U47 lost low end

[madreza]

Hi Guys , I have 2 pretty similar problems at the same time and don't know what too look after

I have a real U47 ( nuvistor ) and a MK47 , both lost their low end
on the real one I tried swapping the capsule and nuvistor with another one, no result !


which way do I have to look ?

Thanks

 

[MagnetoSound]

Blown woofer?

 

[MagnetoSound]

Is the phantom power still balanced (present on both pins 2&3) at the input of your MP1A?

 

[madreza]

they are not connected to the MP1A , So , No Phantom power

 

[Gus]

Are you using the same signal path as when the microphones still had low end?  If so maybe something changed in the signal path.  Do you have other microphones to try to hear if they changed?
Can you test your u47 and MK47 with another system?
Has the humidity changed before and after the low end reduction?
Have you checked your cables the XLR one after the power supply is one to check first

 

[madreza]

thanks for your reply
both mics tested on different system .... same result
the mk47 tested with another PSU , same result
the real 47 voltage tested at the psu , the voltage seems to be good !

 

[tubestation]

Hello,
Do You have a LC Bridge or a handheld induction Meter handy?
You could meassure the Outputtransformer. The Primary Winding should read something like 160 Henry at 100 Hz.Maybee it s better to test the secondary winding first. It should be 160/6,5*6,5 Henry. I hope not ,but a shorted 3pin phantompowered Miccable (Pin 2/3 with Pin 1) or another ,even small Voltage,current,over a longer Time could permantently mag the Core .When the Problem happend with the first Mic,did You use the same 3 Pin Miccable for testing the other? I dont know the Induction of Max Tranny , but it should be in the same Ballpark like the 47. If the Core reads much lower Induction,than expected ,You have a Problem.
Try to put the active Mic in front of a loud Speaker for a Day or longer, this could demag the Core again, maybee. Hopefully another Member, Cj , PRR ,analag or another Expert have a good solution for You how to demag the Core again. All other possible Reasons for a Basslost seems unrealistic for me because it s unusual that the same Problem appears at the same Time with both Mics, so I would test the Transformer first.
Good luck!!
Greetings
Lothar.

 

[madreza]

Thanks for your help
Finally I changed tubes on the mk47 and it solved the problem
The u47 still needs troubleshooting
What I forgot to mention is that the U47 is not mine so it is just coincidence they have the same symptoms

 

[tubestation]

Hello,
Nice to hear that You solved the Problem with the Mk 47. Witch Brand of 6028 Tubes do You use?
A Nuvistor converted U 47 isn t known to be Bassstrong due to the Impedance Missmatch of the Outputtransformer and the NuvistorTube. I didn t heard a Nuvistor conversated 47 by myself but it s common sense that a Basslost does happen due to a much higher Ri (over 20kOhm) of the Nuvistor Tube that can not drive the Bv8 Tranny properly.  But I also hear that some People like it like this for Vocal Recordings.
You could sent, or better bring the Mic to Andreas Grosser in Berlin for a re-Build.He has a popular Fet replacement "Tube", and also a glasstube in a Steeltubehousing as an Option, but it s only possible  if You send him the Mic, he doesn t sell the Replacement just so, - not very Diy friendly, but in Your case it could  be a Way, because a soldering Job to change the Tubesocket and a complete Test of the Mic and the Powersupply should be performed anyhow,- or keep the Mic as it is and live with the not so Bassstrong Sound.
Greetings and happy recording
Lothar

 

[madreza]

thanks Lothar for your reply,
but the problem I have here is like there was a low cut at 800-1KHz ....... it's not just a nuvistor impedance missmatch
I tried to swap the Nuvistor with a good working U47 , didn't solve the problem ..... I have to check the transformer ......
can anything else cause that symptom ?
Cheers

 

[SSLtech]

No idea on this particular problem, but I thought I'd post to say that I recently converted a matched pair of U47's with factory Nuvistor retrofit.

The mod was one which was proposed by David Bock, and involves replacing the BV-8 with a BV-11, then removing the resistive 'pad' inside the power supply.

I did it to one, and gave the mic back to the owner, for his thoughts and comments...

