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Replacing capacitors in between triodes?

Carbsandcaps

Carbsandcaps

Super Member
I’m re-capping a fisher tube amp using pio coupling caps and I’m wondering what about the small caps between the stages of a tube. They’re usually ceramic discs so if I’m replacing the rest of the caps in the signal path shouldn’t I replace these with pio’s? Obviously cost is a factor here but a chain is only as strong as it’s weakest link so are these going to be a weakest link? IMG_9903.jpeg
 
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If those are the flat round disc capacitors, they are usually not problematic and may never need to be replaced.
 
Electrically they are fine. I'm very much not into the whole sound of components thing but ceramics are one of the few exceptions I might make to that. I've replaced them with boring yellow polypropylene and generally preferred the result but its all very subjective.
 
gadget73 said:
Electrically they are fine. I'm very much not into the whole sound of components thing but ceramics are one of the few exceptions I might make to that. I've replaced them with boring yellow polypropylene and generally preferred the result but its all very subjective.
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I tend to agree, I get caught up in the boutique capacitor thing but at the very least it’s a selling point if I decide to sell later on. I just assumed if you replaced that if I was going to put the very best at the end of the line it can only be as good as what’s in the front of the line. At the least I’m going to replace the ceramics with film but I wanted to get some opinions on this.
 
nj pheonix said:
Oftentimes the pios are problematic because they're too large for the space provided (think 10 lbs of crap in a 5 lb container. I tend to think its not worth the effort.
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Not only that,.some have bare metal cases, so you have to make sure they don't touch anything, or at leased put some shrink tubing around them.
I had a probe go just a bit too far into a test jack and touch one once. There was an auditable snap, and the fuse blew.
 
gadget73 said:
0.022uf in silver-mica would be fairly expensive. Do they even exist at that value? Rare to see them over about 1000pf honestly.
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Nope, not at 500VDC rating. I didn't see the schematic, when I posted about silver mica disc cap option.
 
Run-of-the-mill ceramic disc caps are NOT good in the signal path. Nearly all of them exhibit very high values of dielectric absorption, and many - especially the multilayer types - actually change value when the DC voltage across them varies. The latter is a well-known characteristic that's been documented in white papers from various capacitor manufacturers. I've measured it myself, and it's not trivial. Regarding these characteristics, the best caps in the values you pointed out are polypropylene, although they are generally larger and more expensive than other film types. Note that I am not a proponent of boutique parts. I buy components like this from companies like Digikey and Mouser, and I never buy boutique brands. You simply need good quality components that are suited to the application at hand. Knowing exactly how each type behaves electrically is key to choosing the best capacitor for a given situation.

Jack
 
TriodeLuvr said:
Run-of-the-mill ceramic disc caps are NOT good in the signal path. Nearly all of them exhibit very high values of dielectric absorption, and many - especially the multilayer types - actually change value when the DC voltage across them varies. The latter is a well-known characteristic that's been documented in white papers from various capacitor manufacturers. I've measured it myself, and it's not trivial. Regarding these characteristics, the best caps in the values you pointed out are polypropylene, although they are generally larger and more expensive than other film types. Note that I am not a proponent of boutique parts. I buy components like this from companies like Digikey and Mouser, and I never buy boutique brands. You simply need good quality components that are suited to the application at hand. Knowing exactly how each type behaves electrically is key to choosing the best capacitor for a given situation.

Jack
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Wonder why Fisher chose the ceramic disc in this location? I don't think that lead length is a stability issue in that location. Just put a regular Sprague OD, or something, on the board and run leads to it?

Interesting.
 
gadget73 said:
Electrically they are fine. I'm very much not into the whole sound of components thing but ceramics are one of the few exceptions I might make to that. I've replaced them with boring yellow polypropylene and generally preferred the result but its all very subjective.
Click to expand...
Problem with the ceramic caps is that they can be piezoelectric in nature and could be excited at their physical resonances, which could affect sound quality. This would be true if there is enough current flow to cause that. In a small signal interstage application, the effect may well be below audibility. Even if the effect is not audible, I would not want them in my amplifier, because nothing's too good for my little Fi-fi; do you understand? N-n-n-nothing!
 
egellings said:
Problem with the ceramic caps is that they can be piezoelectric in nature and could be excited at their physical resonances, which could affect sound quality. This would be true if there is enough current flow to cause that. In a small signal interstage application, the effect may well be below audibility. Even if the effect is not audible, I would not want them in my amplifier, because nothing's too good for my little Fi-fi; do you understand? N-n-n-nothing!
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I agree. I don’t mind spending more on my own stuff even if it is snake oil. It helps me sleep at night knowing my baby has the best.
 
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