RCA BA-6A problem - SOLVED!

[audiopete]

Thanks for that info Jim, and OMG I hope it's not that. I have a PP 100W 6L6 I can clip in, but first I want to make sure that it isn't the parasitic oscillation issue on the bias line that I suspect. I think I will just try a decoupling cap there on the bias supply and it will either fix it or it won't. 

There is an IT on the Sowter site for sale. I'm always nervous ordering up iron paranoid my diagnosis is incorrect.
- http://www.sowter.co.uk/cgi-bin/sh000001.pl?WD=ba-6a&PN=VINTAGE-COMPRESSOR-TRANSFORMERS-J-to-Z%2ehtml#a263

 

[audiopete]

The other fact that leads me away from the IT is the unit works fine up until threshold, works fine with increasing gain if I pull the 6H6 (no anomalies in the signal) and compresses fine when I added an external negative supply to the 6SK7 grids. I would think a shorted winding would show up somewhere somehow and also have some variation. This action is cyclical and predictable. This makes me suspect the bias circuit interacting with the signal causing an oscillation and runaway peak.
I'm hoping a low impedance path to ground will cure that.

 

[emrr]

I agree I think it would show up without compression if there was an IT problem.  Though, transformer intermittents can be very odd.  We have changing current with gain reduction, which might trigger something, though the pair of 4K7's is taking part of the current load.  Plus I'd expect problems with current going the other way more.  I'd sub ANY semi-suitable IT in there and see.  Many many IT's will work satisfactorily for such a test. 

I suppose some breakdown intermittent in the OT could do it too, though I'd expect to hear it or see it in waveforms. 

Without reading back, I assume you've replaced C1/R11/etc.  I think I said earlier it sounds like what I've seen when C1 is funky, and I know not exactly how, all the 'bad' ones I've ever replaced tested fine, showed no obvious leakage, etc. 

 

[jim-analog]

Greetings,

Pete, relative to your reply #41. Man, it sounds exactly like what I experienced on the problem unit I had. Removing the 6H6 will disable the limiting circuit (and the problem).  I did the same test; I also used an outboard neg supply to simulate.  There was a distinct relationship between what occurred at the moment the threshold was exceeded and the unit should normally go into limiting. What seemed to happen was a HUGE overshoot at that point which would send the unit into hard limiting that then gradually release according to the time constant selected. 

It was a pretty long time (at least 10 years) and hundreds of repairs ago, so my recollection could certainly be wrong in some minor details. I'd really suggest you give it a try using the PP output TX you have. It should only take a few minutes to "fly wire" it in and then run some signal through.

As with my unit, it sounds like you've tested just about everything as thoroughly as  I did. It was almost a process of elimination (as there wasn't much left that I hadn't swapped out).  A really smart tech suggested I give the IT a try, though I thought it had to be something else.  A bunch of other techs/manufactures (some very well known in the industry) all very sharp and generous with their time had given up trying to help me by that point as they were plain out of ideas.

BTW, that looks like the same Sowter TX that I used as a replacement. It worked fine and there were no problems ordering directly from them. I don't know if they have a distributor in the US now, but back then they did not. I was leery about ordering from the UK but it went fine. For sure, getting the old one out of the can was a really awful job.  IF it ends up being the IT, it may be worth it to buy a small, functional, used toaster-oven to try as opposed to my heat gun method.
Run an extension cord outside to keep you shop air healthy. The oven would have to be scrapped afterwards.

Anyway, whatever it ends up being, I hope you find it soon.  It was one of those jobs that just baffled and humbled me that I'll never forget. Best of luck!

Regards, Jim

 

[audiopete]

Yah it's been one of the tougher projects I've had, that's for sure.

My plan is to change the psu voltage divider resistors feeding the bias to the 6H6. Old carbon comp can do strange things. Second is to try a 22uF cap on that line. Third I will try subbing out the IT. Fourth, I throw it in the bay.

I'll report back.

Thanks everyone.

 

[audiopete]

The other data point that sticks with me here is that the test sine is clean everywhere except the bias line. I can see it start to wobble and increase in intensity and I can see an AC spike shoot out there. Everywhere else that sine stays stable until the spike and shutdown, then it comes back up clean and starts the cycle again.  I have some video of my scope I can put on dropbox.

 

[audiopete]

Here's the vid link. This is the bias line between the divider resistors in the psu and the bias resistors in the 6H6. Here I can only see a positive spike, but other places the spike is AC.

https://www.dropbox.com/s/aejazxwfd33uzr9/ScopeVid1.mp4?dl=0

 

[audiopete]

OK did all those tests.

New dividers did nothing.

A 22uF cap on the bias line did clean it up but did not fix the issue.

Clipped in a 100W OT for the IT and lo and behold. It looks like that is the problem. It compresses without diving, although I am guessing because the IT is 10K:10K and the OT is 4Kish:16ohm I have to crank the gain from the signal gen to get it to hit threshold.

Looks like I will be ordering up a new IT.

Thanks for all your time and help.I'll update when I get the new IT.

Cheers and all the best.

Pete

 

[emrr]

Good to know, very strange. 

