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.



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Under what circumstances is a 12AU7 is a substitute for a 5751?

12AU7 substitutes are ECC82, 7316, 5814, CV491, 6189, E82CC, 5963, 6350, 6680 and a few others . Have never seen 5751 as one of them

Yes, I believe the 5751 is a compatible substitute for the 12ax7.

But, I’d confirm it with a call.

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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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



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.




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.


Very helpful atmosphere!   Thank you!

Ah well, that's fine.  I can still get 5751 tubes.

I found the GE black plate triple micas to be the best. I tried the Sylvania Gold too. They both have completely different presentations.

If it helps, here is a post from my Virtual System page:

Swapped out the (phone tubes) GE 5751 BPTM for NOS Sylvania 5751 Gold GPTM.
Firmer and tighter bass, a little dryer than the GE which are very liquid. Definitely a bigger presentation with the music swelling more appreciably. The sound is a like the Telefunken sound. I always feel that grey plates have a slightly grey coloration whereas black plates seem to have a slightly darker presentation... like there are are blacker silences. There is a pretty stark difference between these tubes.

speakers disappear, I can't even imagine the sound is coming from the speaker.
fuller, more beguiling bass, upper bass and lower mids.
spittier dryer treble.
more pluck and snap on strings.
piano sounds more expansive.
treble not as sparkly as GE, less air, seems busy.
color, timbre not as delicate as on the GE tubes.

Switching back to GE:
everything seems a little smaller, quieter and way blacker. More musical,less hi-fi.
definitely more detail and air. I can hear the acoustic here much better.
Crystal Gayle on Waits - Is There Any Way Out of This Dream is simply more musical like a 300B SET. Wispy, breathy trumpet. Recording hiss is more audible.
Looks like the GEs are staying. They make me smile, the Golds would impress the those who like a bigger, rockier sound.

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@oldhvymec : a 5751 is NOT a 12AU7! And cannot be subbed for one! A 5751 is a lower-gain low-noise version of a 12AX7, with the same current rating. They are interchangeable. The best versions have three micas and low microphonics. The 12AU7 has a much higher current rating and is used in driver stages. I own examples of all three of these tubes!

+1 @atmasphere ! Sticking to the original tubes is always best! When buying 5751’s always try to buy the 3 mica version. The 2 mica version has poorer resistance to microphonics.

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@thecarpathian : thank you for catching my mistake! Brain fog setting in! I have made the correction in my above posts! Thanks!

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@ 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

Excuse please, 12BZ7, not 12BH7, Though I use both. Cary and Mac use them both.

I see the actual chart is a Gain of 70, that is a far cry from 19-20 on a 12AU7 for sure. Can't argue with the facts. BAD AudiogoN forum. LOL 


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

The 12BH7 also has double the filament current. Best to make sure the power transformer can handle it, especially if more than one tube is replaced in this manner.

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.


just call during business hours... a real human receptionist will answer and will put you through to jeff, or he will call you back if you leave your number

over the past 25 years i have never had trouble reaching jeff quickly


I did try a few times, but no human being.   I also emailed a couple of days ago.

No response yet.

It's not that I haven't had luck getting some help from CJ over the years.  They have indeed answered some of my questions!   But given the responses can be a little hit or miss, and since I had the tubes ready to try, I was wondering if there was a quick answer so I could try them.

It turns out that the answer is complicated.  I've decided not to try the 12AX7 tubes, to be careful.