Cap composition and placement preferences

[JD0x0]

So I realize I might be opening a slight can-o-worms here, as not everyone agrees on this sort of stuff and we're in 'cork sniffery' territory.. but let's just say you had a healthy supply of Polyester Film, and Polypropylene film caps, lets say 6PS, and 715P's of equal measured values. What is generally your preferred choice of placement for these composition caps in an amp circuit?

I fully realize that PP is more stable than PE, so you may want to use PP in areas where there's more heat, or needs to be a more consistent value, but as we can see in many builds, by different builders, there is often not one universal composition. Many of the Wrecks and Dumbles as well as other high end amps have a mix of PP and PE composition, and even CC and MF resistors (let's ignore that for now)

So, if cost, part availability and fitment were out of the equation, what are some of the community's preferences and experiences, with their, or their customer's amps, using these cap compositions? And if you could, explain your process behind your selections, that'd be even better. I realize there's a lot of subjectivity to this, just looking for some ideas for my next overhaul.


I know the best way is just to try myself, but I'm interested in other's knowledge and experiences with this subject matter.

Yes, I realize I've entered Dumbleville

Also, while we're on the subject, what would be your preference for a small value (1000pF) cap in parallel with a plate resistor. I have ceramic, silver mica, and even Polypropylene film. I know ceramic generally gets frowned upon, except in a few select applications, but I'm most curious about the PP Film 1000pF caps, as you often dont see these being used at all, let alone, in amp circuits. Would the PP Film have better noise rejection than say, Silver Mica, if you oriented the outer film properly?


Thanks in advance!

[David Root]

I'm not aware of any TW Express that used PP caps. The odd Dumble did though.
In this type I generally use older NOS 6PS or (even older) chocodrops. (PP is a bit too bright for me). These were all difilm construction, ie mylar (polyester) and paper. This is cork sniffing I'll admit but difilm construction is nice and smooth sounding to me. Plus they all had outer film end indicated on them.

The smoothest sounding pf size cap is polystyrene, IMO. Some may not agree (cork sniffery again?) but you can demonstrate that they are very linear in response. Compared with a ceramic, the ceramic is a trainwreck in that respect.

Whether or not they would have any advantage across a plate load resistor over other construction I don't know but in signal-only transmission the difference is I think audible. In plate duty so long as you used 500V or 630V rated you should be OK. I have used polystyrene caps as snubbers in Dumble builds, they hold up OK. Tonewise I doubt it would make any difference as snubbers are just dumping high end signal to ground. But since they are all NOS these days and prices are rising, ceramics are a lot cheaper.

[M Fowler]

I'm all over the place concerning what type of cap I will use in a given amp circuit such as Marshall vs. Fender vs. Trainwreck, etc. I like to try different things like precision resistors in a wreck or Fender tweed.

One of my best sounding ultra high gain Marshall circuit is using Xicon caps which are not favored by some people.

Mark