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Old 19th January 2008, 01:59 PM   #1
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Default Tube rating

Hello. I'm considering building Rozenblit's T8 OTL. Seems to have been a successful design and yet the 12AU7s are running very hot, one at 475V on the plate (manual says 330V max for this tube).

I'd like to ditch the 12AU7s in favor of a 6SN7GTA/B which at least has max rating of 450V. Whatchall think? Thanks..
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Old 19th January 2008, 02:05 PM   #2
Colt45 is offline Colt45  Serbia
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schematic?
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Old 19th January 2008, 02:47 PM   #3
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Sorry, imageshack not liking me this morning. Full schematic will not upload. Here's the signal portion:

Click the image to open in full size.
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Old 19th January 2008, 02:50 PM   #4
Sherman is offline Sherman  United States
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It appears there is only 100V across that tube. 475 at the plate, 375 at the cathode. Way under the 330V rating. Am I missing something?
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Old 19th January 2008, 03:43 PM   #5
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Nope, you're missing nothing. Apparently I'm an idiot. I still might try *SN7s here, though. Not a big fan of the *AU7 as an audio device.
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Old 19th January 2008, 04:28 PM   #6
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Quote:
Originally posted by CarlyBoy
Nope, you're missing nothing. Apparently I'm an idiot. I still might try *SN7s here, though. Not a big fan of the *AU7 as an audio device.
I don't have a problem with you wanting to change out a tube type, but why do you have to add a myth/stereotype/nonsense at the end? There is nothing wrong with the 12AU7... and according to Crowhurst, "the tubes are nearly identical, except the 12au7 is a miniature tube type."

The 12au7 can be used in and has been used in many fine sounding amps, it's cheap and NOS readily available; as well as the current production JJ tube which is a great sounding/performing tube too.

So you could build it as it was designed, and then make your decision on the sound quality, or you could swap it out and get a very similar sound from a 6sn7; which should take minimal circuit tweaking because the tubes are so similar.
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