Values for brightness capacitor on volume/input ?

[PeaveyBandit]

Hi All,

Getting some gigs in on my little clone/hotrod Princeton and loving it overall but now hearing some places for tweeks. For example the amp can sound a little too warm at times in the mix of a small band. The problem is when I raise the treble control above about 7 the amp does have more trebles but not the kind I want. Begins sounding too harsh or strident. If that's the right word.

I had none of the 50-120pf style caps on hand that I see used in my other older fender blackface amps. For example the original deluxe reverb uses a 47pf across that volume pot and the super reverb specifies a 120pf for it's bright switches. All I had on hand was a mica 250pf so I tried it. It's the right idea but the problem is it's a little too much. To the point I have to lower my treble control almost to a 4 to tame the glassy trebles this cap added. This of course sounds fine with my Gibson humbuckers but the more trebly guitars like my acoustic with a sunrise soundhole pickup sounds a little too trebly with this 250pf added.

QUESTION: Should I go to a higher or lower value cap and what's a value that will add just a little bit of sparkle to the tone without being too much? And should I get a specific kind of cap? ceramic disc, mica? Am I just going to have to experiment or are there some really common or favorite values for most brightness caps?

This is a princeton AA1164 design with a celestion gold 12" speaker and the stokes and paulc mods.

Thanks!
PB

[Enzo]

UNtil you get a selection of caps to play with, stick 2 or 3 of those 250pf caps in series. That will lower the net value to somewhere around 120pf and 80pf respectively.

[potaetoes]

so i'm new here, and i see this is a pretty old thread, but i thought i'd resurrect it in case anyone was interested.

my solution to a too-warm princeton is a 250pf bright cap in the normal place, plus a 75pf "dull" cap across the two end terminals to smooth out the very top end. result was excellent on my amp - beautiful vibrant treble without any nastiness. curious if anyone else has tried this...?

for the record my princeton is AA1164 with DR OT, GZ34, grid resistors, and a weber cali 12. other than having a bias pot, the rest of the amp is faithful to the schematic.

[dai h.]

could try a resistance (fixed or a pot to tweak) in series with the bright cap

[Enzo]

WHy not, just like the tone control in a guitar.

[potaetoes]

could try a resistance (fixed or a pot to tweak) in series with the bright cap

That was my first approach, but I was never happy with the outcome. The r-c combo seemed to either let through too much hair, or muffle too much chime. I found it easier to first find the right bright cap for just the right chime, then add the second one to shunt the unwanted upper freqs out. Easier to control the bandwidth of the brightness, and seems to keep the volume taper sounding a bit more even as well.

[daz]

Just use the 120pf in series with a trimmer. then you can dial it in as much or little as you wnat and leave the trimmer in there or measure it once it sounds right and put a resistor in it's place. thats what i did, and i ended up with i thinks a .001uf and a 470k resistor. That gave me a tone that doesn't seem to change much at all as i turn the gain down. Adds just enough so it doesn't go the other way and get muddier.

[Baxtercat]

I love it.
Two micas: one to add some treble [down to 250 pf] and another to shave off just the top, the ultra highs.
Thx for the idea, Potatoe...!