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12AU7 internally matched triodes

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passive420

passive420

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Joined 2013
#1
Doing research in preparation to build the Hybrid TubSumo amplifier by Valery here on the solid state forum. Thought this is obviously the best place to ask about tubes.

The front end IPS board has a 12AU7 and Valery recommends each tube having the triodes internally matched.

My question is: Are the triodes generally matched to a degree as standard or will I have to search / ask for them to be matched. I can obviously find matched pairs but is it possible to find pairs that are also internally matched?
 
#2
Some companies charge a premium for a 'matching service'. Personally ECC81 - 3 have a slightly higher performance on the second triode, pins 6 - 8 by design, so any valve that is matched is not to specification. It makes no difference anyway unless you are trying to squeeze an extra little bit out of them so use a better circuit or an ECC83 for more gain or a large plate ECC82 for more current.
 
#5
JonSnell Electronic said:
Personally ECC81 - 3 have a slightly higher performance on the second triode, pins 6 - 8 by design, so any valve that is matched is not to specification.
Click to expand...
Where in the datasheet does it say that? The only difference is that one triode might have slightly better hum performance.

RFT valves have a reputation for good 'twin balance'. Many RFT valves carry other branding, including 'Mazda foreign' (often with blue printing).
 
#7
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#8
The Philips datasheet shows the triode connected to pins 6,7,8 to have a more favorable hum level, but that's it, and it's only on the ECC8x tubes. I've not seen any such mention for the 12AX? tubes in datasheets from RCA, Sylvania, Tung-Sol, etc.

Other options would be the 6189 as a premium version of the 12AU7 and the 5814a which is slightly better still but with a slightly higher filament current. I use the 5814a in some of my designs and have been very happy with the results.

Regards, KM
 
#9
passive420 said:
Doing research in preparation to build the Hybrid TubSumo amplifier by Valery here on the solid state forum. Thought this is obviously the best place to ask about tubes.

The front end IPS board has a 12AU7 and Valery recommends each tube having the triodes internally matched.

My question is: Are the triodes generally matched to a degree as standard or will I have to search / ask for them to be matched. I can obviously find matched pairs but is it possible to find pairs that are also internally matched?
Click to expand...
It depends to a large extent what you want to be alike and what the margins are.
Is it :
- current at a specific working point
- Gm at the same
- hum
And finally, the definition of matched: how close = 1% 10% 20 % or some other value.

Good amps ( as build by the folks that know what they did) was designed so
any tube that was within the quite margins of the day would work. It's one
reason that NFB was/is so successful and universally used.

Your best bet would be to buy a few ones and measure and try them.
 
#11
passive420 said:
I read on Audiotubes.com that the ECC802 version has matched triodes.
Click to expand...
Unlikely to be true. The only way to get matched triodes is to throw away those (the majority) which are unmatched. The only way an 'ECC802' design could be matched is if the maker also sold poorly matched 'ECC82' of exactly the same design; in most cases where two different 'ECC82/ECC802' types are offered they are of different designs.

You have two options:
1. pay a dealer to match them
2. buy NOS of makes which are known to be reasonably well matched and then match them yourself

As has been said, a requirement for matching may suggest poor circuit design.
 
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