This webpage describes a microphone modification of a Marshall MXL-603s as described in David Royer's second Tape Op article in the April/May 2003 issue. (See my description of the modification of the Marshall MXL-2001 here.) A reprint of the article can be found below:
This page is in no way intended to be a step-by-step instruction manual on how to build a tube microphone. This microphone requires a dangerous high-voltage power supply. Do not try this at home. Buy David Royer's kit!
This is a day-by-day account of the project.
March 24, 2005. Three 5840W tubes came today from Triode Electronics. I might make two of these mics, and it doesn't hurt to have a spare tube.
A glass vacuum tube. No matter what the microphone sounds like, the glass and metal tube is compelling.
Here is the MXL603s with guts, tube and a quarter for size comparison.
Little gold diaphragm:
Beautiful, I hate to take it apart.
March 25. A box of candy from Mouser came today. The red 2.2 microfarad caps look particularly tasty.
March 26. Disassemble the 603s. Unscrew the little screws...
and desolder the XLR connector wires from the board.
Loosen the screws holding the white cap to the top of the board. Desolder the wire from the cap and remove the board..
Trace and cut a new circuit board from a piece of scrap using the band saw.
Mount the tube. I used the holes on the scrap board to combine pins according to Royer's circuit diagram.
I had to sand the top and bottom edges of the board so it would fit in the slots. Nice tight fit.
March 29. I sanded the copper traces on a scrap of circuit board to break any contact between existing holes. I mounted and soldered most of the power supply parts. I used existing holes when I could, drilled when necessary:
April 1. I gutted an old computer power supply box to house the mic power supply. I cut components off the old power supply board and cut a piece of it to mount the transformer. I double-checked the wiring. Oops. One electrolytic cap was wired backwards. The mic cable is wired to the board. I used the strain relief from the old power supply. I still have to connect the XLR cable that will plug into the mixer.
April 2. The box is all wired up. The XLR connector is on the left, the four wires destined for the mic are on the right.
The mod calls for a 5 wire mic cable, but what I have is shielded four conductor wire. I don't need to disconnect the wire at the mic to mount it in a mic stand, so I modified the original 3 pronged connector by pushing out two of the prongs and drilling two holes:
The four shielded wires will go through these holes. I can cut the last prong. I don't want to push it out because it is soldered to a brass insert that screws to the case. I need this secure ground since one side of the capacitor capsule is grounded through the case.
Construction is complete and I tested the mic.
It works but I can't do a fair test yet because it hums. Crisp highs and seems difficult to overload, but I have to track down the cause of the hum before I can do a real comparison. Frustrating, but at least it works.
June 12, 2005. I was able to decrease the hum by shortening the wires to the audio transformer but it is still not quiet enough. I got a suggestion from cyberspace to separate the power transformer from the audio portion of the circuit. I'll try that.
Later. I put the guts into a bigger case and got rid of the hum! Physical separation between audio and power transformers is important.