The AM Forum

I was told that a hi voltage oil cap rated at say........24 mfd at 5kv for pulse service is not good to use is a ham transmitter HV supply. Could someone explain why????
Thanks,
Bill

I used those photoflash capacitors in my homebrew transmitter for over 20 years without a problem.  They were rated at 25 mfd @ 4 kv.  

The actual working voltage in the transmitter was about 2600 volts.  I have heard that you should de-rate the voltage of those capacitors about 30%.

The only reason I replaced them was that I found some better quality capacitors, rated for continuous duty, at a hamfest.

I ended up using one of the photoflash caps in my Gates BC1-T to replace the original 8 mfd main HV filter cap.  So far, it is working FBOM.

I would assume that the insulation in pulse rated capacitors is designed for intermittent duty at the rated voltage, not continuous duty.

I have noticed the physical arrangement of the terminals of photoflash capacitors is different from that of continuous duty ones.  Usually, the case of an oil cap is rectangular.  The continuous duty ones have the terminals near the far ends, along the shorter dimension.  The pulse units have the terminals near the middle of the longer dimension so that they are closer together.

I tried to draw this out graphically using the keyboard, but when I hit  "preview", the drawing was totally muddled up, so I gave up on that idea.  For some reason the final display ignored horizontal spaces I inserted with the spacing bar.  I've seen  people draw pictures and even schematics on a computer using a keyboard and standard typing symbols such as _ l ^ < > O, ect, but I'll be damned if I can make it work.  The symbols won't stay in the relative positions I put them in when I hit the enter button.  Can anyone tell me how they manage to do this?