I'm building a Cathode Ray Tube-based oscilloscope. The PSU is very simple: 6.3 V AC for the heater (at 0.6A ), -100 V DC for the negative bias (at less than 1 mA), and +500 V DC for the HT (at about 3 mA).

My 100 W toroidal transformer has dual 110 V primaries, dual 175 V secondaries, and dual 6.3V secondaries.

Wiring the 175 V secondaries will provide 350 V AC which when rectified and smoothed will produce 500 V DC.

Simply connecting one of the 6.3 V secondaries to the heater will work.

My problem is the -100 V DC bias.

Here is my question: Can one of the 110 V primaries be used as a 110 V secondary? There will be no connection between the two windings.

Using one of the 110 V primaries as a secondary would provide about 160 V DC that could be smoothed further and dropped to the required 100 V DC. NOTE: the current requirement for the negative bias is very low and consists mainly of a 100 kΩ potentiometer whose wiper goes to the "brightness" electrode on the CRT.

The transformer (from ANTEK) is highpot tested between all the windings and therefore will provide the galvanic isolation required. There is a screen but that is between the primaries and the secondaries.

The outputs from the PSU will be isolated from enclosure of the scope. The main danger in a scope with this configuration of PSU comes from the 500 V DC HT.