Real AC701k?

[SparkleBear]

Hey All,

I think this a real tube. Im hoping to pick one of these up for a m49... anyone see anything off about this one?

https://reverb.com/item/66449992-ac...-sm69-m269c-schoeps-m211b-m222?show_sold=true

Cheers!

 

[NOON]

The link is to EL84 power tubes.

 

[Khron]

I believe the actual link of the item is this:

https://reverb.com/item/66449992-ac...-sm69-m269c-schoeps-m211b-m222?show_sold=true
But it's already been sold - see how it says "That listing sold. Check out the similar listings below." near the top of the page?

 

[SparkleBear]

That was me who bought it but I wanted to make it sure before paying.

Thanks Khron

 

[Khron]

Oooooooooooouch.....

Wonder how it might fare in a double-blind test against the more terrestrial options...

 

[gyraf]

..that's a lot of money for a used tube, isn't it? Or am I reading it wrong?

 

[RuudNL]

You can buy a really decent microphone these days for $1500...

 

[Walrus]

Doesn't say "sold" anymore??

 

[Khron]

That just about adds up to about a quarter of what my ~120 mic collection is worth

Doesn't say "sold" anymore??

Have we made @SparkleBear cancel the purchase?

 

[LevinGuitar]

I know in many terms they can't be compared, but for a top quality LDC microphone, a good TFK EF86 will do the same job as the 701 for 50€ or so IMHO

 

[SparkleBear]

LOL. oh well...

..that's a lot of money for a used tube, isn't it? Or am I reading it wrong?

I had an offer going for around 1k, but it turned out that it was a used tube. I thought it was only installed for testing but I don’t want a tube that’s been significantly used.

Yea, I had two of these tubes in my parts bin before my house burned down … Neumann select k.

I just want to have a spare for my vintage m269.

I hope I have not lost street credit

 

[rock soderstrom]

I had an offer going for around 1k, but it turned out that it was a used tube. I thought it was only installed for testing but I don’t want a tube that’s been significantly used.

Good decision!

 

[kingkorg]

LOL. oh well...

I had an offer going for around 1k, but it turned out that it was a used tube. I thought it was only installed for testing but I don’t want a tube that’s been significantly used.

Yea, I had two of these tubes in my parts bin before my house burned down … Neumann select k.

I just want to have a spare for my vintage m269.

I hope I have not lost street credit

Since you have the original still functioning well, maybe use the opportunity to measure all possible aspects of it to have a "snapshot" in time in case you ever need to find an alternative.

 

[NOON]

My KM56 has one, currently in good working order. (Fingers crossed while touching wood) There's no room in that tiny LDC body for anything else. Someone I trust had good things to say about one of the reasonably priced alternatives, I think it was one of the solid state recreations. He said no other valve will work, there's particular things they did for low noise etc like the grid coming out the top. I'll go through my emails and see if I can find what it was.
EDIT it was the Phaedrus.http://www.phaedrus-audio.com/AC701%20electronic%20tube.htm

 

[Somkereki]

Don't expect a miracle! This type of electric tube was last manufactured 30 - 40 years ago (or earlier) and it was maybe gassed in the warehouse! All parts of a Neumann (AKG, etc.) microphone must be checked and replaced! Especially the hundreds of megaohm resistors and capacitors too! I have been repairing such studio microphones for decades! In practice, the Russian military 6S6B-V tube has a much nicer sound and lasts forever. distortion is halved, especially the 3rd harmonic. His voice is much warmer. For example, I make the hundreds of Megaohm resistors myself from miniature PCB - ladder-like 0805 size Japanese SMD 47 Megaohm low noise resistors, with epoxy vacuum impregnation. The result is 4 x 15 mm. I check the capacitors with the Danish Radiometer IM6 Megaohmmeter at 150 Volts. (The instrument measures up to 1000 Teraohms! (and its test voltage can be set between 1 - 999 Volts.) V The result amazes recording studio owners so much that it can even be used for vocals! The UM -57 also produces a better sound in microphone than the original EC92. When replacing the AC701 tube, do not forget to raise the heating voltage to 5.5 Volts in the power supply module. It is also necessary to replace the electrolytic capacitors, according to my experience, most of them have already dried up, the capacity is poor! I only use good quality industrial capacitors (Epcos). I have been doing this for more than 40 years. If you only change the electron tube, you will be disappointed! I wish you good luck !

 

[SparkleBear]

Since you have the original still functioning well, maybe use the opportunity to measure all possible aspects of it to have a "snapshot" in time in case you ever need to find an alternative.

I’d love to learn more about testing setups: I’ve always been impressed with your approach. Especially now when I’m starting to build my own designs, it would be good to be able to quantify some of the qualities I’m producing…

Any tips where to start with a testing setup??

Cheers

 

[kingkorg]

There are many here way more knowledgeable about pure technical aspect of testing tubes, like curves, impedances and stuff. Something like @moamps has done with vf14.

http://www.moxtone.com/mU47_U47_part3.html
But i'd do something like Thomas:

https://groupdiy.com/threads/microphonics-in-vf14-and-u47.82644/
Or coming up with some kind of simple testing jig to hit the tube at fixed velocity to excite ringing, and recording it.

And obligatory injection tests through fixed value capacitor matching capsule capacitance.

