AC701 in U67/ELA M251 circuit?

[distorter]

Hi, I want to try putting my ac701 tube in a U67 circuit.
Is it ok to just connect a 2R resistor on ac701’s heater(drop 6.3v to 4v) and change ac701’s pinouts to EF86’s pinouts?

Does some method ok for using ac701 in a ELA M 251 circuit?

Thanks!

 

[dmp]

WHAT???? NO
Neither of those mics were designed for an ac701. Start with a mic that was designed for the ac701 (like the m49) and determine the tube operating point and other characteristics (heater voltage & current). Once you understand how that is done, look at the circuits for u67, elam, etc and you can start to answer your own question.
The u67 has an alternative, the m269, that WAS Neumann's adaptation to use the ac701! (note: the grid-cathode voltage is different!)

 

[pucho812]

You want to spend a lot of money don’t you

 

[Gus]

a search will find the M269

 

[mad.ax]

Neumann made two variation of the U60/67 design with AC701.
The M269, and the M367. The later gets the same KK67 head, with the pattern selection on the mic rather than on the PSU.
Both use the same K67 capsule, and BV12 transformer.
Both shame the U67!
But it takes more than adjusting the heater voltage. The bias current is different, and the PSU needs modification too, because the 210V of a NU67 would fry the AC701...

Axel

 

[fazer]

Actually Klause Heynes talked to me one time about AC701 being used in Germany in tele 251s. I asked him for a schematic for a 251 and he sent me the 12ay7 schematic but I recall there being another tube being used as well. If you got the ability to lower the heater voltage it’s up to the individual.

That said a 249B is the reference mic for the AC701 and the reason for the tube choice was low noise and clarity for orchestra recording along with the M50.

 

[fazer]

Ac701 in a EMA 251 with ELA M950E power supply

The power supply has an adjustable heater regulator. For setting voltage down to 4 volt. Or upward to 8 volts