AC701

[tao]

I want to replace the ac701 tube for my 251, but these two make it difficult for me to choose.I don’t know what the difference is between these two versions.

 

[GeorgeToledo]

The k indicates a build designed for lower microphonics.

The one without the k may be the same low microphonic build type as the k, if it was made in the later part if AC701 manufacture.

 

[TLRT]

Best test method is to measure the noise before installing the tube

 

[tao]

Best test method is to measure the noise before installing the tube

I don’t have professional equipment testing.

 

[tao]

The k indicates a build designed for lower microphonics.

The one without the k may be the same low microphonic build type as the k, if it was made in the later part if AC701 manufacture.

I read a lot of information on the Internet, and there is too little information about 701. These two models should be versions with K, but I cannot be sure about this blue dot 701.

 

[RuudNL]

Keep in mind that the whole production of AC701 tubes from the Telefunken tube factory in Ulm went straight to Neumann, where they tested the tubes for noise and microphony. The rejects went back to Telefunken and were sold. So even if you can find an AC701 today ('New Old Stock'), there is a chance that it is a reject from Neumann...
Just sayin'...

Also I know a (first hand) story of an other German microphone manufacturer who had aquired a large number of AC701 tubes.
"What are you going to do with the tubes that are not good?"
"Sell them!"

 

[rock soderstrom]

Also I know a (first hand) story of an other German microphone manufacturer who had aquired a large number of AC701 tubes.
"What are you going to do with the tubes that are not good?"
"Sell them!"

I would do the same, there were/are numerous applications where this excellent tube can do a top job, even if it is not good enough for a microphone. The application as an impedance converter in a microphone is very special.

The question is rather whether one communicates this also to the potential buyer...

 

[Khron]

The question is rather whether one communicates this also to the potential buyer...

"Let the buyer beware..."

 

[rock soderstrom]

"Let the buyer beware..."

I think you should specify that as a seller. The problem is that these 2nd choice tubes are resold several times over the years and suddenly they are 1st choice again ;-)

With such high-priced tubes I would buy anyway only from well known sources with appropriate test equipment. (...or replace with similar types and save a lot of money).

 

[RuudNL]

(...or replace with similar types and save a lot of money)

Isn't that exactly what Neumann did in the M49-V?

 

[terry setter]

For the sake of complete accuracy, the M49-V (which is a "re-issue") does not use an AC701K or an AC701. You can be certain of this due to the fact that it's heater voltage is 6.3V, which would immediately end the life of an AC701 (either K or not K). Interestingly, the Neumann supply that is sold with the M49-V is designed to sense and adapt the heater voltage to 4V if an AC701 is wired into the microphone. I'm sure that means they knew that some folks wouldn't sleep at night unless they knew their M49-V had a vintage 701 in it. Klause Heyne has a very comprehensive article on the M49-V if you're interested. It cam be found at:

Neumann M49V: Complete Tear Down and Analysis

 

[metalockpick]

M49V PSU uses a current regulator on the filament (many late 20th century and recent PSUs do this). That way, the psu will work with a host of different tubes which all draw about the same range of current (even though the voltages are different). AKG did this with a few mics, others have and do as well.

 

[Drea]

In this confusing world noise is character instead. So now the noisy ones are expensive. Not a dealbreaker.

 

[TLRT]

Keep in mind that the whole production of AC701 tubes from the Telefunken tube factory in Ulm went straight to Neumann, where they tested the tubes for noise and microphony. The rejects went back to Telefunken and were sold. So even if you can find an AC701 today ('New Old Stock'), there is a chance that it is a reject from Neumann...
Just sayin'...

Also I know a (first hand) story of an other German microphone manufacturer who had aquired a large number of AC701 tubes.
"What are you going to do with the tubes that are not good?"
"Sell them!"

No Way they tested every ac701 from the factory ,maybe simple emission test yes,but for noise/microphonics and burning in it would took too much time(testing now 8pcs ac701 so I know