WTB: Crappy line input transformer for Federal AM864

Hey all. Trying to find an input transformer for my Federal AM864 build.

I can find plenty with excellent frequency response, but I'm looking for one that does something like the "speech" versions of this unit. Like 300Hz-5kHz or something terrible like that.

Needs to be 600/10k ohm and be able to handle DC on the secondary.

Might also be interested in a 600/600 ohm version instead/also.

Let me know if you've got something like this, or can point me in a direction. Thank you!

10K secondary has always been an odd value for a PP input, that'll be tough to find.  10K:10K would be easier.  Poor response?  You may have to browse the random no-name ebay offerings for awhile.    I'll look at my list and report back if anything surfaces. 

series capacitor\s on input to limit low end parallel to shunt some hi end.

to emrr:

Been looking at ebay for a while trying to find something suitable. No luck so far, so I posted here, but thanks for the suggestion.

I would be open to 10k/10k as well. Thank you for looking at your list; please let me know.

I'm pretty open to a few different configurations on the input, really. I'm either shooting for something like stock (band-limited 600/10k), a full range 600/10k with a band-limited 600/600 in front of that (this is probably my most preferred option; I could switch the "crappy" in or out of circuit as needed), or a band limited 10k/10k (I'm not anticipating needing all the gain this thing is capable of?).

For now I'd just like to find something so I can plug it in

to shabtek:

I don't think I can put series caps in there. The gain reduction on these works by varying the DC voltage on the grids of the input pentodes, which is fed to the grids via CT on the input transformer secondary. Series caps would block this, yes? Please correct me if I am wrong. Thank you for the suggestion, though, I hadn't considered something like that....maybe I'll mess with that until I can find the right one.

(although I suppose I could put series caps on the primary?)

yes some 'tone shaping' can be done on the primary

my great gramma said you can always get warmer but can't always get cooler

series capacitor\s on input to limit low end parallel to shunt some hi end.

you need a center tapped secondary 1:4 ratio according to the schematic

Maybe I'm misunderstanding, but I'm my ebay experience the vast majority of affordable input transformers out there roll off at 300Hz. Like this one:

http://www.ebay.com/itm/667-9521-00-ADC-AUDIO-TRANSFORMER-600-TO-600-OHM-300-Hz-to-22-KHz-USED-/200945653489

As well as the bajillion chinese telecom ones that you can even find at radioshack. Those are typically about $5 each and much smaller than even a JT-11.

Is it the specific impedances you're looking for that's hard to find?

to shabtek:

I gotcha. Posted that reply a few minutes too soon; I'm also half running a session at the moment so my focus isn't quite all there at the moment. Thanks again for the suggestion; I'd like to try to find transformers that just do it for me, but I'll keep that in mind for when I can't find em.....

to hymentoptera:

The problem that I'm having is finding one transformer with all the things I want in one package. Impedances/ratio yes, but also CT secondary (which the one you linked does not have, but might be otherwise suitable with a shunt cap?), magnetic shielding, tolerates DC, frequency response, and size.

I think some of my difficulty in finding one that fits my needs stems from the fact that I'm not super familiar with all the different brands/types of transformers out there, so I may be skipping over suitable ones simply because what I want isn't explicitly stated in the auctions.

Thanks everyone for their input.

Ah I see. That makes sense now.

Maybe email one of the big manufacturers like Sowter and ask their reps if they already make something that meets your needs?

Even if you can't find the right (poor) freq response, I would imagine you could insert an EQ before the Am864 and tailor the signal to need. Something with LP/HP or shelves? Then you'd also be able to adjust levels going in much easier.

Here's a real Federal in action.  No top loss without compression.

https://c1.staticflickr.com/1/456/18754751280_e31efb921c_o.png

Can I assume this is a plot of the version with "full range" transformers?

To hymentoptera:

While inserting an eq before the limiter would certainly achieve the response I'd like sonically, it would be just another parameter to adjust and fret over, and an eq unavailable to use on something else, should it be needed. I'm a big fan of fewer options in the studio, at least on a piece-of-gear specific level. We have things we can tweak if needed; I'd like this particular box to be more of a sounds-good-use-it/sounds-bad-take-it-off kind of thing.

I want it to sound great and measure "bad" out of the gate.

Dreams said:

Can I assume this is a plot of the version with "full range" transformers?

Click to expand...

Nope....and as far as I'm concerned the 'full range' federal is still a rumor, I have still never seen even a picture of one with better iron. 

You want bad, go fond my plots of the Department of Commerce limiters and then go find one.  I don't think you'd have an easy time finding other iron that did that; maybe winding your own.

600:600 with bad response:  almost impossible to do, maybe no lows but to kill lows and highs in a trans so simple would be a trick. 

I have UTC S5 and Hammond 312 both of which are unused. The only problem is the ratio.  They are  500 :50,000

EMRR, am I correct in reading that graph as 'the processed version is normalized or boosted 10dB' to illustrate how the spectral response varies;

Yes

I just looked through my box of limited frequency response transformers and did not find a perfect match for what you need.  I did find a Chicago Standard 600:600 that is spec'ed at 400Hz - 3kHz +/-1dB and operates from -48 to +18dBm.  HOWEVER when I connected it to my Amber 5500 test system I found the frequency response was FAR better than that.  With 600 ohm source and load it was only down -0.65dB @ 20Hz and -0.08dB @ 20kHz, relative to 1kHz at 0dBm input.  Changing the load to 100k, it was -0.94dB @ 20Hz and +0.17dB @ 20kHz.  So as Doug said, you can't really screw up a 600:600.

I have also heard that many of these pieces were required to have a certain specification (such as 400-3k +/-1dB) but it was fine if they spec'ed better than that.  So if the mfgr had a shelf full of 20-20k they could label them 400-3k with the new part number and ship them off.

emrr said:

Dreams said:

Can I assume this is a plot of the version with "full range" transformers?

Click to expand...

Nope....and as far as I'm concerned the 'full range' federal is still a rumor, I have still never seen even a picture of one with better iron. 

You want bad, go fond my plots of the Department of Commerce limiters and then go find one.  I don't think you'd have an easy time finding other iron that did that; maybe winding your own.

600:600 with bad response:  almost impossible to do, maybe no lows but to kill lows and highs in a trans so simple would be a trick.

Click to expand...

This is very useful info, emrr.  I am looking for an input xformer to build one that stays closer to the original than a hifi recreation but all I can find that is shielded w/ct is at least radiometal or 80% ni.  Do you happen to know if the originals were steel core?

I don't know.