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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.

[klaus]

Not sure what you mean by "replacement" tube, because I am unaware that there is such thing.

The AC701 is unique- it was made by Telefunken specifically for condenser mics offered by three companies only. Any mic originally designed to be powered by that tube will either not work with a substitute tube, will need to have circuitry, cable or power supply modified to even function, and if these hurdles are overcome too, none of these mics will ever sound the same as those with the original AC701.

But I understand the temptation; when a tube costs a lot of money, people will search for, and suppliers will offer supposed substitutes.
That path and its poor results are amply demonstrated in the field of capsules: sourcing Neumann and AKG capsules has become expensive, so it is tempting to be lured by aftermarket "reskinning" offers. Never mind the inevitable loss of value or sound quality.

[blackacre]

There are a few options marketed as drop in replacement tubes that don’t require circuit or power supply changes.

Drefahl Audio and Phaedrus Audio both make drop in replacements that I considered as a back up if only because even after finding an AC701, and spending a lot of money, the tube may not work, let alone be up to spec.

At this point we may be approaching the end of the era of these microphones as they were originally built. Unfortunately my M49 had an M7 capsule that is also shot like every other m7 at this point. Tom is going to switch to a K47 and I am going to frame the M7 in a display case.

Thanks for all your perspective and experience.

[klaus]

There are a few options marketed as drop in replacement tubes that don’t require circuit or power supply changes.
Drefahl Audio and Phaedrus Audio both make drop in replacements ...

Let's be clear: these are transistorized solid state devices that try to emulate the sound of tubes.

[blackacre]

So, not a tube, despite marketed as a tube. Frustrating.

[Spindrift]

So, not a tube, despite marketed as a tube. Frustrating.

You have a mic which is probably pushing northward of $15,000 in value. I wouldn’t bat an eye spending upwards of $1000 or a bit more for a NOS AC701 tube from a reputable source. Devaluing your vintage Neumann with an aftermarket “tube” is illogical.

[blackacre]

I understand your perspective. But it’s not easy to find a tube these days. Further, the prices are closer to $2000 from what I am seeing with no returns or guarantee that the tube will be useable. Facing that there is the prospect of having to go through a few tubes to find a good one. That means we aren’t talking about spending $1000 and being done; we are talking about $2000 to $6000.

All that being said, I have talked to a few people that tried some of the options and were really not satisfied with the results. I think I may have found one, but the hunt continues until I have the mic with a working tube in my hands. I would welcome anyone pointing me to a reputable source for a tube.

[blackacre]

Someone suggested I look for a Hiller MSC2 tube, which from my understanding was the tube originally used in the m49 before they switched to the ac701k. Is it possible to install a Hiller tube without significant modification to the mic? Would this be an option for those that can’t find an ac701k?

[klaus]

(...) Further, the prices are closer to $2000 from what I am seeing with no returns or guarantee that the tube will be useable (...)

Can you cite that source?

Someone suggested I look for a Hiller MSC2 tube, which from my understanding was the tube originally used in the m49 before they switched to the ac701k.

Reverting back to the MSC2 is not a good idea. Though you would only need to adjust the heater voltage down a bit, it'a a noisier proposition.

[blackacre]

Over the last few months as I have been looking online it has been fairly consistent across eBay, reverb and public tube retailers (though they have been out of stock). You may know more reputable and reasonably priced sources, but I do not and the consistent data I am seeing is over $2000 on average. Here are two links, but given that many previous listings are now not accessible, I doubt they will serve as a reference long term. So I will just state that the eBay listing is for one ac701k, as is, with no representation of working condition and no returns, for $2995. The second link is for one ac701k for $1995.

https://www.ebay.com/itm/AC701K-Telefunken-NOS-NIB-Microphone-Triode-RARE-K-Version-RED-DOT-for-Neumann-/254687429600?mkcid=16&mkevt=1&_trksid=p2349624.m2548.l6249&mkrid=711-127632-2357-0

https://www.tubedepot.com/products/ac701

[klaus]

During times of high volatility in the stock market the spread between ask and bid price can be pretty wide. And so it is in the current, highly volatile vintage tube gear market.

None of your price quotes would have been quite as sensational if you would have added the prefix "ask" to the numbers you cited.
The last "bid" i.e. actual, price for a New Old Stock AC701 in the box was $1k (https://www.ebay.com/itm/384470037240).