Here is a one-of-a-kind unit using a re-purposed Thordarson transformer. IT works great, and the vintage Shurite (made in New Haven CT!!!) DC voltmeter is a bonus.
On the ‘business-end’ (topmost image) you can see XLR output jack (for connection to the input of your mic preamp), on/off switch, pot for controlling the DC voltage that mic receives, and at right the cable-exit for the seven-foot cable harness that connects to your double-button carbon mic. At right: red and black wires connect one-to-each button, and the clear wire connects to the metal shell of the mic.
For an explanation of WTF this thing is, and why you might possibly need it in your life, see this previous post.