Gates SA-134/MO-3638 Issues

[andYz00m]

Or better yet, see if you can get that shop in Nashville to sell it for you on consignment. They'll probably ask a couple grand for it and since they like to sell things "Untested As is" there's no need to worry ourselves over such details - just make you some money man . . . .

and yes, I'm being sardonic on that last suggestion, for the most part . . .

The worst crime.

I'd gladly donate to CJ. I'll pull it right now and mail it. Who do I summon? (Newb here)

Hoping to find the exact replacement but if not, I'll bug you nerds for a suitable alternative.

 

[andYz00m]

Sounds like I'm going to be able to get a replacement! It's a different model but similar.

The original in this unit was AO-3039T with 12K:600 and +25dB maximum level (not sure what that means)
And the replacement is AO-8554T with 15K:600/150(CT) and +10dB maximum level.

Getting the info on the parts from this sheet.

Anything I need to adjust for this replacement? Any issues here?

 

[beatnik]

The maximum level is the output level at which the transformer starts clipping.
It looks like the replacement transformer has less headroom than the original, so it will start distorting sooner at low frequencies but it should still be decent enough for mic preamp use as long as you don't push it too much.
One thing you have to consider though is the output transformer on this preamp has dc current through the primary. And strangely the datasheet doesn't specify this parameter. So in the event the replacement transformer cannot take dc current through the primary you will have to modify the circuit by adding plate load resistor and coupling capacitor to the 6sn7 tube. It will change the sound somehow but it will still be a fine sounding preamp. If you can get the replacement transformer free of charge, give it a try

 

[andYz00m]

Thanks for the info.

I cant find any other information out there about these transformers besides that catalog which doesn't have an actual data sheet, just basic info.

So in the event the replacement transformer cannot take dc current through the primary you will have to modify the circuit by adding plate load resistor and coupling capacitor to the 6sn7 tube

Why is this? I dont understand the DC voltage value of this transformer. I thought one of the features of the output transformer is that they block DC voltage.

I'll try this one out and see how it goes.

 

[andYz00m]

Where should I send this thing?

 

[beatnik]

If you look at the schematic the high voltage from the power supply is connected to the output tube plates through the output transformer primary winding. So there is dc current in the transformer winding.
Compare it to the redd47 schematic for example, the output tube plates are connected to high voltage via a resistor and they are capacitor coupled to the output transformer. The capacitor is blocking dc to the transformer.
Search for member CJ on the forum to donate the transformer for reverse engineering. He has been posting recently in the lab

 

[emrr]

That replacement won’t work, I’m certain it’s not rated for DC, for sure not 25mA. 25mA is a lot, the original is a very specific costly type that’s not easily substituted.

 

[andYz00m]

Poop. Thanks for the info, Doug. I'll try and see if we can find the exact part.

Will send the busted one to CJ (once I find him).

I do see the 6SN7 plates (pin 5, 2) being connected to the opt, but that same lead is connected to the cathode and grid of the 6SJ7. I assume C9 blocks the DC to (or from) the opt?

Sorry for the questions, I'm still very new to tube basics.

 

[mjrippe]

@CJ - are you following this? Interested in a teardown/rewind?

 

[andYz00m]

I talked to CJ and he said he doesnt have the time for the teardown! I can do it myself if someone gives me sosme tips. I know it's probably a long shot but I would also love to maybe repair it!

 

[beatnik]

very unlikely you will be able to repair it without a winding machine, but you could probably manage to dissect it and take notes on the construction.
every transformer manufacturer I know of will turn down a reverse engineering job, but you might find some available to rewind the transformer if you provided them with the blueprint - although it will likely cost you as much as a brand new current production transformer. Also take into account that the original casing will be unusable if you cut it open, and the cost of a new casing will have to be taken into account.

to reverse engineer the transformer, you will have to cut the bottom of the casing and bake the whole thing in order to remove the potting compound, then it's a matter of unwinding and counting the several turns of wire making notes of the winding arrangement, wire size, insulation and laminations. done by hand it's probably gonna be very tedious but I guess it's not impossible. Have a search about CJ teardowns on the forum for more insight on the process.

to be honest I am not positive you will be able to find someone that can rewind it or make a good replacement for less than what you would pay for a Sowter or Lundahl replacement. You should perhaps try contacting Cinemag if you are in the U.S. as they might have a suitable transformer and it won't cost you the extra for overseas shipping.

Or as I suggested before take the alternate transformer you have been offered and modify the circuit for capacitor coupled output stage.

