Tube tester

[nickb]

Question:
Is the main transformer, factory or is it rewind?

This was the thing that jumped out at me. This seems to be poorly done rewind and the crucial insulating varnish step has been missed making it potentially dangerous.

[vintagekiki]

I know from experience that miscellaneous repairman when rewind burn transformers, use existing wire, just discard the damaged (burn) part of the wire.
With such transformers, it is very important to check that no breakthrough happened primary coil to core (chassis)

[Mick Bailey]

Emission testers like the one you have operate on a fairly low HT and it's practical to derive the voltages safely by other means if the inbuilt transformer is beyond economic reclamation. The same with the heater voltages. Most of the testing of audio-related tubes is usually done with heaters at 5v, 6.3v or 12.6v so you could limit the selections to these values. One way to do this is to use a variable-voltage regulator IC and switch between trimmers to give the heater output voltages you want. This means you have to have a DC supply and a convenient way to do this is to use an external laptop power supply. There are some really neat DC-powered boost converter boards available at low cost from China to give you the 48v and 71v HT. (I paid £2.70 for one that gives 350v at more than enough current for this)

Alternatively you could power the whole instrument from an AC transformer (maybe 18v secondary) and use a regular voltage multiplier cicuit to give you the HT. Tap off the 18v and use a bridge rectifier and smoothing capacitor to provide the source for the heater voltages.

All of this works - I have a Mity-Mite emission tester that uses a capacitor-diode multiplier for the HT and switchable trim pots/regulator for the heaters.