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

@ 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.

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 

Thanks

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.