Within a day, I got a message back, saying:

DO THE SAME MOD TO THE OTHER MIC.... OMIGOD, OMIGOD, OMIGOD!!!!

I asked tor both mics, so I could form my own impressions, and I have to say that the transformation is nothing short of STUNNING. The low end is strong and "mighty". The output level is also returned to nice and strong (the nuvistor output is always lower, and often requires a LOT of gain, sometimes bringing preamp gain into the picture.)

The transformer I used was an AMI from Oliver Archut, and nothing went to waste, because the old BV8's were transplanted into a couple of U47/EF14 'clones' which I've been working on.

As you were...

 

[tubestation]

Hello Madreza,
Did You swap just the Capsule or the whole Head? Did You switch to ommni, and have the same Problem?
You can messure the 10n ,the 1uf Bosch Mp and the 100 m Resistors if they are still in specs. If any of this Parts lost there Value a Basslost takes Place .A standart DMM does not work. You need a Meter that can messure up to 200 M Resistance, a Capitormeter and a Inductionmeter that can handle at least 200Henry and works with a Messurefrequency at 100 or 120'Hz. I hasitate a bit to give You some fast Tips because Your Mike is a very dedicated and sensible Item and can be spoilt quiet easy. The leads of the Transformer are very fragile and break easy.The first Thing I would do is trying to get some Help from Someone who is used to fiddle around in such Sensible Units. Than Messure the Secondarys of the Outputtranny (at the Powersupply between pin 2/3 of the Output xlr) hopefully You get something like 2 H at this Point,than test the 100M resistors by lifting one Side of them, otherwise you don t get the correct Reading, same for the Condensers. Please be very carefull ! ! I hope the 10 n or the Bosch 1uF is the Problem, but it s quiet impossible that the 1uF Drops to 15nF or something like this ,to create a Highpass at 800 Hz. I think and hope it s the Capsulepolarisation 100M Resistor, or the Gridresistor.
I was reading a little Bit about Transformercore demag and Jacob wrote that it is the only possiblilty Way to get rid of it , is to disamble the Transformer. Than You demag the Lams with a bulk ereaser or You can swap /turn every second Lam to neutral the remanent Magnetism.
I would never do this by myself, if You bent the Lams even slidly, They loose there Perm constanly !! This would be a Disaster and a Way of no return, ( so handle them like Glas ) or they need to get aniled again, so ask a real Expert like Max, Moby, CJ ,Oliver from Tab or Andreas Grosser for Help. I Rewind of the whole Thing could be the Result ,if You disamble Yourself , because the Isolation of the Magnetwire breaks very easy because it s rotten after 50 Years.
I just wrote the Post from SSLtech, this is a good Way to go also and i am sure it will sound very good with the Bv11 Tranny,but first i would try everything else, including to send the Mic to Andreas Grosser for a vf14 fet Replacement.and Transformer, Circuit Test. Normaly i am very open for modifications, Upgrades etc and don t see things out of a Collectors View,but in this Case,even if the Nuvistor is already a downgrade, i would not touch the Mic like this before i tried everything to get it back as original as possible, - just my 0,01 Cent s.
I heard a few original 47s with the Andreas Grosser Retrofit Tube, direct compared with my VF 14 U 47 and i must say it sounds a very little bit different in the Highs and Lows at high Levels, the important Mids sounded equal to us, it s a very close and very good Replacement, much better than the Nuvistor with the Bv8 ,i think.
I don t want to make advertisment for him, i don t know him by person.
Good Luck
Greetings
Lothar

 

[Gus]

Have you checked the output transformer?

