Plexi bright cap

[mtlin]

Does the bright cap on a Marshall plexi act as a high pass filter? If so, what is the cut off frequency?

[jlummaa]

I don't think it cuts low frequencies but more like adds highs (which can be the same thing after all), and the more it adds the lower you have the volume knob. There are several different values of bright caps, i think values between 500 pF - 5600pF were used along the years and in different models.

[mtlin]

How can a passive component add highs?

[jlummaa]

It can pass them through where they, without the component, wouldn't otherwise be let. (I am not sure if this is good english)

[mark norwine]

How can a passive component add highs?

It doesn't "add" in the passive vs. active sense. So you are correct...it can't.

But what is does do is allow the highs to "sneak around" the volume pot. So while the vol pot is "attenuating all frequencies", some of the high[er] frequencies are given a free pass, so they're present in the post-potentiometer mix......giving the feeling of having been "added".

Ever notice how the effect is lessened as the volume is turned up?

[mtlin]

Cool. Does anyone know what the frequency cut off is for the stock value?

[mark norwine]

It's not a "cutoff"...it's a rolloff

[jlummaa]

Cool. Does anyone know what the frequency cut off is for the stock value?

I don't know how to calculate it but for those who know you may want provide your bright cap value. As i said there are several stock values used.

When i wanted to figure out what value i liked most i just bought several of them and tried them all. I think i have 250 pF there now and that's for playing the amp with Les Paul. With a strat you don't need a bright cap.

[DGDGBD]

I don't like anything over 100 pf w/HB pickups--any more is way too bright.

[Geetarpicker]

When a bright cap isn't really a bright cap...

I have like a 3500pf "bright cap" in my '68 Plexi Superbass 100. When the cap is that BIG it basically passes most everything except the low bass. In effect it rolls off the sub lows just a tad, which helps keep the Superbass IMHO from being too swampy when I run it on about 6 or 7.

[Unabender]

Removing the bright cap makes the amp work better when using overdrive pedals, IMHO. Easier to get a good lead tone.

I have never added one on my DIY amps, either. For the same reason.

[mwm523]

When a bright cap isn't really a bright cap...

I have like a 3500pf "bright cap" in my '68 Plexi Superbass 100. When the cap is that BIG it basically passes most everything except the low bass. In effect it rolls off the sub lows just a tad, which helps keep the Superbass IMHO from being too swampy when I run it on about 6 or 7.

Hmmmm.. this is an interesting theory. I wonder if it would help with the flabby bass response of a JTM 45. Anyone ever try it?

[Geetarpicker]

Should work. The Superbass was basically a continuation of the JTM100 style amp with the lower gain style preamp. It would be worth a try in a JTM45 IF you aren't cranking the amp up all the way and want tighter bass.

[John Phillips]

There isn't a fixed rolloff frequency - it depends on the setting of the volume control.

Changing the value of the cap affects how far down the treble pass comes at any given setting though - larger lets through lower frequencies, not so much 'more treble'. The 5000pF value Marshall used in some later models is ridiculous, it's so big it lets through a lot of mids as well and actually changes the apparent taper of the pot and makes the sound very strident and harsh at low volume settings. IMO 500pF, or at most 1000pF, are the best values, they give enough for the classic Marshall bright crunch.

[mtlin]

There isn't a fixed rolloff frequency - it depends on the setting of the volume control.

Changing the value of the cap affects how far down the treble pass comes at any given setting though - larger lets through lower frequencies, not so much 'more treble'. The 5000pF value Marshall used in some later models is ridiculous, it's so big it lets through a lot of mids as well and actually changes the apparent taper of the pot and makes the sound very strident and harsh at low volume settings. IMO 500pF, or at most 1000pF, are the best values, they give enough for the classic Marshall bright crunch.

I see. Do you have any idea of what frequency range the 500pf cap rolls off at?

[John Phillips]

Do you have any idea of what frequency range the 500pf cap rolls off at?I've never thought about it really. It's totally different if the volume control is at 2 or 7 (for example) anyway.

The range it works at is the one that sounds right at any setting of the volume control, compared to other cap values :). Sorry for probably not helping much! ;)