Neumann M49V tube mistery...

[Jen]

 

[Jen]

o

 

[kingkorg]

I have seen an analysis of this. Somewhere. It may be in a book at work, and this is a weekend (yay!).

I think the first issue is: tube grids leak enough that a few-hundred-meg is as high as you can go and stay stable. OTOH, small FET leakage is so low (smaller device size, cooler operation, and lower voltages) that you can run gigs of grid resistor.

But it seems to me there was a more obscure reason why some amps use 100Meg and some amps use 1Gig.

> noise level gets down with increase of grid resistor. could anyone elaborate? Low pass behaviour with the tube input capacitance effectively in parallel, or something?

Yes. The thermal noise in the resistor is shorted-out by the capsule and grid capacitance. 30pFd at 1KHz is 5Meg reactance (???check my math). So the noise of a 100Meg resistance is attenuated 100/5= 20:1 lower, a 1Gig resistance 1,000/5= 200:1 lower. The 1Gig resistor has 3 times higher noise voltage than the 100Meg, but that still makes the 1Gig 3 times lower when loaded with the capacitance. (I think that is roughly right, neglecting a few 10%s and √2s.) Of course when you get gate resistance noise down below capsule acoustic-resistance noise or tube noise, increasing the grid resistance hardly affects total noise output. That may be another reason not to use Gigs on tubes: their self-noise will typically be higher than a good FET. Huge resistors on grids don't improve noise, cost more, and let the tube drift off of optimum bias. (At some critical value of grid resistance, a tube will "runaway", bias drifts far away from what you expect. Which-way depends a lot on stray gas, cathode temperature, grid-cathode dimension tolerance, etc.)

Here's PRR's take on higher value grid resistors in tube mics. These 6s6b related tubes have higher grid current.

 

[kingkorg]

I unfortunately had a different experience.
From a batch of 20 6B6B-V, only 18 (great performance) were noiseless and low microphonic (tap on it , it will sometimes ring like hell),
But none can go easily to rumble frequencies, unlike the AC701 i have.
I thought it was the grid current .
Maybe i did something wrong in the settings...How did you bias it ?
How did you test the grid current and measure the pA ?

You just have to listen harder and you will hear the grid current. That is if you have a set of golden ears. Remember, it's voltage that counts, forget about the current and gain of particular tube. (Mark the sarcasm, i don't want anyone to think i've gone nuts.)

Copyright @klausheyne
Tue, 07 December 2004

Quote:


"That's why I never bothered to invest in a regular tube tester, but still use the most reliable and cheapest method to test a microphone tube: ear detection. Wire up the tube, plug in, set the correct voltages, let run for seven days (I use seven, not two!), and I know that 90% of all tubes tested this way usually will remain quiet for at least a year or longer."

https://repforums.prosoundweb.com/index.php?topic=1150.0
*the original thread is closed for comments

 

[Gus]

Why not just measure the grid current?
EDIT
Build a test circuit lets make it a simple cathode bias type like a 251
Plate resistor(s)
grid to meter set to DC other side of meter to ground
Find the input resistance spec for the meter set to DCV the higher the better 10meg or higher
cathode resistor to ground
measure the voltage calculate the current



a good read
https://www.radiomuseum.org/forum/grid_current_and_vakuum.html
EDIT
use the DMM input resistance as the resistor
https://dannyelectronics.wordpress.com/2016/01/28/the-nano-ammeter-you-already-have/ or put the following in the wayback machine add http to the front
://www.electronicdesign.com/files/29/14692/14692_01.pdf

https://web.archive.org/web/2012032...ctronicdesign.com/files/29/14692/14692_01.pdf
http://www.diy-electronic-projects.com/p212-Picoammeter-circuit-with-4-ranges
and even lower needs a cap without/very low DA
https://www.edn.com/femtoamp-fa-measurements/

 

[GeorgeToledo]

This is a fantastic article about Hiller and the development of subminiature tubes, such as MSC2 and the related designs which followed. There are many historical and technical details within:

(auto translated)

https://www-jogis--roehrenbude-de.t...l=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
(original)

http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Hiller/Hiller.htm

 

[TLRT]

Why not just measure the grid current?
EDIT
Build a test circuit lets make it a simple cathode bias type like a 251
Plate resistor(s)
grid to meter set to DC other side of meter to ground
Find the input resistance spec for the meter set to DCV the higher the better 10meg or higher
cathode resistor to ground
measure the voltage calculate the current



a good read
https://www.radiomuseum.org/forum/grid_current_and_vakuum.html
EDIT
use the DMM input resistance as the resistor
https://dannyelectronics.wordpress.com/2016/01/28/the-nano-ammeter-you-already-have/ or put the following in the wayback machine add http to the front
://www.electronicdesign.com/files/29/14692/14692_01.pdf

https://web.archive.org/web/2012032...ctronicdesign.com/files/29/14692/14692_01.pdf
http://www.diy-electronic-projects.com/p212-Picoammeter-circuit-with-4-ranges
and even lower needs a cap without/very low DA
https://www.edn.com/femtoamp-fa-measurements/

Or use an electrometer

 

[mrbutterskin]

To measure grid current over a mic resistor like the ranges we see in mics like 60-250meg, you need a multimeter like a 34401a with a 10gigohm input impedance mode. Or electrometer like mentioned here which have insanely high input impedances.

