A nice big piece of Pye

[Jon A.]

Originally Posted by Electronic M

Paper caps have DC working voltage. Film caps on most sites often have Both AC and DC working voltages*. It is the DC that matters. (IIRC on my links I added a 630VDC filter for the film and a 450V for the lytics so you won't see a DC voltage selection column unless you nix that criteria in those links). You can always go higher voltage on film and to a point lytics as well.

Paper and film caps are not polarized, but if you want a rough idea of the DC polarity on them there is a convention....The 'outside foil' band on the end of paper caps is traditionally connected to ground. The outside foil wraps around the cap shielding it (thus why you want to connect it to ground when trying to optimize performance). Somewhere on youtube I once saw a video on using a scope to find the outside foil of a cap.

*In tube amp circuits the DC rating is what matters since most paper caps have ~200-600VDC on them and maybe 5 volts AC signal (until you get to TVs and sweep circuits then it jumps sharply). If on your tube set there is a cap between the two terminals of the power cord or between 1 terminal and the chassis then AC rating becomes important. Many film caps at made at paper cap ratings are made as line to line or line to chassis RF noise suppression/safety caps thus the AC ratings being available.

The non-stocked stuff can be annoying, but what sucks even worse is stuff with a low stock...I was going to buy IIRC 10 .0047uF 6KV caps for some electrostatic sets I want to finish (+ a few extra for the future) and at checkout found only 3 were in stock the rest would be back order....So I dropped it to 3 and now I've got my fingers crossed that it will be enough (or at least the 2 in series I used in a roundy as an emergency sub for a 2KV part are the right value and that I can take them and can come up with something different for the roundy).

Right, I just realized they have a DC working voltage too, totally different from the AC ratings. I have a list tacked to the wall here that I've been referring to when adding to my shopping cart. I went with a slightly higher voltage for each one, and sometimes slightly different values as the originals were not available. One of the paper caps is rated at 0.0005 uF and the lowest value Panasonic poly film cap available is 0.001 uF so I guess I'll have to use two in series.

Hm, I see lots of people have made videos on how to find a cap's outside foil, but more importantly the video I saw a while ago shows that many with a band end have been marked incorrectly. I had to dig a little but I got it:

https://www.youtube.com/watch?v=BnR_DLd1PDI

This fellow mentions that one should be fine using a digital storage oscilloscope that goes down to 5mV per division but he demonstrates with an old Tektronix 547 so my BK Precision 1479A should work just fine for this application.

I noticed two caps in the parts list that were specifically marked AC, and they're going from line to ground so I selected Kemet X1/Y2 caps to replace those.

In my latest order my shopping cart was loaded up for a while and one electrolytic went out of stock in the meantime, but that was easy enough to replace.

Onto resistors now, and then I'll have to pull a Santa and check my list twice before placing the order.

[Jon A.]

Originally Posted by Jon A.

Hm, I see lots of people have made videos on how to find a cap's outside foil, but more importantly the video I saw a while ago shows that many with a band end have been marked incorrectly. I had to dig a little but I got it:

https://www.youtube.com/watch?v=BnR_DLd1PDI

This fellow mentions that one should be fine using a digital storage oscilloscope that goes down to 5mV per division but he demonstrates with an old Tektronix 547 so my BK Precision 1479A should work just fine for this application.

It finally occurred to me yesterday that the circuit in this video is far more complex than it needs to be. A double-pole, double-throw momentary rocker switch and the ultimate computer (Star Trek reference unintentional) - you know, the one above our shoulders - is all that is needed.

On a side note, I'm wondering how the heck I can make a scan of a dial scale that's nothing more than white lettering on glass. I would have to take it to a print shop because it's too big for a flatbed scanner and it has to sit perfectly flat. I'm wondering if even they would know how to make a scan of such a thing.

[MadMan]

Originally Posted by Jon A.

That doesn't seem like the best idea, to be honest.

Well if it's legit too wide for a regular scanner, a print shop might be able to help. In all honesty, whatever scan you get would not be perfect enough to just reprint. You'll need to photoshop whatever result you get. If the glass is clear, and the letters are light colored, probably put a sheet of black paper behind the glass when you scan it, to give the letters some definition on the scan, bc, you know, the top of the scanner is invariably white.

The real question is, how do you plan to print the new dial onto the glass?

[Jon A.]

Originally Posted by MadMan

The real question is, how do you plan to print the new dial onto the glass?

I've heard of printing on clear acetate film and sticking that to the glass, and I don't recall saying what you quoted.