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Does anyone know why the ac701 is considered one of the best tubes for a condenser microphone? I have read what specs I could find on the web. I can understand why the grid wire exits the other side of the tube away from the other outputs.

I also understand that the german Neumann microphones used the ac701 and the export models used the ef86. I have not heard the ac701 in a microphone I have heard ef86s in microphones.

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anonymous Mon, 06/23/2003 - 04:05

The AC701 and AC761 has marginally better noise characteristics than the EF86, and this is why only AC701's vere allowed in microphones used by German national radio.

But in my opinion, it's not that sonically different from a triode-coupled EF86. But the EF86's electric characteristics are somewhat different, so you need to use a different design around it. There's no way it could be used as a drop-in replacement.

Jakob E.

rmburrow Fri, 11/29/2013 - 07:34

Ten years later....yes the AC701 powered mics sound great, but so do the EF86 powered mics sound great also. Part of the sonic difference is undoubtedly the way the tube grid is coupled to the capsule. The U67 has some LF rolloff circuitry at 400 megohms before the capsule...the SM and M series have none.

As for the tube itself, the AC701 is structured to place the grid by itself close to the capsule. The other tubes must be positioned to get the grid near the capsule while keeping other connections away from the high impedance grid. The noise from a AC701 is very low. The noise from a 7586 nuvistor triode, EF86, or 5879 is comparably low. The 5703 subminiature triode has characteristics similar to the AC701 but has a 6 volt heater.

If your mic uses a NKM or NSM type supply, it isn't difficult to modify the supply for 6 volts...but that supply needs to be marked so that AC701 powered mics are NEVER plugged into that supply. I have not tried the 5703 in place of a AC701, but I have modded a mic to use the 7586 nuvistor in place of the AC701.

Maybe owners of AC701 powered mics should email or write JJ Electronics and encourage them to manufacture the AC701 from the original Telefunken specs. As far as I know, Telefunken has no interest in manufacturing this tube so someone should "take up the slack" and build it...the market is there, and for the VF14 as well...