Disconnect the capsule, measure the voltage across the grid resistor (use shielded cable with as little length unshielded as possible where you have to break out the leads). The grid current we're concerned about is initial electron velocity current and gas current. IEV current will show up as a negative voltage across the grid resistor while gas will show up as a positive voltage. In some instances these two can cancel each other out so you have to do measurements at various specs to determine if gas current is significant or not. You can reduce the IEV current to almost zero by raising the plate voltage while keeping the same plate current. This way, you can measure the effect of gas without IEV current interfering.

There are other grid currents like grid emission and interelectrode leakage (ie leakage across the mica). Grid emission is generally not a problem unless you are driving the grid positive with respect to the cathode, which in a mic we never are, or the grid somehow gets very hot itself. In that case, gold plated grids (like found in 6s6b-v) help keep emission down.

 

[granger.frederic]

To measure grid current over a mic resistor like the ranges we see in mics like 60-250meg, you need a multimeter like a 34401a with a 10gigohm input impedance mode.

Ouch!!! That's what i thought, very expensive! I have to trust the datasheets...

 

[TLRT]

Ouch!!! That's what i thought, very expensive! I have trust the datasheets...

Check Keithley 617/619 cheap in USA.

 

[Gus]

About low end phase shift in the article
what has less lowend phase shift a well designed transformerless or transformer out circuit?

 

[LevinGuitar]

Found a brief test between the V and the C M49 mics

 

[e.oelberg]

I'm pretty sure this is a 6s6b nice to see a feedback loop between diy community and neumann

 

[RuudNL]

I have repaired (early?) M147 microphones in the past and I would swear that the tube was a 6SB6!
(Anyway: it looked exactly the same.)
Later they used the 6111 miniature tube.

 

[soliloqueen]

Found a brief test between the V and the C M49 mics

oh wow, there is something high up in the V that I really don't like. Wonder what that is. Maybe just an issue uncovered by more high end extension? I hear it in the C too, but it rolls off the part that's really annoying

 

[GeorgeToledo]

oh wow, there is something high up in the V that I really don't like. Wonder what that is. Maybe just an issue uncovered by more high end extension? I hear it in the C too, but it rolls off the part that's really annoying

Yeah, and it’s in units I’ve heard in person as well. I’m glad to read someone who is well regarded point this out, because I have gotten some flack on it.

My conclusion is that some people are not as sensitive to the sonic change, for whatever reason. People do sometimes do have differences in what stands out to them.

 

[kingkorg]

It is a 6С6Б-В (as has long been surmised here). However, a special production series (from the tube factory in Novosibirsk in a specific year). Since Neumann removes the labelling on the tubes, the year of production is unknown. Neumann has a lot of these things. They will last for the next few decades. Whether it absolutely has to be this version is unknown. Neumann keeps the 6С6Б-В test series with different production years and manufacturers secret.

Even if Neumann suggests that the power supply unit for the M49V provides a higher voltage - this is not true. It remains 4 volts, but with a higher heating current. The tubes run stably from around 3 volts. At 4 volts they don't get as hot and produce even less noise. Long-term tests have shown no negative effects such as cathode poisoning in the event of long-term underheating.

The supply of 6С6Б-В will never be exhausted. There are some people who have hoarded thousands of these tubes. They can still be bought cheaply in Russia (approx. 6 euros each). I expect the prices of the Telefunken AC701, RFT AC760, Hiller MSC4 and NEC 6D-H3 to fall sharply in the medium term. They can all be replaced with the 6С6Б-В if you use a modern power supply unit that can provide approx. 200 mA heating current.

 

[RuudNL]

Interesting! So the filament voltage is always 4 volts...
On another well known 'gear' forum , I found:

Q: Does this mean that there may be new service possibilities for vintage mics? For example, would the new power supply possibly work (with adjustment for the tube)?

A: (K.H.)
Yes, Neumann thought of that as well: you can power a vintage M49 with the new supply.
You can even replace the current tube in the reissue with an AC701 (without the need for any additional circuit changes!), then flip a switch inside the mic and power the mic with the new supply, or use an old N52, or NN48, or a Hamptone, or whatever supply you prefer.

That explains why I could never find that 'switch' on pictures of the M49-V.