My mic would fire up ok, but after 10 or 15 minutes it would quit on me; did I have a bad tube, was the power supply crapping out, maybe a cabling problem? Lets check it out!

I popped open the power supply and checked for any obvious problems, burned components or traces, cold solder joints. Seeing nothing unusual I powered her up and checked voltages. Everything seemed fine.

APEX 450 Power Supply Click Image For Full Screen View

Below is the schematic for the 460 power supply, not exactly the same as
 

APEX 460 Power Supply Schematic Click Image For Full Screen View

the 450, but close enough for rock 'n' roll. Note that the pin out is backwards. Also, pin 3 is not ground, it is the variable voltage that controls the polar patterns. And, I measured 6.6 volts on pin 2 for the heater. Problem? Lets disassemble the mic.
 

Hold the mic body firmly and unscrew the bottom.

Slide the outer case off. This will expose the pcb and all the screws holding it to the inner case. Inspect all the screws carefully, mine were all damaged to some extent when the mic was originally assembled. At this point there is no need to remove the grille, black screws heads on mine. Remove all screws on the inner case, including the two on the pcb. Gently free the threaded ring, this is where I ran into trouble! There is a screw in the lower threads (click for a detailed look), tighten this one.

Component side of the pcb, marked SK.8 under the transformer.

Here's the valve, I subbed a Sovtec 5751 for the 12AT7 that was in the mic.

Lets have a look at the schematic for the 460, there is a major difference. The 460 amplifies the signal, then utilizes stage 2 as a cathode follower, coupling via C8 to the transformer. The 450 couples the signal off the plate via C8 to the transformer. I favor the 460 layout, should be far less signal loss. Interesting variation here, pin 5 (as are pins 6, 7 & 8) is grounded, effectively eliminating  half of the heater, hence the 6.6 volts from the power supply. I'm a valve guy, and I didn't realize both sides of a dual triode are independently heated. The 450 requires less heater current, less B+ current; BUT, at the expense of more signal loss. Thus more gain, and potentially more noise, required down the signal chain. My vote goes to using the cathode follower.
 

APEX 460 Mic Schematic Click Image For Full Screen View

So, what was wrong with my APEX 450? I don't know! In removing the board I pulled a couple of solder joints apart. Did I have a cold joint, that when heated opened, and caused the problems? The 12AT7 seemed to work fine after I cleaned up all the solder joints. In the process of pulling this thing apart several times I've noticed that some of the lower board joints have left impressions in the insulator on the lower end cap. I had to compress things each time I re-assembled the mic and this may well have been the problem.
 

Check the end pcb for long pins, trim as necessary. Click image for a better view. You can see where I pried on the end cap, not a good idea! Never force things.

Pull the grille and check the capsule to make sure it is mounted tightly.
 

The capsule was loose on its base, tighten as shown if necessary.