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Old 11th June 2009, 10:02 PM   #1
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Default Old big/long plate 12AX7's & 12AT7's

I'm thinking most of these tubes must be really old like early '50's or late '40's, because I believe they were making small plate 12AT7's as early as the mid '50's. Anybody have an opinion on how these tubes sounded as opposed to the later small plate versions? I just picked up a sleeve of International Servicemaster labeled 12AT7's (so only know that they were made in USA) that have plate structures as large as, say, 12BX7's which have a much larger bulb and double the filament current rating. I also have a sleeve of Westinghouse branded 6AX7's which have very large plate structures, also. Generally speaking, I figure the big or long plate construction (more like the octal 6SL7 et al) probably should be an advantage sonically.

Haven't yet listened to them, but their grid windings generally sound pretty tight (usually means less microphonic) as per my 'tap' test.
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Old 11th June 2009, 10:29 PM   #2
SY is offline SY  United States
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Conversely, several highly regarded tube amp companies in the '70s and '80s sought out the tiny plate 12AX/T7s that came in a slightly fat bottle. Though the indicated country of origin varied widely, I understand that they had an Eastern European source.

These days, despite having maybe 20 or 30 different kinds of 12AX/T7s in my stash, I reach for the newer JJ versions. Significantly less distortion, low microphonics, low price. What's not to love?
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Old 11th June 2009, 10:45 PM   #3
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I haven't tried JJ tubes as of yet. Maybe I should give them a try. I was using the Chinese 12ax7 for a while, but eventually became rather dissatisfied with them - their level of detail was exceeded by more than one type of NOS tube I had. Btw, have you tried substituting 5751's for 12ax7's? I've recently switched to them on the input stage of my home brew phono preamp and am currently using a NOS pair that kicks *** as far as detail retrieval, etc. Don't want to say exactly which 5751 it is because it's getting kinda scarce and expensive and I'm stocking up, but I can say I tried it because someone in a 12ax7/5751 comparo said it had the absolute best detail of any he tried, but was too 'relentless' for his taste. So, I initially also found it, but effectively fixed that to my satisfaction by adding extra damping (mostly viscoelastic) materials to my preamp and power amp tube and chassis dampers. I should mention that this stage is operating as the first phono gain stage, without feedback and before any equalization.
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Old 11th June 2009, 10:59 PM   #4
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Hi,

Quote:
Conversely, several highly regarded tube amp companies in the '70s and '80s sought out the tiny plate 12AX/T7s that came in a slightly fat bottle. Though the indicated country of origin varied widely, I understand that they had an Eastern European source.
Probably Polish and East-German ones from RFT and so on.
Never really liked those, no close tolerance and pretty flaky.
Very, very cheap though.

Favourite ECC83 is still the unbeatable Telefunken.
RCA 7025 are very nice too.
ECC808 is better still, E283CC also but but run a different pinout.

Cheers,

P.S. 5751 is really a different tube.
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Old 11th June 2009, 11:21 PM   #5
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Ok, so I'll say what type it is, because I see that fdegrove has been quite specific. It's the GE black plate 3 mica silver clip 5751. I had some all along but only recently rediscovered them & tried them in place of some Philips late '70's 5751's and was suitably impressed. With the extra damping (= less overall microphonics in system), the 'relentlessness' fairly quickly resolved itself into a very good sense of neutrality and the best detail retrieval I've heard out of my system to date. Nothing has been lost as far as spaciousness, either, if the recording has it. In fact, the 3 dimensionality is more convincing than before, overall. There simply appears to be less 'interpolation' or 'smoothing' with this 5751 than any tube I've tried in the phono preamp input stage so far.
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Old 11th June 2009, 11:52 PM   #6
316a is offline 316a  England
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Sounds like what's in your Servicemaster boxes are 6414 . I've seen this sort of thing before , Edicron for example rebranded 5965 as ECC81 at one point . No doubt they were either having quality problems with Ei or had restocking issues

cheers

316A
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Old 11th June 2009, 11:58 PM   #7
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Hi,

Know that one but good luck finding some stock.

You're observation is however quite correct.
Given a sympathetic phono stage these are awesome little valves.

Not too many phonostages where actually designed around a 5751 though (C-J maybe?), so make sure the mere substitution doesn't alter the RIAA correction.

Cheers,
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Old 12th June 2009, 12:05 AM   #8
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Hi,


Quote:
Originally posted by 316a
Sounds like what's in your Servicemaster boxes are 6414 . I've seen this sort of thing before , Edicron for example rebranded 5965 as ECC81 at one point . No doubt they were either having quality problems with Ei or had restocking issues

cheers

316A

Yep.
Many years ago quite alot of computer valves of the 6xxx series found themselves rebranded as there was a shortage of decent quality valves of the ECC8x series.

That 6000 series quite often sported big plates in bulbs the size of a 12BH7A and slightly taller.
Microphony was of no concern, neither was noise.....

Cheers,
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Old 12th June 2009, 03:35 AM   #9
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Quote:
Originally posted by SY
Conversely, several highly regarded tube amp companies in the '70s and '80s sought out the tiny plate 12AX/T7s that came in a slightly fat bottle. Though the indicated country of origin varied widely, I understand that they had an Eastern European source.

These days, despite having maybe 20 or 30 different kinds of 12AX/T7s in my stash, I reach for the newer JJ versions. Significantly less distortion, low microphonics, low price. What's not to love?
The inconsistent quality. I had to stop carrying the ECC83S because the reject rate was approaching 50%! I simply couldn't afford to carry them, and I still don't.

BTW, I like RFT (the East German made tube you mentioned) stuff. They make as good an EZ81/6CA4 as was ever made, and their 12AU7 is a very well balanced, quiet tube with (IMHO) excellent tonal quality. Guitar guys kill for their ECC83s and EL34s.

They were relabeled (most often labeled "made in England") to avoid letting people know they came from behind the Iron Curtain.

Anyway, that's my $0.02 worth on it
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Old 12th June 2009, 05:23 AM   #10
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I checked the bulb size on these puppies - it's a standard size for a 12AT7. But check out that plate construction, assuming file attaches properly which it is not doing in preview.
Attached Images
File Type: jpg 12at7fatplate.jpg (20.2 KB, 600 views)
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