the A-73-C transformers in my WiaD 10EE was bad. When I called the customer service number for Stancor (the manufacturer of this transformer), I learned that it is now obsolete and they could not recommend either a replacement or substitute transformer.

Fortunately, through the archives, I learned that Spence went through this same problem back in 2002. Spence had found a company called Electra Print that was willing to wind a replacement transformer for him. Armed with this information, I e-mailed Jack at electra-print.com and explained what I was after. Jack remembered the project. Unfortunately, he also remembered throwing out his build sheet for that transformer about a year ago. To aid in his effort to build a modern replacement for this transformer, I sent my dud transformer to Jack so he could determine the exact number of turns, etc. in the original transformer.

In just a couple of weeks, I had both a new replacement transformer and the core from my old transformer in hand. The new transformer looked great and performed properly when I installed it in my machine.

One change we made to the replacement transformer was to eliminate the unused center tap to further improve reliability of the new transformer. Jack also assured me that he will make sure to keep this build sheet.

My experience has been quite positive. If you need a replacement transformer, you might consider dropping Jack a line. (I have no connection with either Electra Print or Jack, other than as a satisfied customer.)

Regards,
Jim
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on edit: I've had difficulty connecting to the electra-print.com address mentioned in the 2006 post. Dunno the firm's current status, but here's a link to a profile on the (then) owner I found. ~TW~

Jack Elliano | Electra-Print Audio | ZoomInfo.com
 
"http://www.clarisonus.com/Archives/Trans/Stancor61.pdf"

From the Stancor specification, both the A-53-C and the A-73 are 1:3 turns ratio with a 10 ma. maximum primary current.

Either should work in a 10EE. The center tap of the A-53-C's secondary should be taped (insulated) and ignored.
 
The replacement interstage transformer that A.E.S. (tubesand more.com) sells is a very good unit. I have used a bunch of them over the years in audio applications and radio transmitters. They are wound with slightly heavier wire than the A-53c and use modern insulation that wont die sitting on the shelf. They are wound 3:1 with center taps on both windings so depending how you flip them around and which taps you use, they can be step-up, step-down or 1:1. there is no dedicated primary or secondary winding, so you can use them either way. I have used them for class-A interstage (1:3 step-up) as well as class-b driver (1.5:1 step down) transformers.

I don't have a Monarch, but that little transformer is something I know well.

And........they're cheap!

Frank
 
"And........they're cheap!"

Inexpensive, yes ... cheaply made, no.

They are not hermetically sealed. Even the originals weren't.

I believe these, from tubes and more, will last a long time.
 
I put in some E-mails to both Hammond Mfg and Electra-print, but got impatient and ordered a Hammond 124A just for fun.

The 124A appears to be closer to an A-53C than anything else, be interesting to see what it does.
 
"OK, so was it standard practice in 50's audio transformer terminology to specify 'impedence' at 1kHz? If so, that's useful info."

The General Radio 1650 impedance bridge * always used a 1 kHz (then, it was called 1 kc) internal oscillator as the ac frequency source, although for special purposes an external oscillator could be employed.

Other GR equipment was employed for sweep-frequency measurements, such as would be common for frequency response measurements of audio transformers.

In the instant case of a 10EE (or EE1000) application, 1:3 turns ratio audio interstage transformers were employed as thyratron triggering transformers, one such transformer in the field regulator, one such transformer in the 10EE's single-phase armature regulator, and three such transformers in the EE1000's three-phase armature regulator.

The transformers are all, to the best of my knowledge, identical, and are generally under low stress.

The thyratron's trigger point is set by a dc bias which is applied to the transformer and is passed through to the thyratron's grids.

In this way, a single bias supply may be employed in both the single-phase 10EE armature regulator and the three-phase EE1000 armature regulator, as a dc bias is valid over all 360 degrees of the nominal 60 Hz power input.


[ *] To quote GR: "The IET / General Radio 1650-A Impedance Bridge is a self-contained impedance measuring system, which has five bridges for the measurement of capacitance, resistance and inductance, as well as the generators and detectors necessary for DC and 1-KC AC measurements."

Resistance/capacitance/reactance and conductance/inductance/susceptance may be measured as the bridge is universal and is capable of direct or inverse measurements.

This bridge, and bridges like this one, employ super-precision standards of resistance and capacitance for their measurements.
 
I got the Hammond 124A. Tiny little thing, but looks well made. Windings appear to have no paper between each layer, possibly epoxy impregnated.

Will try it in a few days.

I'm curious to dissect one, and as cheap as they are I very well might do that.
 
Re General Radio bridges, I have two and still use them. Their main limitation is that the capacity range does not cover the huge electrolytics we have now because they didn't exist when the bridges were made. To my knowledge, impedance specs are always at 1 kc unless otherwise noted.

I wind transformers. When I want a good one, I use Kapton film between windings and Heavy Armored Polythermalize wire and impregnate them with transformer baking varnish or epoxy, which makes them nearly immortal.

In the early 1970s I made a general purpose DC supply with two 12 volt supplies fed by the same Variac that can be seriesed or paralled as required. A few years ago one of the transformers died and I got another from Stancor. Besides being appalled at the price, instead of being layer wound, it was just wound one layer on top of another without insulation in between. It has held up, but 12 volts doesn't strain things much.

The old Atwater Kent receivers with a row of 01As almost always have bad audio transformers. I took one apart and found that the joints between the windings and leads must have been soldered with a corrosive flux. All had green corrosion products, presumably copper nitrate, chloride, sulphate, or whatever.

Peter, I have long speculated about substituting SCRs for the thyratrons. They would require different triggering transformers, but that is doable.

Bill
 
First of all, I don't know where I came up with the 10K/90K impedence numbers for an A-73C that I mentioned earlier. That may be correct, however the A-73C says on it:

S.PL. (7K-15Kohm) to P.P. Grids

So if I'm interpreting this correct the primary is suited for a single plate with 7 to 15K impedence to be coupled to push-pull grids, no impedence specified on the secondary.

The little Hammond 124A is working just fine. All the adjustments worked as normal. Machine runs normal, and the transformer isn't getting hot at all. How long will it last, I don't know, but I have a suspicion it will be just fine.

So far I have tried it in the armature circuit, not the field. I'm assuming it will be fine there too.

I'd love to try three of those in the 1000EE, but won't have a chance to do that for a very long while. I better stock up before they get discontinued.

They will probably work just fine in a module drive to replace the A-53c in that.

As for SCR's instead of tubes, it would take a little more than just different grid transformers to make it work nicely. The triggering circuit would have to be more refined, precise and have more feedback (as in current limiting). Also would need to ramp up the armature voltage on start up.

Probably easier to just buy two off-the-shelf drives one for the armature on for the field if one wanted to go DC solid state.
 
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if anyone still needs one of these transformers, the A-73-C, I just removed one from a piece of older test equipment - it is good - I put it on ebay, but you can PM me directly and save some $$ -
 








 
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