What do Oil Caps do and how do they effect sound?

Oil caps are one of the original types of capacitors - usually using paper wound with foil, bathed in an oil dielectric. Often in a rectangular, oval, or round can with two terminals on one end. There are modern day equivalents, Jensen makes some nice ones (copper foil in oil, etc.), as does others (for even more money, e.g. Audio Note silver foil in oil....). Usually vintage caps are referred to as PIO. (paper in oil or sometimes poly in oil). They work great and are often used as filter caps in a tube amp power supply (probably their best application). Also good in crossovers, and sometimes as coupling caps between (tube) amp stages. Fairly easy to find the old ones, commonly they are available from 200v - 1000v ratings, sometimes higher. Usually values range from 1uf to 20uf but sometimes as high as 40 or 100uf for more modern ones. Modern applications include motor run caps (for DC electric motors).

-Ed

Correction in order here bomby:

"845 tube. This is a radio broadcasting type tube & the grid is biased at 1500V! "

No, sorry - not correct. max PLATE voltage on 845 is 1250v, and ASL doesnt' run them that hard. They would not bias the grid higher than the plate(!) and infact the grid is going to have about 70-80v of negative voltage applied, to run them in class A1.

And let's stop talking about 'rail' voltage w.r.t. tube amps - it's called B+. Rails apply to sand amps.

Eldartford makes a good point I was going to mention - B+ on tube amp is somewhat irrelavent in any case, since it's almost always going to go out through an OT, which has a massive step down ratio. (5000 : 8 or more)

The only situation which B+ would be used to drive speakers directly would be a high voltage transmitting tube output connected directly to the panel of an electrostatic loudspeaker. There were a few commercial attempts at this I think, none sold today that I know of. It's an idea which has a lot of merit but unfortunately is rather difficult to implement well, or at all. There are some more worthwhile and successful attempts being made by the DIY tube crowd, but that doesnt' apply to commercially available designs. Most of the time you would need a bigger tube that even 845 to do it as a SET amp (the only way it would be worth doing), meaning either a tube like a 212 or 849 or maybe 250/450TL, or else a more modern planar triode ala Eimac or Svet ceramic types possibly. (talking 1500-3500+v on the plate of those tubes)

-Ed