A little help for rusty tech

[punkin]

 

I'm embarrased to be asking this but hey, it's been awhile getting down to the component level again since the dawn of the computer and black boxes.

I'm building a pepper shredder...the guitar/signal input cap calls for a 100nF cap (i'm calling it .1uF). It's there to couple audio and block what DC might be there in on the input i suspect. The schematic also makes no reference to a polarity requirement. So there ya go.

I went to mouser and ordered a 35V .1uF AVX Radial Tantalum for this application...the cap looks to have a positive and negative polarity. Which way do I put it in?! Gawd I feel old! I can hardly see the marking on this little booger.

[aron]

I would use a .1uF film cap if possible. If you have to use a polarized cap, well, the answer is to orient the cap with the positive toward the side with the higher voltage. In this case.... hmmmm positive towards the grid of the tube?

[punkin]

thanks...I've been reading on the web and it seems these things are a bit sensitive to reverse voltages. Hard to believe at these capacitance levels and for simply very low voltage DC blocking.

Anyone know if they respond to audio differently in reverse connection?

[frequencycentral]

I built a Pepper Shredder a while back (using two 6111 subminis). For that cap I used a film cap, as Aron suggests, so no polarity requirement. I've built quite a few tube pedals (oops) and have always used that type of cap for the input cap, and for all the DC blocking caps immediately after any plate.

[R.G.]

Anyone know if they respond to audio differently in reverse connection?

Yes, they do. Electro caps with reverse bias are a kind of sloppy rectifier. They begin conducting at some ill-defined reverse voltage.

Electrolytic caps should always be used with a correct DC bias across them that is bigger than any signal that will come through, so the voltage across the cap is always greater than zero in the correct direction. All of them will withstand a minor reverse voltage of varying amounts, but it is "withstand", not "work properly".

[flo]

You can measure the DC voltage on the pepper shredder's tube side where that input capacitor needs to be placed. It's probably going to be 0V because (I think) the input tube grid should be at 0V. 
The input (on the other side) is also at 0V.
So it actually does not matter how you put in that input capacitor in this case.

I'd put the + side on the tube side because it is the most likely to get a positive voltage.
Better: Use a film capacitor as the others also suggested.
Even beter: In my Valve Junior preamp, the input does not have such an input capacitor because there is no DC voltage to block so I guess you can even leave it out. (don't hold me on that one though.)

http://www.tube-town.net/diy/lov/lov02-peppershredder.pdf

[punkin]

Thanks all, I really appreciate the advice...off to Rat-Shack I go. Hope they have what I'm looking for.