 

[andYz00m]

Welp, broke it down myself. The Aluminum is actually quite easy to cut through with a utility knife.

The nasty wax shoe polish tar garbage is disgusting and my garage now smells like an old shoe factory, so there's that.

Inside was a normal looking transformer with two leads on either side and a ground on the secondary.

The red lead (which on the schematic for the unit this came from carries the B+ voltage) was severed right at the winding, but even the little bit of lead left accessible still showed the primary as open, meaning the lead probably got severed in the disassembly.

Lemme know if you want more info/photos.

 

[andYz00m]

Just got off the phone with David from Cinemag, which is actually very close to me! First off, incredibly nice guy and answered the phone himself!

We talked about the circuit and the trasnformer specs and he recommended this one: https://cinemag.biz/output/PDF/CM-2810.pdf

Seems close and will work for single ended cfg.

Doing some more thinking about this, I believe I solved the mystery:

When the original "tech" took out the 6X5 rectifier tube and replaced with a diode bridge rectifier, at one point that new cfg maybe caused a surge or other issue with the B+ voltage which fried the primary of the original opt. I have since replaced the original power supply circuit and if I cant get an original opt from my source then I will try something like this solution from Cinemag.

Looking for thoughts.

 

[beatnik]

Now the sucker is out and all naked it's really calling for dismemberment. Even if CJ cannot take it maybe he can give you a little guidance on the process. I would be keen on giving it a try myself but overseas postage might be too expensive.

Since you mentioned a non original power transformer it could be that the original 6X5 shorted and killed the pt. Then they replaced the 6X5 with a solid state rectifier that has a much higher voltage output and that probably caused the damage in the output stage, like emrr mentioned earlier on. It could be the 6SN7 output tube is also toasted so unless you can check it with a tube tester you should definitely replace it with a known working tube once you fit in a new output transformer.

I am not sure about that Cinemag CM-2810 as there is no mention about d.c. current capability.
CM-9589 and CMO-10/600-150 are the only ones I can see on the Cinemag list that have d.c. current capability as an option, however it doesn't mention what is the current rating. It's very possible they can custom make them with the required value.
I would suggest getting in touch with David again and explain you need an output transformer for single ended output stage that can take 25mA d.c. current through the primary (I am taking as a gospel the value of 25mA mentioned by emrr earlier on is the correct figure)
David is indeed very nice to deal with and hopefully he can provide you with a suitable transformer.

 

[CJ]

well since you have done most of the work i wanted to avoid, i guess i could take a crack at it,
will PM addy if you are still interested,

 

[andYz00m]

well since you have done most of the work i wanted to avoid, i guess i could take a crack at it,
will PM addy if you are still interested,

I see, I paid the shoe polish tax. Haha. I feel like maybe I will turn into Venom or something after huffing the blackness.

All good, will gladly send. I'm invested now.

Since you mentioned a non original power transformer it could be that the original 6X5 shorted and killed the pt. Then they replaced the 6X5 with a solid state rectifier that has a much higher voltage output and that probably caused the damage in the output stage, like emrr mentioned earlier on. It could be the 6SN7 output tube is also toasted so unless you can check it with a tube tester you should definitely replace it with a known working tube once you fit in a new output transformer.

Probably right, I have replaced the tubes and power supply and such since all this happened. Bowie hooked me up.

would suggest getting in touch with David again and explain you need an output transformer for single ended output stage that can take 25mA d.c.

I did mention that to him and he suggested the one I posted earlier. I havent ordered anything yet but its a good call to verify the exact set up on this circuit with David before grabbing something from him.

Thanks, dudes!

 

[CJ]

FYI Gates Input

who makes this stuff, RCA?

from

https://worldradiohistory.com/Archive-Catalogs/Gates-Harris/Gates-Catalog-1953.pdf

 

[andYz00m]

Yeah that is from a Gates catalog, but I can't find any datasheets or any other real technical information. I've heard people say its UTC iron but no idea. The actual transformer inside the can has some markings on it, but maybe you can identify once you see it.

The input on this remote amp/preamp is "AI-10379" which I found in another part of a catalog as AI-10379T.



Here is the one from my unit:

 

[emrr]

The 134 has Triad iron. All the natural aluminum is Triad. In the octal SA consoles you get UTC or Triad, it’s obvious to the eye which. Later you get Triad, ADC, and some others. Not much UTC.

 

[CJ]

that "3 "has the same weird font , so triad seems like a match.