3-500Z Arcing

A bit off topic for the general group, but I wanted to share a video of an arcing 3-500Z tube and a tip in troubleshooting with shorted radios/amps.
(Note: the 3-500Z is a very popular compact power triode used in many older tube KW amps.)

This defective tube, was inside a Kenwood TL-922 linear amp that I bought at the Belton Hamfest a few years ago.

When powered on, the dual 15A fuses would blow *violently* (vaporized metallic link).
My troubleshooting adventures sent me on a few costly and depressing wild goose chases, including suspecting bad power, bad filament transformer, bad filter capacitor bank, and bad diode strings.  sigh.

This was one of those outstanding bench projects that gnaws at your soul.  You'd go to bed knowing this project had conquered all of your skills.  It remained unfulfilled and sitting beside the bench with no solution in the works.- for a few years!


Part of the issue for me s that I hate working on circuits that have high voltage (3KV). 100V even makes we wary.  Yep, I'm a chicken little.
For troubleshooting I have a trusty benchtop variac but it's only fused at 8A. The 15A shorted fuse current draw from the amp presented an issue that would normally blow my variac setup.

Luckily, I have a 2nd TL-922A that I used as my primary amp in the shack.  So I finally got off my arse and lugged it up on the bench alongside the questionable one, to do some A/B comparisions.

I wanted to share an old trick that has been used by many a well seasoned repairman:  Place a lightbulb (or better yet an old car headlight) in line (i.e. in series) with the power cord.  If the device under test is drawing excessive current (i.e. shorted), the light buld will dissipate the energy by lighting up brightly, vs blowing up your fuses, your internal circuits, or the house circuit breaker.


See https://antiqueradio.org/dimbulb.htm


So I used this strategy to vindicate the power supply and finally after lots of online research, determined that the 3-500Z was likely a culprit.  To make this harder, there's really no practical way to test the 3-500Z other than testing it in an amp.   I couldn't even find someone who tests them for a fee.

So by slowing increasing the voltage to the amp using my variac and the in-line AC lightbulb, I was able to observe the 3-500Z short out (plate to grid) and also safely compare it with a known good 3-500Z tube.  The defective one would actually start arcing in a rhythmic frequency.  As the arc shorts out the 3KV HV, the in-line bulb lights up as the HV is discharged.  The HV starts building up again until the HV reaches an arcing potential and the cycle starts all over again.  A relaxation oscillator.

Anyway, i think it's pretty rare to see an arcing 3-500Z tube, and wanted to share with the group, in hopes this might be helpful to someone someday.  Yea, I had to bypass two safety switches on the amp, which is not usually advised, but that was the only way to see the tube in action.

What still mysterious, is that the amp contains two 3-500Z tubes.  BOTH of them are shorted.  The guy I bought them from said the amp was driven a distance from one house to another.  So, were the tube subjected to excessive car vibration during his travel and damaged the tubes?.. or was I duped and these bad tubes were intentionally placed in the amp when it was sold to me?

Here's the video of the arcing:

IMG_3472

73 
Dave W5OC