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Is there a lot of difference among 12AX7s

141.161.133.241

Posted on May 25, 2012 at 08:18:30
peterhh


 
I was thinking of learning to build simple amps for music and also for guitar. I was wondering if there's much difference mong 12AX7 preamp tubes. When I was a kid (in the 60s) it seems most integrateds would have six of them, like our Fisher X100b or a Sherwood amp I picked up on ebay, both of which were very sweet and clean sounding (sadly I had to sell them). Are vintage ones better, and why? Sorry if this is a dumb question!

PS - for guitar use you want to be able to overdrive the preamp, so differing characteristics when overdriven would be relevant. Thanks!

 

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Every ingredient that goes into a unit makes a difference.. , posted on May 25, 2012 at 09:17:54
FenderLover
Bored Member

Posts: 8306
Joined: July 31, 2007
Contributor
  Since:
May 17, 2009
to a certain degrees. Certainly, "voicing" the input and gain stage in a guitar amp is critical to the final tonal response, range, OD, and harmonic content of the amp.

If you have a stable platform with the final tone close to what you seek, then swapping tube brands/sub-types is a nice way to fine-tune the unit. But, I find swapping tubes will not compensate for a poor or unstable circuit.

 

Any dual, hi mu triode...., posted on May 25, 2012 at 09:24:45
suffers from the manufacturing inequalities and difficulty of making a dual tube in one envelope that is well matched from side to side, ie both tubes, the same transconductance, mu, the same plate resistance, the same cathode current. They can't be made perfect, close but not perfect. Yes, there are differences from brand to brand. It was rumored once that the Chinese were evacuating tubes with foot pumps! I don't know if it was true but it came from a reliable American engineer that did consulting work for the wonderful Red Chinese. The best thing for an audiophile to do is get a transconductance tube tester and check the tubes for mu, ie gain, and distortion. Ray

 

I guess Joe S's old post answers all my questions, posted on May 25, 2012 at 12:30:25
peterhh


 
and more. Well, he doesn't go into overdrive ...

 

RE: Any dual, hi mu triode...., posted on May 25, 2012 at 12:32:19
peterhh


 
Thanks! Do you recommend an affordable tube tester?

 

RE: Every ingredient that goes into a unit makes a difference.. , posted on May 25, 2012 at 12:39:49
peterhh


 
Thanks! Do you have a favorite preamp tube for a Fender guitar amp?

 

RE: I guess Joe S's old post answers all my questions, posted on May 25, 2012 at 12:50:04
Joe Appierto
Audiophile

Posts: 677
Location: Central NJ
Joined: January 3, 2004
Some of these blurbs go into guitar applications.

No affiliation with the seller in any way.


Joe

 

RE: testers, posted on May 25, 2012 at 12:59:50
Look for a Weston 981 Type III or a Heathkit TT1. They both were designed by the same engineers. Weston and Heathkit were owned by the same company. The Eico 666 and 667 are also nice if they have been taken care of. Just don't buy an emission tester and insist on a transconductance tester with a built in oscillator that applies a signal to the grid of the tube. Testers this old are gona need some repair or rejuvenation like replacing electrolytic caps and maybe a few worn out tube sockets so get yourself psyched up for that. Ray

 

Yes sir!, posted on May 25, 2012 at 13:24:25
FenderLover
Bored Member

Posts: 8306
Joined: July 31, 2007
Contributor
  Since:
May 17, 2009
For typical BlackFace front ends, the Raytheon BP 12AX7A or 5751 are the tubes I like in the input stage. Which is most critical in these amps.

 

Sonically, Yes, there is a lot of difference among 12AX7 tubes...., posted on May 25, 2012 at 20:19:27
Interstage Tranny
Audiophile

Posts: 2737
Location: Eastern
Joined: October 4, 2006
Your ears will let you know. It's a fun journey, but nowadays good old tubes can get pricey. Enjoy the ride...

 

Would you care to divulge....., posted on May 26, 2012 at 00:12:09
Uncle Mike
Audiophile

Posts: 1749
Location: Eastern Pa.
Joined: June 20, 2003
The reliable American engineer that witnessed these manufacturing processes, or is this all strictly hearsay?

 

RE: Would you care to divulge....., posted on May 26, 2012 at 06:48:19
I overheard it at a hamfest in Springfield, Missouri. The guy was selling high voltage mercury rectifiers. He had worked for International Rectifier and was doing consulting work in China. Ray

 

RE: testers also B & K 747, posted on May 26, 2012 at 12:52:01
Mechans
Audiophile

Posts: 1217
Location: East Coast
Joined: May 23, 2004
I use a Hickok and BK 747 which is a SS tester and very good for a simple reliable emissions style tester. There are usually some for sale on e-bay for less than the tubed transconductance models. I think they also claim transconductance but I don't think any one uses them for that.

Steve

 

RE: testers also B & K 747, posted on May 26, 2012 at 21:36:32
Jim McShane
Dealer

Posts: 5129
Location: Chicago
Joined: January 13, 2003
B&K 747s are NOT just emissions testers, they are true transconductance testers. They just don't give you a readout in umhos, they give it as a figure on an arbitrary scale.

I use mine for Gm all the time - the 747 will test a lot of tubes that older testers won't.

 

Thanks! nt, posted on May 27, 2012 at 15:19:45
peterhh


 
nt

 

Thanks for the link! nt, posted on May 27, 2012 at 15:37:08
peterhh


 
nt

 

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