Tubes to never use in audio
Jun 3, 2008 at 1:27 AM Thread Starter Post #1 of 28
wower

wower

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I hope I've come to the right forum. Several times it has been bandied around on head-fi about certain tubes to never use in hi-end audio reproduction. I have searched extensively but can't find the references again. I ask because I'm looking at a tube output stage CDP which uses a 12AX7/ECC83 which I think was listed as one of the offenders. (Something about poor linearity.) Or was it 12AUX which had the problem? I'm so bad at these things. Please help!
 
Jun 3, 2008 at 2:47 AM Post #3 of 28
Just because a tube was used in a guitar amp doesn't mean it would be an appropriate tube for a high fidelity amplifier. Guitar amplification is one place where people seem to want distortion.
 
Jun 3, 2008 at 4:03 AM Post #6 of 28
Thanks for the responses all. Perhaps those were rare desenting voices I read. I know through my own research the 12ax7 is used in a wide verity of audio gear but its guitar amp roots do beg suspicion. Needed to come here for the down low.
 
Jun 3, 2008 at 4:45 AM Post #7 of 28
The 12AX7 is a high mu, low Gm dual triode. It is used extensively in guitar amps, but has also found its way into a lot of hifi audio gear. In many cases this is likely out of habit (particularly when used as a cathode follower as a tube output stage to a CDP where it is a terrible, though not uncommon, choice), but in others it is used due to the high mu, since it can be used in circuits with a lot of feedback while still maintaining a reasonable amount of gain. It is not particularly linear compared to the best of tubes, and is generally a second rate choice to the 6SL7, though it is smaller which makes it easier to fit in small cases and on circuit boards. I wouldn't use it anywhere but in a guitar amp, but others disagree with this.

12AU7 is a lot like a cruddy 6SN7. Used a lot, but again not particularly linear. It was designed as a cost saving measure. Has it's uses, but again, not my favorite.
 
Jun 3, 2008 at 4:53 AM Post #8 of 28
I would STEER CLEAR of the 12ax7.

It was designed with high gain to allow lots of feedback, not good linearity. While a little feedback can quite nice and do great things to a good circuit, this tube was designed for LOTS of feedback.

Feedback is like fertilizer on your lawn. A little here and there and everything comes up nicer and greener. Too much, and your house just smells like poo.

The fact that some sell for hundreds of dollars is more a reflection of the tubes value in guitar amps than hi-fi applications. try to sell a guitarist a 6dj8...
 
Jun 3, 2008 at 5:00 AM Post #10 of 28
Quote:

Originally Posted by penger /img/forum/go_quote.gif
This might be a little off topic, but I think it's got some merit so I'll post it anyway. =P

My question is then... for an amp that has 12AX7's... what's the alternative?


A different amp? only kind of joking.

Some amps can be reconfigured for other tubes, or MAY be able to accept the other tubes in the 12a?7 family. check with the builder for the final word.
 
Jun 3, 2008 at 5:08 AM Post #11 of 28
Let's take my SP Extreme Platinum for example. While I can run 6SN7's, mine came with a ECC1 adapter that can take 12AX7's and 7AU7's I believe. Assuming I want to use the ECC1 adapter, what would be a good "tube of choice"?
 
Jun 3, 2008 at 5:18 AM Post #12 of 28
Wow, okay, I defer to your expertise, but this leads to more questions, specifically in regards to tube outputs on CDPs:

[I ask for forgiveness beforehand for committing the transgression of not modding gear myself. I just really appreciate the knowledge and help.]

Looking at the Jolida JD-700 CDP it says it uses the 12AX7.
However, looking the Underwood hi-fi modded one, they list the tube compliment after modding as NOS Mullard milspec CV4024 (left side), which is a 12AT7 / ECC81 variant if I have my facts right. Am I missing something??
 
Jun 3, 2008 at 5:24 AM Post #13 of 28
Hmmm, the RS B52 uses 4 12au7 and can use 2 12ax7 in place of the 12au7. Maybe the badness of this valve is being overstated here.

My EAR 864 uses 3 12ax7s in the 834 phono and a 12ax7 and 12au7 in the pre-amp section. I guess Tim de Paravicini (designer of the 864) AKA "The king of tubes" needs to be brought up to speed on how bad these valves are.
 
Jun 3, 2008 at 5:29 AM Post #14 of 28
Quote:

Originally Posted by penger /img/forum/go_quote.gif
Let's take my SP Extreme Platinum for example. While I can run 6SN7's, mine came with a ECC1 adapter that can take 12AX7's and 7AU7's I believe. Assuming I want to use the ECC1 adapter, what would be a good "tube of choice"?


I think the 5751 and 7025 are better - they both are audio-specific versions. The 5751 has lower gain that a stock 12AX7, but is generally a compatible swap.

But are you happy with the 6SN7? I have a preference for octal tubes and use them whenever possible. Cheap 6SN7s aren't that great, but the good ones are very good.
 
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