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R/E/P => Klaus Heyne's Mic Lab => Topic started by: dbalcaen on January 17, 2013, 09:40:45 pm

Title: Quiet U47, 15-20 db down
Post by: dbalcaen on January 17, 2013, 09:40:45 pm
Hi.  I am a tube amp guy and a U47 came into the shop with an odd problem, it is quiet.  Really quiet. 

I am looking for "typical" output voltages from a U47, or electrically equivalent, mic to compare to.  Here is the story:

Sounds great with the volume cranked up, but at least 15-20 db down below other large capsule condenser mics (fet).  This is the first U47 I have investigated, so no background of experience with this beautiful mic.  Has had Nuvistor conversion, and power supply is at 93.7 volts DC.  Looks like someone along the way replaced the bridge rectifier as it is "tack" soldered in place.  Physical inspection, and testing of all capacitors and low value resistors.  I do not own a meter to test a 100 Meg ohm resistor.  As an experiment to check the circuitry I removed the capsule, injected a 20mv peak to peak 600hz sin wave to the grid side of the capsule contacts through a .047mfd capacitor.  Measured 200 mv peak to peak AC on the plate of the Nuvistor, so it seems to be amplifying.  The coupling capacitor to the output transformer shows, 200mv p-to-p being applied to the primary of the coil.  20mv across secondary, and it did not change when I repositioned the output impedance from 50 to 200 ohms.  As a comparison, I took my cheap Shure shop mic and sang loudly into it and measured 20 mv p to p on the output.  All measurements done from my oscilloscope and are apx.  I think 20mv at 600ohm is -40dbm.
As this is the first U47 I have ever worked on, I cannot comment on the output level other than to say it is really quiet, and certainly not as the literature and common use states as a high output mic. 

Any comments on measured voltages/dbm would be gratefully accepted. 

As an experiment, and with no capsule connected, I thought I would see just where the Nuvistor began saturating and clipping the signal.  I increased the input voltage from 20 mv all the way to 2 volt, and the output just climbed ever larger and NEVER did clip!  Wow, this tube is being run way, way down in it's operating range. 

Cheers

David Balcaen, Saskatoon Sask, Canada
Title: Re: Quiet U47, 15-20 db down
Post by: klaus on January 17, 2013, 10:28:42 pm
Hello David,
Even if everything was working fine inside the mic and power supply, you will not get much of an audio output when the supply voltage is 17VDC too low. Nominal with Nuvistor-tube: 110VDC.

So, start changing the dropping resistors across the elco caps inside the power supply until you get 110VDC output, with the mic connected, and measured inside the mic- at Tuchel connector pin #5.

Let's talk again once you have brought the supply voltage into compliance.
Title: Re: Quiet U47, 15-20 db down
Post by: dbalcaen on January 18, 2013, 01:51:53 pm
Hi Klause.

You were correct.  I altered the PS dropping resistors equally until voltage at the mic was 110  (actually 113) and the mic now performs perfectly.  There was nothing wrong with anything else.  Obviously due to the single supply providing plate, filament, cathode and bias, being 20 volts low did not allow the mic to behave properly.  I now know, first hand, that any reduction in voltage to the mic reduces the output of the microphone in a very big, non linear, downward fashion.  The 110 volt is critical.  I thank you for your input.  The owner of the microphone was overjoyed at the  20 db  increase in gain from before I made the PS mods, and compared to another Neuman, this old U47 just sounds so sweet and warm.  Cheers,  David Balcaen.