 

[jim-analog]

Greetings,

Great Pete,  glad to hear it!  Your persistence wins out in the end.

Regards, Jim

 

[emrr]

Since the IT is a 1:1, it could be interesting to know if it works ok with pri/sec reversed.    Of course there’s no CT, so B+ would  have to be resistive only.  Which leads to an even simpler thought, what about with the pri taken out of the DC path?  It’ll certainly still work.  Might help reveal the failure mechanism. 

 

[noulou]

Cool! Well done Pete.

What are your plans for the old transformer? Cj if we send you these do you think we would be able to rewind them to spec?

Fwiw in my opinion the sowter IT sounded nothing whatsoever like the original.

Nikos

 

[emrr]

Fwiw in my opinion the sowter IT sounded nothing whatsoever like the original.

I'm not surprised.  The original is likely quoted on the (unobtainium) RCA spec sheet as 50K:50K, wire size much smaller and DC resistance much higher than the modern version. 

 

[beatnik]

Has anyone tried Lundahl LL1635 ?

https://www.lundahltransformers.com/wp-content/uploads/datasheets/1635.pdf

 

[emrr]

Has anyone tried Lundahl LL1635 ?

https://www.lundahltransformers.com/wp-content/uploads/datasheets/1635.pdf

The test source Z and the DC resistance both suggest it's far too low an expected impedance for this job. 

 

[emrr]

I'd be tempted to put a Hammond 850Q in there.  Spec is right in all ways, L is high, and response is pretty much flat even with a 40K source and a 40K load.  A Hammond 835 would probably also work fine, though L is a lot lower. 

There's a thread about BA-6 transformers, Lassoharp and I tried and measured a bunch of different things.  Lots will work. 

 

[beatnik]

Thanks Doug

the Hammond is also much cheaper than Sowter I guess would be worth give it a try

 

[vari-mu]

    Several years ago I was engaged in the manufacture of transformers for limiter RCA BA-6A (input  ,IT, OPT).
Interstage  transformer has been described here : https://groupdiy.com/index.php?topic=55693.msg764627#msg764627
  To test in a real circuit, a complete compressor circuit module was manufactured :

The transformers turned out very good for me, the band 18Hz - 100kHz 0dV in the real circuit.

 

[jim-analog]

Greetings,

If there are other options for a replacement IT for Pete that may be less expensive or easier to get that would be great. However, I had two BA6A units only a few dozen serial #s different on the bench at the same time when I completed the IT replacement with the Sowter. Honestly, any difference the IT may have caused was swamped by the inherent differences between units of that age. None of the measurements I took (FR, THD, Gain(s), noise, etc.) showed very much at all. I couldn't tell if one unit sounded any better or worse than the other either, nor could my client when they were returned to him.

Perhaps in a brand new, close tolerance, highest quality component, optimized layout, master craftsmen build that statement may well be true, but not in the case of  40+ year old units (though both were rebuilt at the same time by me).

No special loyalty to Sowter here,  just a satisfied customer who couldn't fault their IT in any way in that application.
  I'd be quite interested if anyone did tests that showed a significant, measurable difference between the various replacement ITs.

Regards, Jim

Fwiw in my opinion the sowter IT sounded nothing whatsoever like the original.

Nikos

 

[noulou]

Hey Jim,

Do not get me wrong. This is by no means a diss to sowter and their products. Their transformers are not inferior to their vintage counterparts. I have prefered and installed them in various pieces of gear despite having access to old, expensive transformers.

My experience has been with a pair of really closely matched ba6as. I had been using them before the one unit developed the fault and they had been extremely similar in tone and character.
However, after the switch the situation was quite different. The sowter unit lost all the low and low-midrange thickness that I associate the ba6a sound with. I would even go as far to say that in my opinion the interstage transformer is where the ba6a gets most of its signature sound from. It was definitely not a frequency response issue and the sowter transformer is performing perfectly well in the circuit.
But, as anyone who has fooled around with vintage transformers could probably tell you, the modern "better" transformers usually sound a little anaemic. Almost always cleaner in comparison.
I do not know if it the differences are in inductance, dcr, bh curve, phase response, parasitic capacitance or what have you. But it is definitely there.

Nikos

Greetings,

If there are other options for a replacement IT for Pete that may be less expensive or easier to get that would be great. However, I had two BA6A units only a few dozen serial #s different on the bench at the same time when I completed the IT replacement with the Sowter. Honestly, any difference the IT may have caused was swamped by the inherent differences between units of that age. None of the measurements I took (FR, THD, Gain(s), noise, etc.) showed very much at all. I couldn't tell if one unit sounded any better or worse than the other either, nor could my client when they were returned to him.

Perhaps in a brand new, close tolerance, highest quality component, optimized layout, master craftsmen build that statement may well be true, but not in the case of  40+ year old units (though both were rebuilt at the same time by me).

No special loyalty to Sowter here,  just a satisfied customer who couldn't fault their IT in any way in that application.
  I'd be quite interested if anyone did tests that showed a significant, measurable difference between the various replacement ITs.

Regards, Jim