well it had to happen sooner or later. needed TE to fix a problem.
my national sw-54 receiver suffers from a well know design fault. the dial lamp is powered from a 6.3v tap on the 35z5 rectifier. problem is that the plate current for all the tubes also goes thru the poor little #47 lamp. no shit....see this link
https://www.philcorepairbench.com/blown-pilot-lamps-and-35z5-open-filament/ .
all is well unless you switch it on an off again too quickly (or if the power company decides to balance their load and switches our development from our north substation to the one south of us). when that happens the full current necessary to recharge the filter caps zaps the lamp.
have been thru half a dozen #47's in the past two months. tried a pair of back to back 6.8v, 1 watt zener's across the lamp to clamp the voltage (based on a recommendation from an antique radio repair website). it sorta' worked but eventually the diodes would fail short.
decided to try the trick from one of daves videos and make a voltage clamp using the B-E junctions from a pair of transistors. had a bag of 65 fairly beefy transistors that have a BE reverse breakdown that measured at 10.2V. but first needed to see what the voltage across the lamp looked like in normal operation. didn't want to screw up and blow up a real scope, but no worries. the fluke is a scopeMETER. so it is happy doing floating measurements as long as you are not more than 300v above ground ( i think that is what the manual said).
you can see that the voltage drop just from the filament tap peaks at about 4.6v, while the peak with the plate current thrown in hits 9.4v. so a 10.2v clamp should not affect normal operation, but should kick in and stop a quick on/off from killing the lamp.
and damn if it does not work just great. radio sounds the same as before and multiple attempts to kill the lamp or the transistors did neither.
winner winner chicken dinner.
ps if anyone wants to know what those transistor are.....dunno. have a bag of about 60 of them. they are definitely silicon npn. the part number stamped on them crosses over to nsn 5961-00-230-8853. cage code 98230 comes up as......ummmmmmm......No Such Agency. wonder what the hell they came out of? am watching right now for black helicopters.