Dim Bulb Limiter question (1967 Vibrolux)

[Steelwitch]

Hey everyone,

I just built a dim bulb limiter (it works!).. on one hand I am happy to be done blowing fuses while troubleshooting amps, but also getting familiar with what to expect from the voltage drop across it. With standby on it's passing 120v and with standby switched off it pulls the mains down approximately 20vAC. the bulb is 150w incandescent. The original issue with this vibrolux is that the fuse blew on the owner's gig. Ok, I removed output tubes and the light was completely dim (probably due to the lack of current being drawn from the output stage) but still got something to short out. This amp has been serviced recently so my feeling is it's likely the rectifier tube. I replaced it and it hasn't happened again. Is it feasible to stress test under a lightbulb limiter with the voltage being drawn down by 20v? Is this indicating an issue somewhere else? or is it normal? thank you!

[xtian]

You are using a 150w bulb, which will allow 1.25 amps of current. That's enough current to destroy plenty of components, and quick. The point of the LBL is to prevent things from burning up, while allowing you to identify whether there are shorts in the DUT (device under test). I use a 40w bulb, which allows at most 0.33 amps--the bulb will light brightly if there is a short, but then I can turn off the DUT quickly before anything frys.