Optimation PA-250AC Lab-Grade Audio Amplifier - diyAudio
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Old 21st May 2012, 09:35 AM   #1
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Default Optimation PA-250AC Lab-Grade Audio Amplifier

Hello everybody!

I was recently given this amplifier, an Optimation PA-250AC, and I would like to know if any of you all know anything about it. As far as I can tell, it is NOS, or near NOS. It even came in its original shipping box! I also have all the tubes for it and the manual. First of all, this thing is a monster and weighs in somewhere around 90 pounds. According to the manual, it is a lab-grade audio amplifier, capable of delivering 250 watts at 0.01% THD! At reduced power, the frequency response is 5 to 100,000 Hz. It takes 8 EL34/6CA7s on the output stage. It also has 3 impedance outputs, 3.75, 15, and 60 ohms. It's also capable of delivering a constant-current or constant-voltage output, and the meter can measure the output voltage, output current, or the plate current of each individual output tube. It uses both negative and positive feedback to keep distortion to a minimum, and even has overload protection. Overall, it sounds like a pretty amazing amp. Also, the front panel is hinged so you can easily access the circuitry. I know tube amplifiers pretty well, but this thing is far more advanced than anything I'm familiar with.

I've included a few pictures of this beast that I took before I installed the power tubes and played with it. First of all, it sounds incredible! It goes down to 5 Hz with ease, and it has power to spare.

If any of you know anything about this monster, or have had any experience with one or something like it, please let me know! Apparently it's pretty rare, since I couldn't really find anything about it on ol' Google, but just because something is rare doesn't mean it's worth anything.
Attached Images
File Type: jpg Optimation Front.jpg (484.2 KB, 409 views)
File Type: jpg Optimation Rear.jpg (483.5 KB, 391 views)
File Type: jpg Optimation Inside.jpg (481.6 KB, 388 views)
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Old 21st May 2012, 04:41 PM   #2
kevinkr is offline kevinkr  United States
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Rather cool, probably would make a great subwoofer amplifier..
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 21st May 2012, 06:04 PM   #3
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It sure does! The only speaker that I really had to test it with was a subwoofer, and it was fun to take the amp down to 5 Hz and just watch that cone move in and out! As for music, it sounded incredibly good on anything from jazz to hard rock and techno. In fact, it was probably the best sounding subwoofer amplifier that I had ever heard!
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Old 21st May 2012, 07:15 PM   #4
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I know this amplifier. They were used for AC calibration to supply wideband high voltage from an oscillator. The full system was an AC104A consisting of RCD1062 sinewave oscillator. RT1003 ratio transformer. T103 matching transformer. And PA1015 power amp. I have several of these amps in storage and gave away,as scrap, several others when I was forced to relocate. PA250 was also a valid designation for the HV model.

Some versions did not have 100% interleaved laminations in the OPT, while the other ones did. They were supplied in several configurations depending on how the output was trapped for voltage. The meter scales were accordingly different. Some of the amplifiers I had used 8417s while others had EL34s like yours. Optimation was taken over by Julie Research Labs in NY. (Lobe Julie)

Of the amps I do still have (3 or 4?) I had planned on rebuilding with the transformers and 6550s, but it's been quite a while and I may never get to it. One person I know actually listened to music through them and thought they were pretty good. Most all of mine came from Government surplus like yours. I recognize the sticker on the front panel.
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Old 21st May 2012, 07:18 PM   #5
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Julie Labs was my first thought when i saw these. Yes, they are very good when set up right. No one knows what they are so they get junked despite their huge build cost.
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Old 7th May 2016, 04:10 PM   #6
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I see that this thread is from some time ago but thought I'd share photos of the CA-250's I once had that were used at International Telephone & Telegraph. They sounded pretty good and were more than adequate to drive a pair of Innersound Eros ESL's; but it took many, many heavy trips back and forth to a gentleman who I luckily found that was adept at sophisticated electronics engineering to get them up and running up to par. I used 16 Sylvania 6CA7's -- aka, the "Fat Bottles" -- to great success. Holy Toledo did these amps put out some serious power; 600 volts if not more running through the point-to-point circuitry(Same voltage running through the MBTA Commuter Locomotives that run the Lowell-Boston-Providence corridor). Kept a fire extinguisher handy in case; not because they were risky, unless stupidly mishandled by immature hands, but because you never know with that much juice running through something like these. Pretty unbelievable pieces of engineering, thanks to Henry Wolcott.



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Old 8th May 2016, 02:04 AM   #7
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Someone have schematics to post?
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Old 8th May 2016, 11:40 PM   #8
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Originally Posted by smoking-amp View Post
Someone have schematics to post?
The O.P says he has the manual, maybe the schematic is included and he could post it here ?
He didn't specified the full tube complement but I have the schematic of the Optimation PA-1006 which looks pretty close. (very likely another variation of the same circuit ?). This is the only schematic I've ever found and the original was a poor quality underexposed picture. I've cleaned it a little bit and re-adjusted exposition/gamma parameters, it's still far from perfect but mostly legible. From the schematic you can see it's a very complex amp using fully balanced circuits and many adjustments and other refinements. (even the overload indicator uses a special 7323 subminiature thyratron tube !). Engineering specs are probably outstanding but industrial grade/laboratory amplifiers (like the Krohn-Hite, Optimation, etc...) are usually poorly rated by Audiophiles for some reasons I don't know... They looks impressive and being industrial/professional purpose amps very well built.
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File Type: jpg OPTIMATION Amp.2.jpg (770.8 KB, 181 views)

Last edited by Tubologic; 8th May 2016 at 11:47 PM.
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Old 9th May 2016, 01:08 AM   #9
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Thanks for the schematic. I'm curious how similar it is to his patent schematic below:
Definitely some similarity. Have to trace out the feedbacks yet.

Well, I see the same positve Fdbk inner loop, and the "local" output plate feedbacks to the 6GW8 triode cathodes.(also to the crossed triode grids)

In the patent drawing, there are series caps in the "local" plate Fdbks. I have wondered if these were small caps or large ones. Small caps would allow the local Fdbk to increase with frequency (so reducing the global Fdbk with frequency. The PA-1006 does have 22pF caps across 470K resistors, so "local" N Fdbk starts increasing around 3600 Hz. There's actually two similar freq. shaping networks in series. So, in effect, the N Fdbk transitions from global to "local" as the freq. increases.
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File Type: gif Wollcott-1.gif (21.4 KB, 173 views)

Last edited by smoking-amp; 9th May 2016 at 01:36 AM.
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Old 25th December 2016, 10:53 PM   #10
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Default Love these amps!

I found this site searching for some info on the Optimation PA250AC power amp. (I was feeling insecure because I can't seem to find my manual, and I appreciate the schematic post!) I acquired two of these amps back in 1999 from an electronics repairman/genius who got them when a local Motorola factory closed down. So, they were fully calibrated and up to spec when I received them. And it was these amplifiers, speaking through my homemade Morel-driven speaker cabinets, that showed me just how GOOD music can sound! (I moved up from an Adcom GFA 545 II and learned that there was stuff hiding in the music that I was missing.) I added a subwoofer, split off with an electronic crossover, and I have all the fabulous sound I'll ever need. My only regret is that I can't drive the subs with Optimations as well.
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