Neumann M49C repair causing problems...

Recently I serviced a Neumann M49C microphone.
The tube (AC701K) had become noisy and produced 'rustling' sounds.
Since good AC701 tubes are hard to find nowadays (and if you can find them they are very expensive ), I looked for an alternative.
I had noticed that the Great British company Phaedrus produced a solid-state drop in replacement for the AC701.
I had used the solid-state version of the VF14 in a U47 in the past with success.
So I ordered a solid-state AC701 and soldered it in the microphone.
The microphone worked, that is: sort of.
There was audio, but also a very obvious low frequency oscillation, 3 Hz. or so.
I contacted Phaedrus, but they never had experienced this problem.
The people of Phaedrus were very helpful, but could not give a good advice because they were unfamiliar with this problem.
After some experimentation, I found the cause of the problem and more important: how to cure it!
I reported my findings to Phaedrus and after that they released a Service Sheet for their AC701 used in a Neumann M49C microphone.
And that 'one case'... that was me!

Ha, youdaman, Ruud! 

Ruud, what is your take on this Phaedrus replacement: How close is it, and what do you feel is still missing compared to an original tube?

yeah, please tell us more about your experience with the phaedrus solid state tube.

I didn't even know solid state replacements for tubes existed... 

So, yeah, thanks for reporting this!

Well, I am not a specific 'tube lover'... 
I think the Phaedrus alternative sounds good, they even emulated the production of second harmonics that tubes are usually known for.
Noise is very low, probably lower than produced by the original tube.
They produce a 'standard' version, and a 'high-performance' version.
The solid state AC701 sounds a lot better than a 5840 tube, I used to test the M49 with before I had the Phaedrus replacement.
(With increased heater voltage of course.)
Also noise is significantly lower than the best 5840 tube with the lowest noise I had here.
So, to keep 'old' microphones alive, I think the Phaedrus solid state tubes are a good alternative.

http://www.phaedrus-audio.com/phaedrus_electron%20tubes.htm

Interesting!
What is the price of those "replacement solid state Tube"?

Prices; Go to above link.

RuudNL, Did you use the DIY or Red Dot tube?

Hi Ruud !

did you ever used a russian 6S6B in a m49 or similar circuit ?

e.oelberg said:

Hi Ruud !

did you ever used a russian 6S6B in a m49 or similar circuit ?

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AEG Moscow use these tubes in many of there own microphones. I've ordered 10 of 'OTK' stamped Military Grade Tubes and will test it out.
Will reply how these sound.

Hi Eric !

I did the same so let's give it a try. I will build a m49ish circuit with a samar audio 6:5:1 tranformer. Where are you living in Germany ?

e.oelberg said:

Hi Eric !

I did the same so let's give it a try. I will build a m49ish circuit with a samar audio 6:5:1 tranformer. Where are you living in Germany ?

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I've build three M49, two with EF762 and Lundahl 1935 and one with EC71 and Haufe T8974 (BV11). i think noise is ok (similiar to my lowest noise ef806s tube) but is getting better i think, so i will try the russian tubes.
i life in Forst (Brandenburg) near Cottbus  : ;D

e.oelberg said:

did you ever used a russian 6S6B in a m49 or similar circuit ?

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That's the one in the standard Flea M49, no?

RuudNL said:

Well, I am not a specific 'tube lover'... 
I think the Phaedrus alternative sounds good, they even emulated the production of second harmonics that tubes are usually known for.
Noise is very low, probably lower than produced by the original tube.
They produce a 'standard' version, and a 'high-performance' version.
The solid state AC701 sounds a lot better than a 5840 tube, I used to test the M49 with before I had the Phaedrus replacement.
(With increased heater voltage of course.)
Also noise is significantly lower than the best 5840 tube with the lowest noise I had here.
So, to keep 'old' microphones alive, I think the Phaedrus solid state tubes are a good alternative.

http://www.phaedrus-audio.com/phaedrus_electron%20tubes.htm

Click to expand...

Hmm the price is not too bad considering that you probably never have to replace it, right?
Which version did you take? The normal or red dot?
Would you also recommend these for new builds that normaly calls for an AC701?

micaddict said:

That's the one in the standard Flea M49, no?

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Hard to believe after all these years, still not a single picture of the Flea 49 tube or guts.

Precious bunch these Flea owners...

Winetree said:

RuudNL, Did you use the DIY or Red Dot tube?

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I used the "standard" version.

Any expected complications when the Phaedrus replacement is used in an M49b?

The M49b uses the same LF feedback (with different biasing), so I suppose the problem will be the same in this case.
For the M49 (without suffix) I wouldn't expect this problem, because there is no LF feedback from the output to the backplate.

Sounds very interesting. An alternative for my M49c builds? But I don’t understand the explanation for the Red Dot versions. They “soak-test them for 300 hours”. It’s a solid state circuit, is it not? Why test them for 300 hours?
If it’s a FET-circuit, they would test the FETs for noise before they put them in a glass tube, not after.
Maybe just a way to charge £100 extra for those who are willing to pay.
I’ve tested (and burnt in) 18 5840-tubes for my two M49c and I’ve found 3 or 4 that have lower noise than the rest. But I must say that they are all highly microphonic. That was a bit disappointing.
I will order two ‘AC701 tubes’ but not the Red Dot versions.

stelin said:

It’s a solid state circuit, is it not? Why test them for 300 hours?

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Agreed! A solid state circuit works, or it doesn't...
If you test it for an hour and it still works as expected, there is no reason to assume that it can't work for 25 years! (Or longer.)