API-Sowter Transformers

[CJ]

Brian just emailed me to tell me that the Sowter 9820 has been added to his web site as a replacement for the API 2622. I had sent him the specs after taking one apart, and it looks like he already had a cross!

Also, he is doing the API output. I took down all the notes but misplaced the winding resistance. If anybody has a loose API output, ala Tommytones, etc, could you please measure the resistance of any coil? (they all ohm out the same due to quad winding structure).
Thanks!

I am going to send Brian all the details on the output so he can do one.

He also told me that he is working on the BA6a innerstage!

Here is some of the email:

"
I dissected the BA-6A interstage transformer and found a surprise
inside! It appears to be a Nickel core with a very small air gap.
Quite a surprising thing to do because the gap kills the permeability
and hence the inductance. This transformer sits in such a high
impedance circuit (Assuming the modified circuit with series resistors)
that it needs all the inductance you can get. Presumably they decided to
go for a high Nickel core for low distortion (good idea) but they put
the gap in it to avoid saturation if the dc gets unbalanced. They made
the gap with a very thin tape (a few microns) and have a very well
engineered brass clamp to hold it tight. This would be to get the
highest possible inducance. I should think this unit needs to be left
on all the time and adjusted for dc balance every time it is used.

Best regards

Brian Sowter
SOWTER AUDIO TRANSFORMERS (E A Sowter Ltd)
The Boatyard, Cullingham Road,IPSWICH, IP1 2EG England

 

[jrmintz]

Hey CJ,

I've ordered a pair. When I get them I'll put them in some 312s and compare them to the real thing and see (or hear) what they're like. This is exciting.

:green: :thumb: :sam: :guinness:

 

[CJ]

Cool!
So I did not rip apart your last one?

:guinness:

 

[DrFrankencopter]

Hi CJ...I had one handy (tommytones API OP Xfmr), but didn't have a great meter. I got 8.5-8.7 Ohms on each tap.

Think I'll order some of the 2622 replacements...if the price is right.

Cheers,

K

 

[CJ]

Thanks Kris!
Sounds about right.
cj

 

[jrmintz]

------------------------------------------------------------------------Cool!
So I did not rip apart your last one?

It was my last one - I'm going to hook up with Soundguy. He's not terribly far away and he has an original 312. So we'll report back on how it goes.


:thumb: :thumb:

 

[kruz]

Excellent guys.. and thank you very much seth for your generousity..
now guys.. before ordering some let's hear from seth about the comparison..
then we could do a group order, there are really nice discounts on quantities at sowter
i maybe could handle that..

 

[Bauman]

So Mr. Ham,

going to get some of those 9820 to US?

cheers!
Fabio

 

[Peter Simonsen]

Just messured 3 "Tommytones" API output X-formers

All messured 10.9-11.0 ohm using a Fluke 112 true rms multimeter

Kind regards

Peter

 

[Bauman]

That's great to hear! :grin:

btw, thank you CJ and Seth for the great work on this one!!! :sam: :sam: :sam:

 

[TomWaterman]

This is awesome!!!

:sam: :guinness:

Thanks to Seth, CJ and Brian Sowter!
I'll be getting some of these for my API pair!

Cheers Tom

 

[soundguy]

I dont mean to be the rain cloud, but I think some of this excitement could be saved until someone actually hears it. Its my understanding that this was an existing part and as such, just about every one of the manufacturers out there has an input transformer that works well with a 2520. The question here is wether or not it sounds like a 2622...

dave

 

[Flatpicker]

[quote author="Chae H Ham"]...If there were a definite secured demand of say 40/...[/quote]Well, I could possibly go for a couple. What's your selling price going to be?

 

[PRR]

> working on the BA6a innerstage! ... adjusted for dc balance every time it is used.

Waste of time. It isn't the "normal" bias that may be unbalanced, but the balance as the push-pull variable gain tubes go into gain-reduction.

