To settle microphonics I apply a dab of silicon to the back of a ceramic disk.
  
I did an experiment with a testboard and dipswitches that selected a number
 of picofarad range caps and various types. I found that, (to my ears at least)
, the ceramics sounded the smoothest, what was surprising is these are
 also the cheapest. What you use in the pF range seams to be the most
 noticable for capacitor types in a 1987 circuit. I also played with values
 to get rid of the infamous ice-pick-in-you-ear tendancy the Brilliant
 channel can have. With lower values the shrillness went away but it also
 shifted the peak if the highend to higher but thankfully not as loud, 
unfortunately that also killed that signature Superlead tone.
I then tried adding resistance in series with the standard value bleeder
 caps to restrict the amount of signal going through them, that worked great.
 I found that to add a 178K in series with the 470pF on the volume pot
 and a 150pF in series with the 470pF on the mixing resistor and I was
 able to retain the right grind that is the signature of the Superlead Brillant
 channel but without the annoying shrillness. It worked for me but as 
always, YMMV.

    I just tried a similar setup recommended on the Kinman site for a "bright"
 cap on the guitar volume control... it makes so much sense that you slap
 yourself on the forehead and wonder why you hadn't thought of doing
 that yourself a long time ago. D'oh!
 
    I've been going up and down with the values and adjusting the value
 of the parallel resistor (when used)... which as you said will move the
 frequency response curve up and down, along with adjusting the mix 
between the direct signal and the "enhanced" signal.
 
    Maybe as the rest of us start experimenting with similar circuits
 we can post the "recipes" that work for us! (Whenever the archives
 are back up and searchable, plug in the keywords "marshall" and "cap"
 to see all of the posts on the topic of these 470pF caps in a Marshall...
 a very popular subject! Many of those posts recommended using a
 smaller value or removing the 470pF caps altogether but I doubt if any 
of them suggested using a 150k to 180k resistor in series with the caps.)
 
    Now that you brought this up I bet that it is one of those fine-tuning
 tweaks that Ken Fischer used in his Trainwreck amps! Not necessarily
 used in those specific applications, but the idea of using a series resistor
 to lessen the effect of a cap...
 
--Thanks!
 
Steve Ahola
 
P.S. IMHO mica caps will conduct a very strong and clear signal, and
 one or two placed judiciously in a BF/SF amp can be a definite
 improvement. But in a Marshall the soft, diffuse response of a ceramic
 cap is usually more appropriate. It does depend on the sound that you 
are looking for...

I do not like them in stage coupling applications except for tone
 stack/controls. Possibly in parallel/series, ala' BFDR Ca.'65 or so,
 they do sound good in series with the .022's to the PI. Wacky.
 
Jason C. Arthur

I'm thinking this over because of the modified tweed twin copy that I 
built. I wanted to make it practically bulletproof, so it wouldn't need 
any work for many, many years except tube changes & bias checks.
 I used silver mica caps in the pf range and 2-watt metal film resistors
 on the entire board. The amp has great tone and I love it, but the truly
 magnificent amps that I've played in my lifetime have had a little
 something more. An indescribable quality that makes you feel like your
 ears are being sprinkled with magic pixie dust, and the meat of the tone
 is grabbing your gut. I wonder if replacing the micas with ceramics
 and putting carbon comp resistors in the signal path will get me closer
 to that point. No matter what, though, I ain't gonna use no carbon 
comp plate load resistors or voltage dividers.
 
Shea

Ceramic disk caps yes. But for resistors, first try this compromise
 before you go to the noisy carbon comps. Try 2 watt carbon film
 resistors. If that doesn't wax your surfboard, then give in and go
 to 2 watt carbon comp, but after going with ceramic disk. But yeah,
 the improvement of just going to ceramic disk will surprise you, it's imediately
 noticable, I don't know why.
Kelley

Somewhat like the difference in sound between using alkaline batteries
 and zinc-carbon batteries in some effects boxes, carbon comp 
resistors add a subtle softness to the sound that is pleasing to the ear.
 This is not the case with carbon film or metal oxide, which sound
 slightly colder in comparison. The difference is mild, but it is noteworthy.
 With this being evident, if I work with a vintage amp, if carbon comp
 comes out, carbon comp goes back in.

now mind you, I'm very partial to the Marshall circuits, but I use ceramics
 almost exclusively.. The one place i'm real big on silver mica caps, is 
between the plates of the PI -- since this cap is there to supress 
oscillation, the SM's seem to do a better job (don't have to deal with
 microphonics, which could defeat the purpose, and since you are
 essentially 'cancelling out the ultrasonics, the SM probably do a better
 job of that here as well). -- actually, that is the only place I can't hear
 the difference between a SM or a Cerm. type at all.

excerpt in regard to ceramic caps follows:
 
"By placing the same ceramic capacitor in an HP filter circuit, the roll-off
 of harmonics can be circumvented. In this type of use, the voltage
 across the capacitor is highest at low frequencies. Thus nonlinearities
 will show up as higher harmonics, which are readily passed by the filter." 
 
"The data for the HP test show much stronger distortion at the lower
 frequencies, where the voltage is highest. We are not sure what 
should be interpreted as the common distortion-producing source
 in these two tests. One thing seems quite 
clear, however, and that is the simple fact that you cannot "work around"
 the distortion problem in ceramics. Our feeling is that they should simply
 be avoided anywhere near an audio signal path and probably just
 avoided altogether for audio. For example, some listening tests have
 indicated that they can produce audible distortion when used as supply
 bypasses, let alone coupling!"


The ceramics were quite visibly non-linear. The author suggested
 that this may be why many guitarists prefer their sound. This Jung
 article goes a little step further and posits that the non-linearity allows
 higher harmonics to pass through. Harmonic generation always grabs
 my interest, and I think it should be discussed more here and on techs' 
websites.




  


Kelley