RCA 86A1 interstage

http://www.sowter.co.uk/schematics/Rca86a1Schematic.jpg

I've got an 86A1 open here that's showing low plate voltage on V2. DCR across its half of the insterstage primary shows ~10K ohms. I haven't troubleshot too many audio transformers at this point in my life. If there were a problem I'd expect either open or shorted windings, not a somewhat higher resistance like I'm seeing here.

Thoughts? Any tidbits I might be overlooking? I'm mostly hoping this transformer hasn't kicked the bucket, though this seems unlikely to be the case. If needed, should I seek a rewind (and from whom?) or just purchase a replacement from Sowter?

Sounds like its failing, seems common with those.  Rewind will cost a fortune, Sowter can be stuffed in original can I think. 

Now that I think about it some more I guess that this failure mode does make sense, the winding's basically starting to go open I suppose.

I'd like to see what a rewind would look like, if for nothing else than for me to know what it looks like. Any suggestions?

Doc Hoyer is your man.  If I gotta make a bet, I'd say $300-$600 from Doc. 

So I got a price of $650 shipped. Pretty pricey, I ordered the Sowter. Nice guy though, talked at me for a while, sounds like he really knows his shit.

What's the reason behind the significantly higher cost for rewind? I'd expect the inverse if anything, but then I don't know much about transformer winding.

So I got a price of $650 shipped

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wow.  There was recent thread regarding having the Collins 26 interstage rewound and it seems like it was lower than that.  Don't know if they are equally difficult.  Wish we had some L figures to compare.


Nevermind. Apparently the same due to reverse EE labor.  Think maybe the word was 2nd one would be less . .

http://www.groupdiy.com/index.php?topic=38279.80

So I got a Sowter in and am starting to get it put in the compressor. I'm a little bit apprehensive, the Sowter diagram shows this transformer as a single primary and dual secondary, 20k/10k+10k. The RCA's transformer is the opposite, a 2.66k+2.66k/3.79k.

The Sowter sheet lists it as a replacement for the 86-AL or 86-A, their site links to a schematic for the 86-A1.

Put simply I'm having a little bit of fear that the 86-A and 86-AL have different transformer requirements. But my logic and intuition say this may not be the case. I'm going to go ahead and wire it up and see how it runs, but I wanted to post and see if anyone else can confirm or deny my concerns.

The RCA numbers are dc resistance, not impedance.  An 86 is an 86.  There's no 86-AL, that's a typo.  While you have it in front of you, tell us how the Sowter ohms out.  I expect much lower.

Oh, yeah. I knew that one was AC and the other was DC... I quickly read the DC of the Sowter today, I think it was about 60 ohms per winding. If you're interested in exact figures I can check again.

My memory was way off. ~520 on the split side and ~560 on the single side.

That makes more sense.  I don't know fully how the winding DC difference will translate, but it will definitely have less voltage drop difference (lim vs  no lim) than the original.  May or may not affect the knee shape. 

Yeah, I was thinking about that. Is there any reason I wouldn't want to add a little more of a voltage drop to the supply path for that stage to get my plate voltages in line with the manual, if needed?

I don't see that as an issue.  Just question about possible changes in limiting character.  I would expect probably minor, but do report if you have any ear memory of the old one at its best (working).

Unfortunately I have never had a chance to really dig into the unit since I've been at this studio, and the time or two I used it was when it was not working properly.

I'm excited to have it back up. I'm curious to hear how similar it is to our BA6As, those things sound absolutely amazing.

I triple checked those windings and man, my short term memory sure has hard times some times. The reality of this transformer is that the primaries are 350 ohms each and the secondary is 310.

So very roughly 1/10th and 1/20th the DC of the original.    Sowter calls it 10K+10K:20K.  I'm certain the original would be spec'd for a much higher impedance on paper.  But what does it all mean?  Please share any comparisons you can from other engineers there who heard it working properly before, and after. 

Things move slowly around here sometimes... Finally got it powered up. Plate voltages are ~20 volts higher than spec, 257. Listed max voltage is 300V, so this seems like it ought to be fine.

The voltage on these and other tubes jumps significantly higher (~400V) than the rated 300V on power-up and ramps down over about 5 seconds. I assume this is normal and not to be worried about?

Probably normal.  Won't draw current until all tubes are heated fully, so if rectifier is faster then it goes high.  Be sure you have the correct rectifier in it.....

Ok, so it's the current that'll kill them, not the voltage, if the latter is within a reasonable limit?

They're AC heated straight off the 6.3V winding. Also, the heaters are at ~6.6V. Ok or a bit higher than ideal?

bobschwenkler said:

Ok, so it's the current that'll kill them, not the voltage, if the latter is within a reasonable limit?

They're AC heated straight off the 6.3V winding. Also, the heaters are at ~6.6V. Ok or a bit higher than ideal?

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6.6 is the top of the spread, within limits.