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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

Showing 2 responses by yogiboy

yogiboy's avatar
yogiboy
5,486 posts
 

@ oldhvymec

The 12BH7 is a sub for a 12AU7 not a 12AX7!

The 12BH7 requires no circuit modification to substitute for a 12AU7, but the plate resistance is a little lower (5300 vs 7700). Thus, even though it has a lower u (or amplification factor), the gain may be slightly higher depending on the circuit design. [This is mainly true for circuits that use an anode resistor load.] It will also draw a little more current, but the 12BH7 is a beefier tube and can handle it. For most circuits this should be well within the tolerances of the design, but you can ask the manufacturer to be sure.

Tube Gain Acceptable Substitutes
12AX7 100   5751 12AT7 12AY7    
5751 70 12AX7   12AT7 12AY7    
12AT7 60   5751   12AY7    
12AY7 45   5751 12AT7   12AV7  
12AV7 41       12AY7   12AU7
12AU7 19         12AV7
yogiboy's avatar
yogiboy
5,486 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

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