Ba6a interstage

[baadc0de]

Can I use an UTC A16 as the interstage? What kind of impedance ratio did the original have?

Thanks!
B

 

[EvLoutonian]

Sowter currently offer the following for this unit:

9830
RCA BA-6A Compressor Interstage
High performance transformer for repair or reconstruction of the RCA BA-6A Compressor.
Radiometal (50% Nickel) transformer in Mumetal can. Will fit into original 78 x 100 mm can.
Impedance ratio 10k CTK/10k.  Colour coded leads.

 

[emrr]

I keep asking if anyone has built successfully with the Sowter, getting full frequency response, no evidence so far. 


----------edit----------

see later post

 

[shabtek]

A-16 is physically smaller and does not have center tap-- A work around may be possible at the cost of diverging from Ba6a.

the scheme I have for ba6a shows 22k plate resistors (possibly penciled in) for the 1st stage, and 4.7k (penciled in?) resistors in parallel with each half of the interstage primary.--does not appear that c.t. is joined to junction of 4700Ω pair...can someone verify or refute?

 

[lassoharp]

Seems to be the dead end for trying to build a respectable clone.  Sowter's = ~200USD.  Not enough L for the job = poor bass.  A ton of work & money to build one and come up short, apparently way short . . . . . why even bother?     

SA-39 is your alternative for a big Vari-Mu . . . . until somebody does the RCA interstage right.


If you're just building one for a color unit - may as well use cheaper alternatives for all the signal transformers . . . . .   

 

[baadc0de]

Hi there,

didn't check the thread for some time .. thanks for chiming in guys.

I'd like to make a clone really and the interstage seems to be one of the things in the puzzle that really bugs me. It would be good to know if CT on pri is required and what kind of transformer is required. Has anyone done a disassembly of the original, if so how was the transformer made?

There are transformer winders in EU and USA that I think may be willing to tackle this if they knew what specs to go for, no?

 

[emrr]

The truth is likely that if made 'right' it would be more expensive than the market would bear.

 

[lassoharp]

I think may be willing to tackle this if they knew what specs to go for, no?

Dennis "Doc" Hoyer may have rewound some of these before.  I know he has rewound Collins 26W interstages and the RCA 86B which may be similar to the BA6A.  It wouldn't hurt to contact him.

http://www.roger-russell.com/autran.htm

David Geren at Cinemag may be willing to tackle.  I wish somebody would.  Rare vintage iron is not a volume market but I can't see why BA6A interstage wouldn't sell as many as the Pultec output. 



I believe you have some very thin wire (thus easier to burn up and go open) and a higher AC impedance spec (more stray capacitance that fights against high freq response) than most modern transformer makers seem to use or are willing to use.

 

[lassoharp]

I suppose this thread is as good as any place to bring up for discussion and speculation. 

Since UTC manufactured some, what ever became of the design specs?  Did UTC keep them on file permanently or turn them back over to RCA?  If kept, then worth asking if Magnetika still has them?    RCA throw them away when the patents ran out?


According to an older post from CJ Brain Sowter has dissected one.  If fear of it being too expensive is the case maybe it's worth looking at a group buy and see what happens.

Brian Sowter has disected the innerstage UTC. It is high nickel with a gap, which is very weird. Most of the time you see M6 used with a gap.
    I am thinking the gap is for when the compressor is working at max gain reduction where PRR mentioned that the tubes, even if carefully balanced at idle, will exibit a mis-matched condition, thus allowing dc into the core, which reduces inductance sharply and thus produces a thump.
cj

 

[lassoharp]

It would be good to know if CT on pri is required

Yes.  It is a push pull circuit and the B+ is fed to the gain reduction tubes through a CT.

 

[MatthisD]

Out of interest, do you have any inductance measurements of the Sowter version? looking at the components around the primaries I would think that each winding should be 150H or higher to give good frequency response.

 

[emrr]

I don't have BA-6 measurements.  I understand (apples to oranges) the Sowter interstage for the 86 limiter is calculated to be about 100H, my handheld meter shows around 470H per primary winding on two RCA examples, 1860H full primary, testing at 120Hz.  930H on the sec, which is a step down ratio in the 86.  Those are 5300 ohm DCR primary and 3800 ohm DCR secondary, more representative of a 50K+ primary.  56K-68K resistive loading on the secondary, part of which is the stepped output gain control. 

 

[lassoharp]

the scheme I have for ba6a shows 22k plate resistors (possibly penciled in) for the 1st stage, and 4.7k (penciled in?) resistors in parallel with each half of the interstage primary.--does not appear that c.t. is joined to junction of 4700Ω pair...can someone verify or refute?

The schematic I have shows the 4.7Ks (R14 & R15) are joined at the CT.  The 22K plates are there on the factory drawing and form a divider with the 4.7Ks.



I spoke with Doc Hoyer.  The original winding data has apparently been thrown away years ago.  Magnetika doesn't have them.  He said he has enough info from rev engineering them to wind them.  I inquired about the possibility of doing a group buy for those who are seriously interested.  They won't be cheap of course.  How many out there are serious about doing a point to point build?

 

[baadc0de]

I'd be in for two, of course.

 

[shabtek]

I'd likely be in for 2 in a GB.

 

[Woot]

Hey guys,

Interesting info if not a little above my head. Could someone please explain why the current offering by Sowter is not ideal for a replacement?

I may be up for a pair in the group buy

Cheers,

Matt

 

[baadc0de]

Hey guys,

Interesting info if not a little above my head. Could someone please explain why the current offering by Sowter is not ideal for a replacement?

I may be up for a pair in the group buy

Cheers,

Matt

It has been suggested that the bass response will depend on inductance. It's a spec that Sowter doesn't share with us and another transformer from them meant to replace a vintage one in a big vari-mu compressor was found potentially wanting. Neither do they share the frequency response or THD at, say, 20Hz which would be even more helpful. It doesn't mean that it doesn't work in circuit (drip's video proves that) just that some other transformer may work more like the original.

Cheers,
B

 

[Woot]

Thanks for the reply. Has anyone here used the Sowter interstage for the Ba6a and found it not to their liking? So this lack of bass response can be objectively measured as you suggested below?

Thanks again

 

[lassoharp]

Thanks for the reply. Has anyone here used the Sowter interstage for the Ba6a and found it not to their liking? So this lack of bass response can be objectively measured as you suggested below?

Thanks again

There are at least 3 members that I'm aware of that have used them but no measurements have been posted.  One member said they liked it a lot in a point to point clone, another member said they didn't and was asked why with no response.  If anyone has one and is willing to lend out I will build up a tester circuit.  I have access to the original OT and have at least a couple of good inputs that are probably close enough to the original to give an accurate picture of how it will do in circuit.  Doug is willing to provide a set of sweeps.  I was told that the Sowter 86B iron tested very well out of circuit but did not measure up well at all in circuit so there's a bit of a mystery there but that is a different circuit than the BA-6.     

 

[lassoharp]

Good news!  Jackies has been kind enough to lend us a Sowter interstage for testing.   So, here is the test circuit I promised.  The transformer should get here next week and testing at emrr's will soon follow.