Xpress 500p Treble Tone Cap

[PCollen]

Silver Mica, or Polyprop...would it really make a difference ?

[Gibsonman63]

My build was really bright when I first finished it. I started with Silver Mica and went to PolyStyrene (I can't spell, but you get the point) and backed it down to 430pF.

It is hard to put your finger on, but there seems to be a little Mojo there. A little less brittle perhaps. I replaced both coupling caps and both bright caps, so I can't say what effect just the one bright cap would have.

[Roe]

I'm using a high voltage ceramic

[David Root]

You spelled it right, I'm not an Express guy but polystyrenes are smooth and musical if that's what you're looking for.

Dumble guys use ceramics mostly, but in general polystyrene>silver mica>ceramic in terms of smoothness.

The old '50s domino silver micas are a little different, some like 'em, some don't.

[Cliff Schecht]

I've always been a fan of polystyrene caps. I don't think there is a lot of "mojo" surrounding polystyrenes though. They are known to have low dielectric absorption, very low ESR, great temperature stability and consistency in value from capacitor to capacitor. If anything, the high quality polystyrenes I use are so great because I can pull one out of the box knowing that it is the value it says it is. This allows you to get more consistent cutoffs and more consistent sounding builds without having to hand sort capacitors (just resistors ).

[PCollen]

Silver Mica, or Polyprop...would it really make a difference ?

Please not that the inquiry was w/r/t Silver Mica vs. POLYPROPYLENE (Sprague OD 716P). Are polystyrenes the same as polypropylenes ?

[JamesHealey]

No they're not the same... there is no harm in using polyprop it will sound different to mica.. how so i'm not sure.. try it.

[Andy Le Blanc]

different plastics, Polystyrene has place in the HI FI world too.
Technically they all do the same thing, but they all have a different qualitative response.

I like to audition different types in different stages, in the pre the cap types
really stand out, cap type in the signal path can dictate an overall tone

you can also parallel caps of different types.

[Phil_S]

I've read, probably at this forum, if I recall correctly, that the Express sounds very different in a large room vs. a small studio or practice space. The bright thing is a big part of it and one should reserve judgment on the tone of this amp until you get it into a decent sized room. Then again, there is nothing wrong with voicing it to the intended space. I'm just sayin'....

[M Fowler]

The Express shines when you gig cutting through the mix verses playing at home.

[PCollen]

The Express shines when you gig cutting through the mix verses playing at home.

I can't understand why the 'brightness' of the amp, can't be adjusted at lower volumes using the EQ or other implemented control. If you're playing an original TW, I can understand not wanting to mod the amp, but with a clone..COME ON. Get out the soldering iron and get to work !

[M Fowler]

I did all kinds of changes which we have discussed previously on my Ceriatone. But the last and final Express I built for myself with a Larry Mann work of art head cabinet has no issues.

Still I really don't enjoy the Express. I like the Liverpool and Rocket. But once all the guys started building Rockets with reverb I never looked back at the Express. I really enjoy the tone of a octal powered Rocket.

Many people have changed their Express by changing the slope resistor, changing the type/value of tone caps and reducing the Prescence resistor etc.

[PlinytheWelder]

I had a mica cap in mine at first and didn't like the voltage leaking through it, so I got some HV Polystyrene caps from Watts Tube Audio and it seems a lot smoother to me.
I also used some tiny Wima polypropylene caps on the bright switch with good results. The only mica cap that's still in the amp is the 47pF fizz cap .
I may change that out, too.

[Noel Grassy]

Gary,
What are the part numbers for the WIMA caps you had success with?
In their MKP (polypropylene) series at 400v~630v ranges I can't seem to find any within the pF capacitances.
I only looked at the Mouser Bros..



Noel Grassy.

[orrong65]

Here's an answer to your original question from another angle. I'm not an Xpress guy either, but choice of caps makes a BIG DIFFERENCE in the tone of Fender and Marshall amps.

In vintage Fender tweed/BF/SF amps, you can replace the coupling and tone stack caps with polypropylene caps without colouring the tone. For the smaller values (like the treble tone cap, some couplers, and treble bypass caps) silver micas seem too bright and brittle, and the best choice is ceramics, particularly if you have NOS ones like RMC, circle D, Sprague etc. With these the tone is like an "iron fist in a velvet glove". The modern ceramic caps are much cleaner than the NOS. Polys are OK in these places, but they can be a bit "too clean".

I regularly upgrade caps in PCB Marshalls with great results - particularly on pure valve-path amps. It is a simple way of really opening up the Marshall tone. I replace the larger generic 3c poly coupling and tone caps with the box mount ERO MKT, WIMA or RIFA polys for a major tone improvement. The choice of the first pre-amp coupling cap is crucial, as this cap can often be 1000-2000pf. NOS ceramic for a mellow tone, Mullard mustard cap for greater definition, and silver mica for more bite.

And if I want to brighten the tone more, I will use a silver mica for the tone stack treble cap, or for any treble bypass.

For polystyrene caps and other "audiophile" components, I find them too accurate and clinical for guitar amps, but they are great in Hi-Fi amps. For guitar you want a bit of dirt in the tone, especially when overdriven.