capacitor question

[Randall]

I am going to gut my 65 Deluxe Reverb RI of it's printed circuit board and rewire it for p to p. I have a handle on what type of resistors to use in which applications, and I'm good with electrolytics. But I realize I have a weakness in understanding which type of non-electrolytics to use where just from looking at the AB763 schematic. I like Mallory 150 for coupling caps, that much I know. But how does one determine silver mica, ceramic, poly, etc? I do not want to try to improve on the circuit, I just want to understand how to go with convention at this point. Then I can change them out if I feel the need later once I get the basics down.

[Enzo]

They make decisions like that based upon practical needs. No one makes an electrolytic 0.001uf cap. In fact the whole point of electrolytics is that they make a large amount of capacitance in a small volume. You could replace them with film caps, but a 40uf 450v film cap would be HUGE. There is no point to them in small values. The polari9ty thing is to the benfit of the cap, not the circuit. The nature of electrolytics makes them polarized, unless you get specially made non-polar types. You could put plain old not polarized film caps in place of electrolytics if you could find them in sizes that fit.

SO if you have a favorite brand of film cap, great, use it. Those other forms are not film caps anyway. Ceramic discs are cheap. They worked well enough in guitar amps, but I don't recommend them for the most part. Oh the three feedback caps in the trem oscillator can be anything, and ceramics are good enough for that, there is no tone to a trem oscillator.

SMall value caps like the 250pf treble cap in the tone stack or a 100pf or whatever brightness cap can be ceramic. They are tiny and not so prone to microphonics as the large ceramics. Many people prefer to use silver-mica caps in those places. Silver micas tend to be smaller value caps. The amp only has a couple small caps, the bright caps, the treble tone stack caps, and the little 10pf cap across the 3.3M resistor where the reverb mixes. SO the added cost of silver mica should not be an issue. And really, going to s-m for those couple caps is not going to upset your original-purity approach.

The thing is really practicality driven. If you can find a 10pf film cap, go ahead and use it. Or a 250pf one. The reason Fender usede ceramics for those few and film caps for the rest is a matter of what is reasonably available or not extra costly.

A quick look at Mouser and I found for 250pf 500vcaps:
film - none
mica - $1.41 each
ceramic - 14 cents each

Adding over a dollar and a quarter to the cost of that part in each and every amp they made with that part (most of them) for the benefit of some tiny perceived tone shift was not good business.

SO make your parts list. Go down the caps and fill in each cap with a film cap if you can. Then fill in the blanks with mica or ceramic.

And for heavens sake, don;t WORRY about it. It is just a guitar amp, changing a couple film caps to mica or ceramics to film is not going to make your Fender sound like a Crate.

[Randall]

Thanks Enzo, as usual that was very helpful!

[loudthud]

One problem with ceramic caps is that some formualtions will change capacitance with changes in voltage. Some ceramic caps will be called "NPO". These are stable with voltage. Mica is very stable with voltage. It is said that the change in capacitance with voltage causes ceramics to sound brittle, but then there are people that like that sound. It's worth the effort to try both to see what you like or if you can hear the difference.

[kleuck]

It is said that the change in capacitance with voltage causes ceramics to sound brittle, but then there are people that like that sound..

Yep ! I like to use non-audio (hi-voltage filtering ones) ceramic caps in audio path, specially at the end of the preamp, under high AC voltage.
Just like CC resistors, in the right place, they can add some life to an amp.

[stormbringer]

Just a couple of follow-up points:

- silver mica caps are expensive at mouser. There are less expensive ones at tubedepot.com.
- most guitar-amp-oriented parts stores like tubedepot sell the electrolytic caps that we have seen in amps before. These include can caps and axials from brands like sprague and F&T. But consider using modern radials by nichicon and panasonic. These work well on eyelet boards but perhaps not on turret boards.