Yet Another DIY U67 Mic

[Ó Draighnáin]

First foray for me into the valve mic world. Made plenty of other valve equipment.

Took some risks, the layout is nothing like the original for the vertical circuit boards, High impedance board includes an extra cap (I moved C2 up there as well). I accommodated components I had at hand, including a rather large 470uf cap. That cap basically goes from transformer to anode without much extra wire involved. After getting the power supply voltages in range plugged it in and it sounded like I was generating mostly hum but there was signal getting through. Got out the isopropanol and cleaned up (hadn't done that yet), reflowed the hi-Z joints and to my surprise I had the basis of a potentially very workable mic.

I still want to tweak the power supply. For those that might know, is there any reason I should drop the shunt (R4) resistor value instead of raising R2 and R3?

The power transformer I used had a 230V secondary (240V mains here) and also given the low current draw provides a high voltage post rectification. I had to raise R2 and R3 to about 22k. I could drop these back and and also drop R4 (down from 68k)?

Plate voltage is currently about 82 VDC, so in the ball park. The filament voltage is not a problem.

Any ideas appreciated.

 

[Ó Draighnáin]

So I have psu voltage of 209V under load for the HT. Pretty close to the required 210V. Plate voltage is valve dependent, neither of the EF86 valves I have gives 75V on the plate (one is low one is high), Need to ensure 60V back plate though so can't fiddle with the HT too much.

Herein (back plate) lies the source of my stupidity. Firstly, have to remember to read up on reading voltages at high impedance nodes. Secondly, mis-wiring. I mixed up the 120V figure 8 wire and the 60V back plate wire. Me thinks I might have a cooked capsule. Spare capsule change in pending I suspect. Main take-away so far is that the layout is workable.

At least the second mic underway will benefit from all this.

Other mistakes made (and corrected) will not be disclosed due to fear of derision.

 

[Ó Draighnáin]

Capsule still good, all fixed, nice an quiet.