Looking for RCA transformer or OP7 for parts

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It's a needle in a haystack I know but I need the input transformer for
an RCA OP6. The code is k900 849-501 it's a cylinder approx. 2.5" tall
x 1.5" wide.

In leu of that, a half eaten "parts" OP7 would be a great find.

Also, if anyone knows the "MI" # for this tranny (or if it would even
have an "MI" #) that would really help the search.

TIA,
DA
 
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Scott,
I have no idea how you learned all of this disparate and often archaic
info but I'm sure glad you did. You should just open an ask mister
wizard booth and leave Usenet to the argument junkies.

The inevitable question is how much will a better transformer change
the sound of the OP6. You know the scenario... people (maybe even you)
think 2622's are lowbrow but 312 cards are worth more (and
theoretically sound "better") with them than with Jensens.

If the Junky xfmr was a UTC (seems to be the best guess) they made (or
perhaps still make) a very similar can called an "ouncer" that Stephen
Sank said would work but they're known to be some of the worst sounding
in the UTC line. Perhaps it's perfect?

If you don't mind answering the "what would you do if it was your
decision to make" question I'd appreciate it. I've taken your advice
before and I've always been happy with the results.

Also, can I email you off list for a similar but unrelated issue?
Perhaps I can actually pay you for your time for a change.
Thanks,
Don
 
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db2100 <donambory@mac.com> wrote:
>It's a needle in a haystack I know but I need the input transformer for
>an RCA OP6. The code is k900 849-501 it's a cylinder approx. 2.5" tall
>x 1.5" wide.

It's a pretty junky step-up transformer. I don't think you can get
anything that bad.

If you want something that sounds good, try a Jensen KE-115 in that
location.

The original has 250 and 30 ohm inputs with a grounded center tap.
You'll want to lift the center tap for better noise rejection anyway,
and you'll probably never want the 30 ohm input. So anything with a
250 ohm primary and a high-Z secondary will be fine.

If your unit was originally shipped for use with crystal microphones,
the input transformer and a couple resistors will be gone, replaced
with a DC blocking cap, a shunt resistor, and a shunt capacitor.
Refer to the schematic in the manual for how to undo this.

>Also, if anyone knows the "MI" # for this tranny (or if it would even
>have an "MI" #) that would really help the search.

It does not have an MI number, it has a K number.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 
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db2100 <donambory@mac.com> wrote:
>I have no idea how you learned all of this disparate and often archaic
>info but I'm sure glad you did. You should just open an ask mister
>wizard booth and leave Usenet to the argument junkies.

If you want to pay standard consulting rate for the advice, you're welcome
to. I take paypal, even.

>The inevitable question is how much will a better transformer change
>the sound of the OP6. You know the scenario... people (maybe even you)
>think 2622's are lowbrow but 312 cards are worth more (and
>theoretically sound "better") with them than with Jensens.

It will change it a lot. I think it will change it for the better. It
won't change it as much as replacing the output transformer would.

Remember these things are designed for voice-grade applications. The
input transformer has a _lot_ of parasitic capacitance in order to get
as high a ratio as possible. This gives you basically free gain, but
it also pretty much cuts off the top end.

Sowter might still make a high ratio transformer like that as a specialty
item.

>If the Junky xfmr was a UTC (seems to be the best guess) they made (or
>perhaps still make) a very similar can called an "ouncer" that Stephen
>Sank said would work but they're known to be some of the worst sounding
>in the UTC line. Perhaps it's perfect?

RCA made a lot of their own transformers. They did use some UTCs,
although I think the ouncers and sub-ouncers are too new to have been
used in those. They also have since been discontinued so you won't be
able to get them.

The ouncers are also smaller than the original transformer.

Hmm... maybe some of the Tamura Microtran transformers might do it? Again
they'll probably be better than the originals, but a lot less so than
the Jensens. They won't sound the same on the low end either since the
are made with much more modern core materials.

>If you don't mind answering the "what would you do if it was your
>decision to make" question I'd appreciate it. I've taken your advice
>before and I've always been happy with the results.

If it were my decision, I'd put Jensens in. But then, I always threw
out those cheap transformers on Altec gear and put Jensens in too,
which turns out to devalue them considerably. Also those godawful
Beyer things in the Altec mixers...

I forget... is the transformer on this an octal-socket thing, or is it
mounted through a 5/8 stud? I remember it being a round can type.

>Also, can I email you off list for a similar but unrelated issue?

Sure!
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 
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I was told it's attached with 2 screws under the can (not on flanges)
but they could be studs, I haven't seen it yet, I had it sent directly
to Mr. Sank.

I got a couple of emails from this post so it's possible I could find
an original but if not I found this:

http://www.sowter.co.uk/acatalog/SOWTER_PRO_VINTAGE_TRANSFORMERS_6.html

3rd from the bottom, # 9835 for the BA-6A

Unfortunately the BA-6A schematic that's floating around the net is
"simplified" and doesn't have the input xfmr's "k #". Anyone know? If
the OP6 and BA-6 are the same then that's my second best.

As for the issue of parasitic capacitance and loss of top end, is this
inherent in the xfmr design or can this be controlled by the way it's
wired, etc?
 
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db2100 <donambory@mac.com> wrote:
>
>I got a couple of emails from this post so it's possible I could find
>an original but if not I found this:
>
>http://www.sowter.co.uk/acatalog/SOWTER_PRO_VINTAGE_TRANSFORMERS_6.html
>
>3rd from the bottom, # 9835 for the BA-6A
>
>Unfortunately the BA-6A schematic that's floating around the net is
>"simplified" and doesn't have the input xfmr's "k #". Anyone know? If
>the OP6 and BA-6 are the same then that's my second best.

No, they aren't the same thing. This is a much higher grade transformer
than was used in the OP6. The OP6 uses a communications grade transformer
intended for voice applications only.

>As for the issue of parasitic capacitance and loss of top end, is this
>inherent in the xfmr design or can this be controlled by the way it's
>wired, etc?

It's inherent in the transformer design. It's easy to make low ratio
transformers. It's hard to make high ratio ones. Some fancy winding
techniques can reduce the problem, but at the expense of cost. Jensen
is doing some other tricks that they won't describe in detail, either.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."