Counterpoint SA-12 outputs 2SK134/2SJ49 work?? - diyAudio
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Old 18th January 2009, 08:06 PM   #1
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Question Counterpoint SA-12 outputs 2SK134/2SJ49 work??

Hi gang,

Relatively new here, but been building hi end audio since the early 80's...

I have just dug out my old Counterpoint SA-12 after 10 years of storage and found a bag of MTM12N10's strapped to the side. Mmmm...seem to remember something about one channel failing??

Well, upon removal of the FET caps, I see that it has been repaired before with IRF130's and the /\/\ mentioned above. Well, knowing how fussy this amp is with using matched output devices, it's time to fix it properly. I have searched the web for the last 2 days and can't find any hard evidence of reasonable subs. I believe the original MOSFETs had PTC characteristics, and that seems to be hard to find in complementary pairs. Then I remembered I have 2 flats of 2SK134/2SJ49's sitting in my pile of valuable junk in the shop and thought I'd dig up the specs on those. Can't find temp curves but it looks from reports, that these had positive temp coef. as well. They are rated at 7A instead of 10 or 12, but they were still sucessfully used in pairs in 100WPC amps.

Your thoughts? Can I just drop these in, rebias to around 100-200mA and go?

Cheers

Paul
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Old 19th January 2009, 01:25 PM   #2
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Have a look here:-

http://www.datasheetarchive.com/
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Old 26th August 2009, 11:06 PM   #3
SA-12 is offline SA-12  Canada
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any update to this thread
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Old 28th August 2009, 05:17 AM   #4
anatech is offline anatech  Canada
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Post Whoa there! Careful with mosfet outputs in these amps

Hi Paul,
The only sane way to rebuild this amplifier is to order a set from altavistaaudio, refrain from any of his upgrades.

I have hand matched a set of outputs for an SA-220 and also an SA-100 (same as yours almost) in the past. It took me two days of careful work and 50 of each mosfet to get a set I was happy with. In fact, it turned out to be a tighter set than any I've seen in these units. Since there is no source resistors used, the match is hyper-critical. Not really a good business plan if you ask me. Use the original numbers though! Right or wrong, they worked in this amp.

I used to do warranty service for Counterpoint in Canada. I am pretty familiar with them by now.

I am developing a modification to the output stage. I have already designed a couple other critical modifications to the basic circuit, the power supply and the DC offset balancing circuit (no, not active). The sound quality and stability has been vastly improved without losing the good part of the "Counterpoint sound". You can still identify it by sound only.

BTW, the original circuit requires that the 6922 tubes also be carefully matched. There is no getting around ridiculous amounts of by hand work if you plan to restore it. Some day when I'm happy with the output circuit change, I'll offer it as an upgrade. Right now there are two upgraded amplifier prototypes in use with a very happy Counterpoint owner. They are a few years old and still going fine, one is loaded with a very poorly matched set of outputs as a test. The new outputs are BJT types, and not a normal output stage either. Please proceed with great care here.

-Chris
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Old 20th October 2013, 04:10 PM   #5
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Default It's alive...it's alive

Well crap! 4 years later.....

I pulled it out of the shop last week and started working on it again. Life just got in the way 4 years ago and I never did get back to it, and I didn't get any replies while I had it on the bench so I just parked it again.

So I fired it up on my test light bulb and watched output offset, and watched for smoke , and nothing..it worked, about .02vdc offset on each channel...but then I noticed that R52 was cooked. Shut everything down and checked the resistance...should be 2.7K but it was burned open. Aha, no B+ to the 6922's, that would kill the sound. Replaced that and fired it up again..this time after the relay clicked off (45 seconds..way to quick) I saw the output voltage wonder all over, up to .5VDC but it settled right in at around 0, +/- .03VDC. Checking the bias today and setting it...and I found the cause of the cooked B+ R52, there is a shorted 6922 when it heats up. Changed all the tubes and everything seems to have settled out.

So that's the update...and I'll keep you posted as I make progress. The outputs were fine and everything seems okay.

Chris, I recall in other posts you mentioned a few design flaws in the low level stage..something about local feedback with the first tube's cathode R, or maybe it was the long tail pair's ground point...and there may have been others. Could you summarize what you have discovered please and thanks?

Cheers

Paul
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