AMI U47 Kit review

[Niketouille]

Hi,
One of my client ask me to assemble his U47 kit from AMI. As now the build is complete I wanted to keep trace of the few details that went wrong.
PSU is from AMI, tube is EF800, capsule is drefahl audio. The kit is premium. Mic has remote polarity selector on the psu.
Mic is working now and sounds quite good. Finishing is superb and package contained everything needed to complete the build.
The body had a slight alignment issue with the fixing hole and I had to enlarge the 3 top hole in order to fix that.
Also there are some mistakes in the schematic : C4 is 1nF and pin 4 and 7 from the connector are swapped.
When time has come to check psu voltage I find out that it was impossible to get that 105V. I had to replace R8 on the psu board with a 47k/2W resistor to allow the trimmer to reach 105V.
Tiny detail, badge is not provided but body has the hole for it.
In use the only concern is when you switch from one polarity to another. Sound tend to disappear for a couple of seconds then come back. It comes from C4 ( again... ) charging/discharging time.
Any questions are welcome otherwise I strongly advise you to build such a kit.
Also If any mic guru want to explain me the use of C4, I would be very happy.
Cheers

 

[RuudNL]

Also If any mic guru want to explain me the use of C4, I would be very happy.

Where C4 is the capacitor connecting the front and back membrane?
Well, you want the AC voltage from the back membrane to reach the grid of the tube.
But you DON'T want the (positive!) polarisation voltage (on the rear membrane) to reach the grid!
(The grid should always be negative referred to the cathode)
C4 blocks the DC voltage.
When you switch the polarisation, it takes some time to charge (or discharge) C4.
After that, the situation is stabilized and you hear sound again.