Different level sweeps using REW to capture frequency response, harmonic curve profiles and THD at those levels. Phase shift...

Recording some music through that cap, again different levels, different source material, both compressed and not compresed stuff. Maybe individual instruments captured with transparent mic, or even DI (piezzo steel string guitar - sharp transients). Pink noise!

You can then compare alternative tubes, and even do null tests, judge if your ear/head combo plays tricks on you.

I'm hesitant to say record some material through speaker under strictly controlled conditions simply because humidity, placement, and temperature are variables that can swing results considerably.

Noise shouldn't be that much of a deal to figure out, as tube mics are somewhat noisy so just recording constant low spl klick at fixed position and comparing should do it. I'm not obsessed with tube noise as long as it's at reasonably low level, I don't reach for tube mics if the noise might be an issue anyways. But that's me. I am primarily keyboarist, I love synths, we have a knob for noise, and it is sometimes even desirable. But that's another topic.

Ideally you would need several original tubes, just to see how much they vary between specimens, so you don't get hung up on one aspect that could be specific to that one tube. Or capsule, transformer, capacitor...

For anyone not believing all of this would be sufficient to bring you close to the target, i'm afraid the original would be the only alternative. Of course i might have missed something.

 

[tomas.borgstrom]

Don't expect a miracle! This type of electric tube was last manufactured 30 - 40 years ago (or earlier) and it was maybe gassed in the warehouse! All parts of a Neumann (AKG, etc.) microphone must be checked and replaced! Especially the hundreds of megaohm resistors and capacitors too! I have been repairing such studio microphones for decades! In practice, the Russian military 6S6B-V tube has a much nicer sound and lasts forever. distortion is halved, especially the 3rd harmonic. His voice is much warmer. For example, I make the hundreds of Megaohm resistors myself from miniature PCB - ladder-like 0805 size Japanese SMD 47 Megaohm low noise resistors, with epoxy vacuum impregnation. The result is 4 x 15 mm. I check the capacitors with the Danish Radiometer IM6 Megaohmmeter at 150 Volts. (The instrument measures up to 1000 Teraohms! (and its test voltage can be set between 1 - 999 Volts.) V The result amazes recording studio owners so much that it can even be used for vocals! The UM -57 also produces a better sound in microphone than the original EC92. When replacing the AC701 tube, do not forget to raise the heating voltage to 5.5 Volts in the power supply module. It is also necessary to replace the electrolytic capacitors, according to my experience, most of them have already dried up, the capacity is poor! I only use good quality industrial capacitors (Epcos). I have been doing this for more than 40 years. If you only change the electron tube, you will be disappointed! I wish you good luck !

I like AC701k better than 6s6b-v and I think it's very impressive that a lot of them are still working perfect.

I do not recommend routinely replacing anything inside a Neumann, AKG or other vintage microphones except electrolytic capacitors. Replacing everything with modern components will alter the sound and decrease the value of the microphone. I service old tube microphones and I rarely need to replace a resistor, except for the really old U47's and other microphones from the 40's and 50's. Cleaning the microphone and servicing the PSU is important. The capsule often needs reskinning and sometimes the microphone calls for a new tube.

 

[TLRT]

I made many custom testers for different mic tubes,they are actually mic circuits terminated with capacitors and the output goes to spectrum /audio analyzers. With this I can get 100% user satisfaction(except the post lose the pack)

 

[Somkereki]

Sorry, I didn't want to force anyone to do what I wrote, I only wrote it because of many years of experience, with the intention of helping. My experiences are supported by the measurement results of serious instruments. My instruments (not an exhaustive list) Neutrik A2 -D Audio test & Service System, Tekktronix TDS 224 oscilloscope with FFT module, Lecroy WaweAce 204 oscilloscope, HP 3456A (6.5 digit tabletop multimeter, true RMS) BUT the high Megaohm resistors almost always they are faulty and especially very noisy! I even found a noisy leakage current path between the styroflex capacitors connecting the capsule to the grid of the tube! If there is a hairline crack in the polystyrene-insulated connector holding the capsule, I turn a new one from polystyrene or Teflon. I put the microphones without a cover in a whiskey tin for the duration of the measurement, so that no electric field disturbs them. These microphones require a lot of patience! It's no use insisting on "original" parts, if the defective ones have to be replaced! I even replace the tube of the power unt gas-filled stabilizer with a self-made extremely low-noise semiconductor stabilizer for two reasons: When switched on, they do not ignite for a few tenths of a second and the entire unstabilized voltage goes to the microphone circuit! The other important thing: Gas-filled tubes have a very low-frequency "flicker noise" that partly passes through the output matching transformer and partly causes intermodulation. Which causes an unpleasant sound in today's digitization. I try to reduce the noise to a minimum in all devices, especially the inaudible < 20Hz flicker noise! Worth it ! Good luck for everyone !
Ps: I reserve my opinion on the AC701 tube, because I came across more than 100 pieces, and most of them were defective, even though they were bought for 400 - 500 dollars on Ebay as an original selected new tube! Among the Russian electron tubes, I did not find a single defective piece, even though the 100 pieces I bought were made in 1977. (They had the purple quality control stamp on them.) By the way, it says a lot that they can withstand 100 G acceleration. Even their glass dome is special, less fragile than the others.