 

[tubestation]

Hello,
If You test the Capitors, don t just look if They still have there rated Value in Farad, look for the dissapation Factor as well. If they have a greater  Faktor than lets say 0,2 (a good one reads something like 0,002 ) they are leaky and DC isn t blocked anymore. In Case of the Outputcouplecondenser You have DC send to the Tranny and this causes a permanent magnetisation to the Core.
For a Transformer Test You could disconect the Condenser at the Side that goes to the Transformer
Than apply different Frequencys with a  very low ImpedanceGenerator to the Secondarys ( via the 3pin XLR  on the Powersupply.)  At the Place where the Condenser have been before You can now connect a Milivoltmeter and look if every Frequency You send from Your Generator is equal. If the Core is the Problem You will see a big drop at the lower Frequencys, or if You don t have a Milivoltmeter use a passiv Di Box and send the Signal to Your DAW. I think it s better to apply a Testtone to the secondarys because i don t know what happens if You drive the high impedance Side of the Tranny with a low Impedance Generator. But this will work also ,probably , but than You have a very low Impedance at this Point. If the Tranny is the Problem You can try to send pink Noise with a high Level to the Tranny for a few Days and than test again. This probably will not solve the Problem completly but if Your Bassresponse gets better You can be shure that You need to demag the Core.
If You have not the expected Henrys , when  You messure the Induction of the Transformer, than you could have a short in the Windings. This Test should be done first.
I hope I did not talk Nonsense.
Lothar

 

[CJ]

you can check the OPT low freq pri inductance with a generator and ammeter,

use Ohms Law and Inductive Reactance formulae,  E/I=R and XL = 2 pi f L

apply 10 volts @ 10 hz  thru the current meter into the pri,

math goes like this, use 1 ma ac as example for current,


10 volts / .001 amps - ac = 10,000 ohms reactance,

divide 10,000 ohms reactance by 10 hz and again by 6.28 ,

so 10,000/10 = 1000, 1000/6.28 = 159 Henries,

let say you read 10 ma at 10 hz and 10 volts,

10 / .01 amps -ac =  1000 ohms reactance

1000 ohms  reactance / 10 hz = 100,  100/6.28 = 15.9 Henries,

i do not know what the pri Henries should be on the OPT, maybe Gus knows,

here is a chart for figuring out inductance using 10 volts at 10 Hz


ma  H
1 159.2
2 79.6
3 53.1
4 39.8
5 31.8
6 26.5
7 22.7
8 19.9
9 17.7
10 15.9
11 14.5
12 13.3
13 12.2
14 11.4
15 10.6
16 10.0
17 9.4
18 8.8
19 8.4
20 8.0

 

[tubestation]

Thank s CJ for this nice Lecture ! Great Drawing ! Now I get it !
Is it possible You made a Typo? , because You calculate with 10 Hz / 10V and get a Result of 159 Henries  at 1 mA Current.  In Your Chart You give the same Results for 10v/20 Hz . It did Your Calculation and get  10000 Ohm/ 20 Hz/6,28=79,62 Henries. Or did I get something wrong again ?
Thank You very much,
Greetings
Lothar

 

[CJ]

here is the correct table for 20 hz, i will change the name of the erroneous chart above to 10 hz in order to give ammends by posting 2 charts for the price of one, 

chart for 10 V-ac and 20 Hz

ma--Henries
1 79.6
2 39.8
3 26.5
4 19.9
5 15.9
6 13.3
7 11.4
8 9.9
9 8.8
10 8
11 7.2
12 6.6
13 6.1
14 5.7
15 5.3
16 5
17 4.7
18 4.4
19 4.2
20 4
21 3.8
22 3.6
23 3.5

 

[Moby]

Thanks for your help
Finally I changed tubes on the mk47 and it solved the problem
The u47 still needs troubleshooting
What I forgot to mention is that the U47 is not mine so it is just coincidence they have the same symptoms

Hi. If you cant measure the inductance of the primary then u can desolder the red wire and put the line sweep through Bv.08 . It should be pretty flat from 20hz sourced from lowZ generator. Just my 0.2c

 

[squib]

Or ... just poke a tone generator into the input of the head amp and sweep the rrspony of the ehole shebang

 

[tubestation]

Hello,
Is is really the Case that a permanently magnetised Transformer Core causes a Drop of Induction ?  Do only the Low Frequencys drop,because  f cut = R/2pi * L  or are all Frequencys affected also and get distorted ? And Why ? I think because of the interupted Hysteresis Loop. Is a H or B Change ?
Thank s
Lothar