What do you use for high voltage paper caps?
 

I'm working on recapping a 721ts and I noticed a few paper caps rated at 1000 volts. My caps supply generally are rated at 630 volts. How do you handle this? Series?

You can series two caps at twice the capacitance to double the voltage, or just buy 1000V or 1600V caps. Check ebay or justradios.com or other cap suppliers.

Orange drops rated 1600 VDC. Or, you can get the 6000 VDC tubular jobs from Allied or Just Radios...

I used two 630V caps of twice the required capacitance in series for two of the 1000V caps in my 721TS, just as John recommends. They work just fine.

I'm wondering if ceramics might work in those circuits? But I think I'll just series them.

Ceramics will work fine in some circuits, but they drift in value much more than papers, and they're temp-sensitive. I always try to replace paper with paper, except for the AC line spike supressor caps (the ones that go directly from the line connector to chassis ground where the AC comes in). You should use special ceramics for those; not just regular ones. DEFINITELY don't use a ceramic in place of a paper in the horizontal oscillator, unless you like to sit with hands permanently resting on the horizontal hold, that is... They're bad news in the RF-IF as well. I use mica there.

Charles

Personally I wouldn't use ceramic as a line safety cap, there are specific caps rated for that duty. Look in modern electronics, it's the square one near the AC input marked with either an X (for across the line) or Y (line to ground). Here's a nifty PDF which explains the difference: http://www.newark.com/pdfs/techartic...ty_2008-01.pdf

Just Radios carries 1000 VDC polyester film tubular caps in a number of popular values. That's what I use. I also buy their 1000 VDC micas and their 6000 VDC tubular ASC caps for electrostatic sets for applications that need them.

Surely you didn't mean that quite the way it came out? If you really want paper caps, you have to pay $$$ and get them from the "high end" audio crowd.

Anymore, I don't replace the line filter caps. That is, I remove them. Who really needs 'em? If someone will give me a compelling reason to put them in a set, I will change my attitude.

Is not thier purpose to take any voltage surges, thereby protecting whatever is farther downstream? I guess it's only an issue if you have a set with an unobtanium power transformer, but all John's sets are pretty run-of-the-mill...lol

As I understand it, line filter caps are there to filter out interference from the AC line. Perhaps those were more important years ago, when cars had magnetos, etc.

The 721 has a few other caps with high voltage ratings. The oddball that I recall is a mica around the horizontal adjusters, 5 pf/1.5KV, which I replaced with some ceramics in series. Not my usual style, but that's all I had on hand, and it's holding up fine, so far.

Phil Nelson

I believe the purpose of these caps (and sometimes a resistor) is, as Phil suggested, to either prevent RF interference from coming in on the AC line, or from the set putting RF out on the AC line. The caps are way too small to have any effect on 60hz AC voltage spikes or surges, other than the very high frequency noise. IMO.

I plan to replace the line caps on each set I restore. Since I am a radio operator, RF interference (in or out) is an important consideration.

In the late 1970s when the CB radio craze was going, Consumer Reports included interference protection in its ratings for a while. Sony sets were the worst, as I knew... My neighbors all owned Sonys, and they were the landlords and their family. :(

I went crazy trying to figure out why a tube phonograph I restored for a friend of my daughter's a while back worked fine sometimes but was plagued with an intermittent buzzing noise at others, always at night. I never figured out the source of the noise, but I was able to see on a 'scope that it was coming in over the AC line. Adding a bypass cap reduced it to a tolerable level.