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In need of help (again) identifying some mystery (allegedly Fisher 125) transformers

S

Sam Lysinger

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I got these from a friend who got them decades ago from a guy who said they were out of a pair of Fisher 125s. I believe he laid eyes on them as he said he passed on the transformers, but the number of pins on a 125AX opt transformer on the schematic is far greater in number than what is on these. The two transformers are good and have a turn ration for 3200ohms of plate load to 16 ohms speaker, but they have a 48-49:1 tap on them that I have not seen on any fisher schematic I could get my hands on. Does anyone have any idea what these came out of?

Both transformers are 5" by 4.25" by 5.5" tall
The 6 pin transformer weighs 15.5 pounds
The 7 pin transformer weighs 15 pounds

Pics and voltage test data attached.

All insights are greatly appreciated, thanks!
 

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AlexHempel said:
That's some serious iron. What are your plans for it? Trying to replicate whatever it came from?
Click to expand...
I was wanting to run a pair of 6146s on them by putting a 16 ohm speaker on the 8 ohm tap to bump the load up to 6400, but I may be screwed. Interestingly, I did the math the other day and the RCA MI-12158 PA amps (6146s and 70W, perfectly distorted for rock and roll output, but I am learning how to better tweak them) and their plate load was 4300ohms, ironically the same as a Dynaco A431 (with a thinner laminate stack and no ultralinear taps.) The whole 6146 plan hinges on finding out whether these transformer can handle the 70-100w of power. If the 90A and 125A yield something close, then I am in business. If the 3200ohm load is an issue, there are always kt-120s (post supply chain issues, I will wait it out) given that Acrosound published a 6550 circuit for the TO-330 transformer (3300 ohms).
 
Love that plan. Rest of the circuit from scratch or adapted from the 12158? I'm guessing you're also aware of the MI-12182, which ran 6146s as well. At 15 lbs I'd expect they're good for at least 50 watts if not hopefully 70-100, especially if you don't need too much LF. Using an old RCA design would definitely be a fun idea if you're looking for a bit of distortion. For 6550s there are lots of schematic options -- I'd be tempted to do an improved Allen Organ 75 circuit, which would probably be more hifi.
 
AlexHempel said:
Hmm, also looks like the SA-1000 OPTs would have more terminals than these do?
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His one transformer has six taps the other has 7 .So they're not the same .if you look at the characteristic for the 8417 it falls inline for the plate to plate resistance.The other transformer is close as well but with only 3 taps it's probally only common 8 and 16 ohm or 4 and 8 .not to familiar with fisher amps so I don't know what these might of came out of .The sa 1000 definitely has 3 for the primary and four for the output
 
Based on the schematic it looks like the SA-1000 has an extra winding connected to both cathodes with a center tap to ground? To me it seems like this would require at least eight terminals to implement but maybe I'm missing something.

fisheropt.png
 
90A had only 8 and 16 taps, according to spec sheet at radiomuseum.org. 125AX had 4,8,16 and 70V; 125A had 8 and 16, appears to be identical with 90A (again, catalog page at radiomuseum.org). For that "odd" ratio, try using the opposite end as common - I think you'll show 8 and 16 Ohms.
 
Tom Bavis said:
90A had only 8 and 16 taps, according to spec sheet at radiomuseum.org. For that "odd" ratio, try using the opposite end as common - I think you'll show 8 and 16 Ohms.
Click to expand...
I will test that today and report my findings, thanks!
 
AlexHempel said:
Love that plan. Rest of the circuit from scratch or adapted from the 12158? I'm guessing you're also aware of the MI-12182, which ran 6146s as well. At 15 lbs I'd expect they're good for at least 50 watts if not hopefully 70-100, especially if you don't need too much LF. Using an old RCA design would definitely be a fun idea if you're looking for a bit of distortion. For 6550s there are lots of schematic options -- I'd be tempted to do an improved Allen Organ 75 circuit, which would probably be more hifi.
Click to expand...
I speak fondly of the distortion as the amp delivers plenty of gain and shock value, but I find it interesting that the NFB loop in the schematic has never been seen in any of the 12158s I own. That's another experiment, adding it back in to find out why it is missing. I have some MI-12182s as well, they have an NFB loop on the schematic and none in the chassis. At the end of the day, I prefer a cleaner sound. If I go by pure preference, tetrode / pentode comes first for the sharpness, ultralinear second, triode third. I like and own them all, it's a matter of small degrees in preference (the best amp being the one you are currently listening to that you built or serviced yourself ^_^).

For the Fisher transformers, regardless of power tube, I plan to use the Acrosound Ultralinear II circuit due to its differential amplifier driver / phase inverter. I was looking in the bottom of my 12158 after looking at its waveform on the scope and realizing that I can add a balance pot to the driver phase inverter and run it as-is, or just replace its circuit with the ULII. It's a crammed in PtoP mess in there, one has to use a chopstick to find the path the soldering iron will go before touching anything near the driver tube sockets.
 
Tom Bavis said:
90A had only 8 and 16 taps, 125A had 8 and 16, appears to be identical with 90A. For that "odd" ratio, try using the opposite end as common - I think you'll show 8 and 16 Ohms.
Click to expand...

Please see today's attached test results. They could run, but they could not hide from the gurus... We are one step closer to World Domination, MUHAHAHAHAHA!

Big thanks to everyone who has weighed in on this mystery!
 

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TriodeLuvr said:
Don't forget the 6146 is rated for +250V maximum on the screen.

Jack
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So true, crispy smoking tube death if you don't. Few 6146 amplifiers I have seen go above 210v at the screen. You lose tube longevity if you go beyond it. Air cooling these tubes is also recommended by the ham community. I was thinking of converting a Dynaco MKIII to 6146s now that politics is interfering with my tube addiction...
 
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