I was nearing completion on my homebrew tube tester when I was treated to 2 nice pops, each of which took out 2 panel meters, a transistor, and a zener diode. I've attached what I hope is the info necessary to diagnose what might have went wrong. I think I have some rough ideas. In a nutshell, the nodes in my HV power supply are connected to female banana jacks which can be jumpered to either of 2 Hall amplification style VVRs. Each VVR then outputs to another female banana jack via a Vishay/Dale 1ohm 1% 5w resistor. These jacks were not connected to anything, though they do have caps to ground via a switch and RCA jack (which will allow me to listen to tubes with powered speakers, like with those cheap DIY signal tracers). Each VVR has it's voltage read by a 0-600v DC digital panel meter which reads from chassis ground to the VVR output banana jack. The current of each is read indirectly across the 1ohm's by a 0-200mv panel meter. I built everything slowly, section by section, with lot's of testing and variacing along the way and everything worked beautifully, with nothing generating significant heat (if any at all), until I connected the 0-200mv meters across the resistors. Note that I never had more than 1 of the VVRs connected to high voltage at once. With one side connected, I flipped the master switch for the tube tester on. No problem. I then turned on the power (switch in drawing) for the 600v/200mv meter that was connected to B+. I don't "think" anything out of the ordinary was happening at that point. When I then turned on the 600v/200mv meters for the then unconnceted (to B+) VVR, poof. Stupidity lead me to then repeat the process by switching the B+ to the second VVR. Don't know why I expected anything different to happen. One of the 600v meters still powers on, though I don't know if it measures. The other meters, the NTE2973s, and the Zeners are also dead. Everything else is fine. The high voltage is present on the banana jacks to which the VVRs output. Now, however, the VVR knob will only drop the voltage by a couple of millivolts
Both HV secondaries are fused at 250ma, which hopefully would have prevented something worse. My initial conclusion was that I overvalued the use of an isolated power supply for the meters, and that using a single + and - connection to power those 4 meters was a mistake and lead to some sort of short through the meters. I intend to push on, slowly and carefully, but my initial plan is to power each 200mv meter that reads across the 1ohms with it's own 9volt battery. If anyone has any ideas as to what went wrong I'd be grateful for them.Thanks as always!
Joe