I've been searching for info on the output Transformers for the 428B and 429B preamps in the Altec 250A, but am coming up empty-handed.  I know from the schematics that they have a tertiary winding, which makes me wonder if they're the same, or similar to the S-217-D. Anyone know much about these?

 ::cough-Doug-cough::

I actually have a 428B (needs recap) with both the 4629 IT and 16267 OT, which I could test, given some direction, but I am lacking the 16266 for the 429B.

Any info appreciated.


EmRR

Pretty much same as the other catalog terminal board types, but with wired leads.  In most cases.  Not sure what the direct equivalent is on the outputs, if any.  The input is 80K sec like many other Altec.  458 and 459 transformers are the obvious custom with no equivalents from anyone. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Thanks Doug,
Looking at the 458 and 459 schematics, their OT's don't appear to have the tertiary winding.  Is it possible the S-217-D is similar, or is it known to be different?  I could test and compare the one I've got to that or others in the catalogs.

I'm good on the input side, 4665's and Triad HS-1's on hand for that.

EmRR

4665, 4629.  Same thing.  etc etc

The 429B is essentially a PP 6V6 output with a tertiary.  Look up the Langevin 117 data, see catalog.  Put tone through it and measure dB at primary, secondary (loaded) and tertiary.  Gives some idea of ratios, not accounting for core losses. 

458A is 600:120K 25K:600, for very specific design reasons.  There's an AES paper about those amps. 

« Last Edit: January 07, 2020, 03:48:08 PM by EmRR »
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Bumping this, wondering if anyone can help shed some light on the output transformer (peerless 16267)specs for the Altec 428B Mic Pre.

Looking at the Schematic (below), the secondary is very clear, but the Primary and Tertiary are not (to me) I'm hoping other clues in the schematic may be useful. Primary is center tapped, (could be split for other uses) push/pull from V2 which is a 12ay7. And the Tertiary also split/center tappedis wired as feedback and there's a note on gain levels by changing the way the tertiary is wired. "40 DB (or 34DB with series connection of feedback coil)"

I know full specs are unlikely, barring a CJ teardown/analysis, but a ballpark on the Primary and Tertiary would be helpful in choosing a potentially suitable substitute for a recreation.


EmRR

I'm not aware of a current production transformer that's close.  The RCA BA-21A output is the next closest thing I can think of that's semi-common.   Both probably are like a UTC A-26 with a tertiary, something in the 25K-30K range, could be 40K, minor differences in practice given all the other variables in play with a different transformer and different usage circumstances.  Most tertiary are going to be relatively similar for a similar winding. 

Still blows my mind that these Altec and the fan favorite 1567A are matching Z on the input, for mics. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Thanks Doug,
I was looking at this schematic next to a pultec, as the S-217-D was the first thing I could think of that also has a Tertiary feedback winding. Noting V2 is a 12au7 in the pultec, I checked out the datasheets for the tubes, and now have what is probably a dumb question: Is output transformer primary impedance selected to correspond to output tube plate resistance? I see the 12ay7 has a rated plate resistance of 20-25 kohms while the 12au7 is 6500-7700ohms. So if the 16267 has a ~30K pri as you suggest, and the S217D has a 12.5K pri, could the 428B design be modified to use a 12au7 for V2 and a S217D OT with similar results?

Forgive my ill-informed questions and tenuous logic.


EmRR

458A, four 6072 plates driving a 25K pri
12AU7, 2 plates driving 12K5 pri

see a pattern?  4x Z relationship, sweet spot for distortion versus drive, simplistically.  Usually see at least 2x.

implies actually the 428 could be 40-50K.  or like the BA-21A at 2x relationship.

Question is what are you trying to achieve?  Anything is a redesign, so why hew so closely to a particular amp design?  Knowing that would help. 

I have an AWA preamp that is basically a BA-21A, but with a 12BH7 output stage, much more massive output transformer.  These things were done. 

Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Goal is kind of up in the air. I have a 428B (which needs a recap) and was kind of hoping to build it a friend, but it seems unlikely given the mystery/scarcity of the OT. If nothing else, a better understanding of what I have is the goal.

I have a few 4665s and Triad HS1's and would like to build some Mic pres. (maybe even a modest mixer) Haven't done a whole lot in the way of exploring circuit candidates just yet as I had no OTs to go with them...until now that is, as I've come up on some S217D "clones" some of which aren't designated for pultec builds that I'd like to use here. That conversation should probably get it's own thread though.
« Last Edit: March 10, 2021, 03:35:16 PM by thecr4ne »

EmRR

Looks like the stepdown is about 16dB to pri, 50dB to tert.    Looks like roughly 25K, comparing some other known types.    The DCR of the primary is very low considering, about 350, and L looks like 65-70H @ 120Hz.

Notes say the BA-21 is actually 25K also, it's L is about 100H. 

The BA-21 feedback method allows lower and higher gain, the 428B can be modified the way I did the Collins 356A (there's a thread), but only for more gain in any remotely easy way. 

If it were me, I'd either be dealing with the price of another 428B or letting it go in favor of building a pair of something else, maybe maybe not related.  The tertiary winding locks you into a more limited gain control reality than other approaches, and one with a greater sonic change across the gain range due to the changing feedback.    The NewYorkDave preamps done with nice transformers are smartly designed approaches worth considering, 1 bottle, 1 bottle w/o feedback, or MILA.  All smarter than cloning a REDD or something like that! 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


rackmonkey

+1 on the MILA. They really are simple but ideal designs for what you look for in a tube pre. Your iron choices for those are vast.
Whether you think you can or you think you can't, you're probably right.

So all of NewYorkDave's original links are broken and I'm having a hell of a time tracking down even the schematics here. Threads are old and now devoid of images and functional links.  If he's got a new website or anyone has his original documents archived somewhere, it would be worth sharing. Or at least point me in the right direction since I can't seem to find a legible MILA schem anywhere. 

rackmonkey

MILA original doc. NYDave has been out of the DIY game for years now, so you won't find anything directly from him.

I'll look for the one-bottle design as well. But the MILA is his most versatile design, IMO.

Schematic shows 12AV7 tube, which was an overlooked, cheap and plentiful vintage tube that isn't manufactured anymore (as far as I know) at the time he designed it. But you can use a 12AT7 or 12AY7 with minor changes if you prefer something still being made. I've always just used 12AV7 because I have so many of them.
« Last Edit: March 12, 2021, 01:32:07 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.

rackmonkey

One of several schematics I've seen for the "one bottle". There are several versions floating around with different tubes. Looks like I screen-captured this one back in the day.
« Last Edit: March 20, 2021, 03:01:55 AM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.


 

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