R/E/P > Klaus Heyne's Mic Lab

Is there a Replacement for the Telefunken AC701k?

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blackacre:
I have a Neumann m49b that has been acting up. My mic tech (Tom Onofrio) has diagnosed a few problems including the tube.

I have only been able to locate NOS AC701 tubes at insane prices, and with no guarantee or return, and am looking for options to get the mic working well.

I have an AKG C60 with an AC701k that works well and an AKG C0 with an AC701 (non k) that also works well.
Would it be viable to harvest one of those tubes for the M49b? I am open to other suggestions including after market tubes if any of them are good.

klaus:
In theory, transferring one of the two AC701 from your C60 to your M49 is perfectly fine, except that AKG cut the tube's filament wires in the C60 amp very short, so they will not reach the terminals on the M49 mic amp.

That means Tom would need to splice in extensions to the tube wires, which is labor intensive and requires meticulous heat sinking of each wire during soldering.

As prep for the operation, you could make sure that at least one of the tubes remains quiet over at least 48 hrs of continuous operation.

blackacre:
Thanks for the reply. Good to know that the plan may work even if it is a bit more difficult. One wrinkle I learned is that the c60 used a later ac701 that was a little smaller and this will be harder to find a replacement tube and it is a shame to take such a good mic out of commission.

klaus:
That is erroneous information. The AC701 had always had the same form factor/dimensions, from the start to the last one leaving the factory.

blackacre:
Good to know. I won’t name the specific vendor but that information came from an after market tube manufacturer that explained why their replacement tube would fit my m49 but not the c60. My original thought was to put the replacement tube in the c60 after so that both mics worked. Well the hunt continues.

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