I would try the 6N1P-EV. It's cheap and very good. The EV version is long life. It was designed as a true low frequency voltage amplifier. It is 6.3V though with pin n.9 connected to a screen between the two halves. Amplification factor is 35 and average gm 4.5-5 ma/V at 150-180V/6-8 mA. Input capacitance is higher than 12AU7 though.
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Looks a bit like the ECC99 and like that tube it is much taller than the 12AU7. I need something no taller. And its price is crazy!
Cheers
Ian
Lots of ARC amps have used them in balanced driver circuitry. Used to have bunch of Amperex ones which I unloaded when I realized what they were worth.
What about 5687?
Same height as the 12AU7 but with a slightly different pinout.
Vh= 12.6 V, mu = 17, gm = 8 ma/V
New age of insane prices on tubes
If the bottle has to be the same size as a 12AU7, then I think 5687 is going to be the best choice. Unfortunately, it looks like prices on those tubes have gone a bit crazy. They used to be a $4 tube, now it's $15 and up.
Then again, I see people selling 5814 (industrial 12AU7) for stupid crazy prices these days. So maybe the 5687 is still something of a bargain at "only" $15 each.
Possible problem is that the 5687's pinout is different, and its heater takes double the current of a 12AU7.
If the bottle has to be the same size as a 12AU7, then I think 5687 is going to be the best choice. Unfortunately, it looks like prices on those tubes have gone a bit crazy. They used to be a $4 tube, now it's $15 and up.
Then again, I see people selling 5814 (industrial 12AU7) for stupid crazy prices these days. So maybe the 5687 is still something of a bargain at "only" $15 each.
Possible problem is that the 5687's pinout is different, and its heater takes double the current of a 12AU7.
