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.
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.
Attachments
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!
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.
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.
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.
Attachments
Last edited:
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.
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.
Attachments
Last edited:
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.
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.
I found a PA-250 schematic someone sent me years ago, I hope they don't mind me posting it. Looks quite similar to the PA-1006 schematic already posted, except for the 8417 output tubes.
I think they were asking some questions about it. Probably was over my head at the time, I don't recall. Maybe if they are still around they can get some answers here finally.
The interesting thing to me is the apparent frequency selective transition from global N Fdbk to "local" N Fdbk at higher freqs. And the inner positive Fdbk loop for high gain, as in Wolcott's patent. The rest of the design seems like massive overkill and could be simplified for audio. Especially that phase splitter. And use TV Sweep tubes with a low primary Z OT (just 2 tubes for 100 Watt out). Twin/Crazy drive of course too. Still have to make that 0.01% distortion spec...
I think they were asking some questions about it. Probably was over my head at the time, I don't recall. Maybe if they are still around they can get some answers here finally.
The interesting thing to me is the apparent frequency selective transition from global N Fdbk to "local" N Fdbk at higher freqs. And the inner positive Fdbk loop for high gain, as in Wolcott's patent. The rest of the design seems like massive overkill and could be simplified for audio. Especially that phase splitter. And use TV Sweep tubes with a low primary Z OT (just 2 tubes for 100 Watt out). Twin/Crazy drive of course too. Still have to make that 0.01% distortion spec...
Attachments
Hi, I know this is an old thread, but I recently acquired a pair of these amps and I’m hunting for any information on them.
These were ofrom some kind of frequency oscillator.
So far, mine are not working. One motorboats as soon as it warms up. I’m rounding up the needed capacitors to deal with that. The Overload lamp comes in on the 2nd amp as it is powering up.
I have the two schematics posted here and my amps seem to be somewhat of a combination of the two. These do not have V4 and V5 but they use 6CA7 tubes. Also, there are several differences between the components in the amps vs those shown in the schematics. For example; the taps on the power transformer are identified differently in the schematic than in my amps.
Is an owner’s manual available?
These were ofrom some kind of frequency oscillator.
So far, mine are not working. One motorboats as soon as it warms up. I’m rounding up the needed capacitors to deal with that. The Overload lamp comes in on the 2nd amp as it is powering up.
I have the two schematics posted here and my amps seem to be somewhat of a combination of the two. These do not have V4 and V5 but they use 6CA7 tubes. Also, there are several differences between the components in the amps vs those shown in the schematics. For example; the taps on the power transformer are identified differently in the schematic than in my amps.
Is an owner’s manual available?
My buddy just sent me a link to this post. I’ve modded several of these amps for home hi fi and probably have more documentation and schematics for the various models that were produced than anyone. If you have a pair of the right model PA250AC the sound quality will knock your socks off.
A couple PA250AC’s driving a pair of Wolcott Audio Omnisphere’s. Henry Wolcott designed and patented the Optimation PA250’s and eventually sold it to Julie Research. Wolcott Audio Presence Amplifiers are based of the same circuitry as the lab grade Optimations. The Presence Amplifiers are more consumer friendly. Getting an Optimation dialed in can take some time but the results are amazing.
Hey, thanks for the reply!
Mine are on the back burner now. I never got them running. I replaced the caps in one, hoping that would take care of the motorboating- it did’t.
Sadly, the friend that was helping me with these passed away a few months ago and I haven’t done much with them after that.
I would like to give these another shot - maybe after Christmas.
Thanks
Mine are on the back burner now. I never got them running. I replaced the caps in one, hoping that would take care of the motorboating- it did’t.
Sadly, the friend that was helping me with these passed away a few months ago and I haven’t done much with them after that.
I would like to give these another shot - maybe after Christmas.
Thanks
I’m sorry about your friend. If you have the time and interest send me the serial numbers stamped on the input and output transformers of each amp you have and also let me know if the amps are labeled just Optimation Inc. or if they are Julie Research Optimations. There were a bunch of variations of these amps made and many of the schematics left out details in order to protect trade secrets. I’m any case I may have schematics that come close to what you have and can probably help you figure out that motor boating…. Whatever you don’t mess with the 2 pots inside the amps behind the output transformers.