I don't have a plan handy. Assume the tubes idle at 5mA. We can put a pot in there and easily balance that down to 0.1mA unbalance at idle.

But that's at idle. To get 3dB gain reduction we probably have to push the tubes down to 2mA each. But we don't really control their current, we control their grid voltage. And we usually push both grids the same amount. But we "never" have exactly matched tubes. We can match the idle condition. We can match at both 5mA and 1mA, though we will have to test a lot of tubes to find two that match within 2% at two points. But then how about at 4mA or 3mA? They could be 10% unbalanced there, even if 2% balanced at 5mA idle and at 1mA. 10% of 4mA is a 0.4mA unbalanced DC current.

In deep gain reduction it is not such a problem. We push the tubes down to 0.1mA, they may actually be 50% unbalanced there, but that's maybe 0.05-0.07mA unbalanced DC. The worst case is at the start of gain reduction, maybe 2dB to 6dB GR or 1/2 to 1/5th of idle current. Unbalance may be 10% to 30%, so we can have a large part of a mA unbalnced DC.

The plate transformer of a vari-gain stage needs to be large, enough iron to absorb the balance-shift that is sure to happen. It isn't a lot of mA, but it also is usually a very high impedance loop and needs a lot of turns. So DC tolerance is a problem. On the other hand, since it is a high impedance loop, we don't want capacitance, and a large transformer has more capacitance. A gap may be the best way out of a very conflicted compromise.

I think the best tube limiters are designed around the best available transformer technology of the time they are made.

 

[tonymite]

Can anyone comment on the Sowter 9820 API 312 Input type AP2622 replacement transfoermer? Does it sound like the original AP2622 ? or is the magic elsewhere ?? and shall I jsut stick to a CineMag ... and if so .. which is best ?? and which sounds closest to the original ?? if I want that API punch for drums ??

 

[guitarmaker]

I have no idea what an original API sounds like but I have made several 312s with both the Cinemag and Sowter iron. To my ears the Sowter are more aggressive sounding. I prefer the Cinemags myself but I'm recording acoustic instruments mostly. The Cinemags are very smooth, detailed, airy, etc... They sound more "hi-fi" for lack of a better term. I'd probably like the Cinemags on overheads and the Sowter on the snare. Just my impressions.

Steve

 

[Larrchild]

Odd, I measure 8 ohms dcr on my API 312-5 cards. hmm.

 

[tonymite]

[quote author="guitarmaker"]I have no idea what an original API sounds like but I have made several 312s with both the Cinemag and Sowter iron. To my ears the Sowter are more aggressive sounding. I prefer the Cinemags myself but I'm recording acoustic instruments mostly. The Cinemags are very smooth, detailed, airy, etc... They sound more "hi-fi" for lack of a better term. I'd probably like the Cinemags on overheads and the Sowter on the snare. Just my impressions.

Steve[/quote]

I'll be doing the Lundahl 1636>Forsell combination for the gentle stuff .. I'm aiming for the famous API bite for drums and just wondering if the magic is in the tx / op-amp or a combo..

Would I be getting a completely different sound if I went for Lundahl LL1538XL or a LL1578XL ??

 

[edanderson]

[quote author="tonymite"]Would I be getting a completely different sound if I went for Lundahl LL1538XL or a LL1578XL ??[/quote]

one of the first DIY projects i ever did was racking up a set of api 325 cards with LL1578XLs wired 1:10 as the input transformers. comparing those to other api preamps with the more "usual" suspects (jensen 110k and 115k, cinemag 75101) i would say it sounds different, but the opamp and the output transformer make a bigger difference.

ed

 

[tony dB]

having 24 (in the proces of building) 312's on my desk, I would like to know prices (yes this is urgent! :twisted: ). :thumb:

I want to put some Cinemag's, Lundahl's, Jensen's and Sowter's to cover all possible ends of the sound they deliver. First test run done with Lundahl on input and very positively surprised!!
This pre eats my other preamps...