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Ok to substitute 12AX7 for 5751 tubes in CJ Amp?

Hi Folks.

Despite having owned my Conrad Johnson Premier 12 tube monoblocks for over 20 years, I'm just starting to play around with tube rolling recently.  (Previously, when a tube got tired I'd just replace them with whatever CJ recommended at the time).

Playing around with the input tubes:   The CJs come with 5751s (I've been using GE), and as I understand it 12AX7 tubes are higher gain.   However, are they not interchangeable in most cases?  Just don't want to screw anything up in my amp.

Thanks.

 

prof
noromance's avatar
noromance
9,576 posts
 

12AU7 = 5751,

No. I think you mean 12AT7 with an amp factor of 60. The 12AU7 is much lower at around 17 at 250Vdc. 5751 is 70. 12AX7 is 100.

 

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yogiboy's avatar
yogiboy
5,529 posts
 

Here ya go!

RE: 5751 or 12AX7 tubes in a CJ Premier 11A Power Amp?

Before posting here I sent an email to CJ and received the following response: "The voltage amplifier tubes are 5751. This is a military/industrial tube similar (but not identical) to the 12AX7. The 12AX7 will work as a substitute. The inverter is a 6FQ7. They are getting to be hard to find, but we do have those available here."

So the 12AX7 is OK to use, but I take to heart the recommendations here that I stick to 5751 since they are still available. I have made a bid on some GTE Sylvania 5751 on Ebay: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370935303697&ssPageName=ADME:B:BCA:0:3160

https://db.audioasylum.com/mhtml/m.html?forum=tubes&n=254681&highlight=replacing+5751+with+12ax7&r=&search_url=%2Fcgi%2Fsearch.mpl%3Fforum%3Dtubes%26searchtext%3Dpsvane%2Buk%2Bel34

prof OP
3,452 posts
 

 

Thanks for the replies!   (Especially yogiboy).

I had tried to contact CJ but didn't get an answer yet.   If they get back to me I'll post their answer here as well.

 

 

atmasphere's avatar
atmasphere
11,651 posts
 

@prof 

A 12AX7 is a poor substitute. The 12AT7 has similar amplification factor but vastly different plate resistance (much lower).

Whenever a tube circuit is designed, hopefully the circuit is optimized for the tube in mind. When you put something else in there that has different characteristics, distortion will be increased, gain will be affected (which will mess with the feedback) and bandwidth will be affected.

Because of its increased gain, its entirely possible that the amp might oscillate under certain conditions with a 12AX7 installed as a sub.

You're stuck with the 5751.

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