I "hack" cheap "electronic transformers" for use as filament supply for my tube amplifier projects. They operate at about 50KHz so there is no audible hum (a particularly annoying problem of directly-heated tubes). For this purpose I rewire the secondary to get the 6.3V required by the tubes. Works wonders :-)
Now I want to experiment rewiring the secondaries to provide the high voltage for the vacuum tubes (B+). Something in the 150-250V range, a few tens milliamps current. Below is a pic of my first attempt, with stock unit at left).
As a precaution I wrapped the stock primary with mylar tape before winding the new secondary on top. I simply counted the stock primary turns and made a rule of three for the number of secondary turns. After winding, measured inductances seems in the ball park: my secondary has twice the primary inductance, which is 1.4142 voltage ratio.
Does everything seems ok to your eagle eyes? Now comes the light bulb part of the question: In a youtube tutorial on DIY transformer winding the guy puts a regular incandescent light bulb in series with the winding and hook it to the mains. If the winding is ok the light bulb does NOT light.
I guess the theory behind this test is that if the winding is good all the power goes into the magnetic field hence no glow on the bulb. Is that correct? And perhaps more importantly, is that a valid test?
For reference, here is the video URL at the test's start: https://youtu.be/Q6GkSNfAEx4?t=954 (The light bulb tests are between 15:50 and 17:30.)
Thanks for any insights! -Joe