Filiment transformer insulation

[timmy]

With 185vdc on the cathode shared with the heater of a crt would there be worry about this voltage passing a filiment transformer and it's insulation to ground. A triad 13x 6.3 .6 vac does anyone know if this trans would be sufficient ? I guess what I'm saying is a crt with a heater cathode short one would use an isolation trans for the heater so I was looking into a 3gp1 crt for a pilot tv but the pinout of this tube shows the cathode wired internally with the heater and the pilot has up to 185vdc on it and don't know if there would be any insulation problems between the heater and cathode voltage within the filiment transformer.

[init4fun]

Timmy , why not just use an isolator and then that wouldn't involve trying to determine the breakdown voltage of a filament transformer's insulation ?
Yes , your not likely to find one with the connectors you need , since I don't ever recall seeing one for the 3gp1 style tubes (this is likely if , as you say , the heater and cathode are already internally connected in that CRT) but if you were to get an isolator and wire it up with the right connectors this would likely allow you to use your different CRT without modifying the TV's circuits at all .

[timmy]

You say isolator wouldn't that be the same as a separate filiment transformer being its isolated from the heater and cathode voltage from the set itself using the 6.3 v secondary of the filiment trans.

[N2IXK]

When in doubt, check the datasheet on the component in question!

http://catalog.triadmagnetics.com/Asset/F-13X.pdf

It shows that the F-13X is rated for 1500V breakdown between the windings, or between either winding and the core. As long as the applied voltage doesn't exceed this, you are fine.

[wa2ise]

Don't know back in the day, but any decent transformer has "HiPot" ratings, namely the ability to insulate 1KVAC or 1.4KVDC. Vintage transformers may have been built for 600VAC isolation. The reason is to avoid breakdowns caused by transient spikes riding on the powerline. Same thing for X and Y death caps.

[jr_tech]

I just measured the capacitance of a small 6.3 volt 2000 volt isolation transformer and found about 150 to 200 uuf between the 6.3 V winding and the primary and the core combined...
I would think that adding this extra capacitance to ground to the video output, which normally has to drive a much smaller capacitance, would reduce the frequency response, causing lack of sharpness of the display.

jr

[N2IXK]

Originally Posted by jr_tech

I just measured the capacitance of a small 6.3 volt 2000 volt isolation transformer and found about 150 to 200 uuf between the 6.3 V winding and the primary and the core combined...
I would think that adding this extra capacitance to ground to the video output, which normally has to drive a much smaller capacitance, would reduce the frequency response, causing lack of sharpness of the display.

jr

Good point.

Perhaps add a couple inductors in series with the secondary legs to block the high frequency content from being shunted to ground through the winding capacitance?

[old_coot88]

Originally Posted by N2IXK

Good point.

Perhaps add a couple inductors in series with the secondary legs to block the high frequency content from being shunted to ground through the winding capacitance?

Or, put inductors in the primary legs and float (unground) the xfmr. frame. That would allow using higher value inductors without dropping the 6.3V too much. Just a thot.