Apparently Marshall knew what they were doing

[alivegy]

In my never ending quest to never leave well enough alone I clipped the bright cap off of my JCM800. I think I like it better with the cap in place. The amp didn't get dark by any means and the treble knob did become a bit more useful. It seemed to add a "low-mid" frequency to the tone that i didn't like as much as it is right between the low and mid pots so you can't really dial it out well without affecting something else. Since I tend to play the amp with more of a clean setting it lost a bit of it's "karang" that these amps are known for. I of course clipped it out, rather than desoldering one end so I guess it's off to radio shack tomorrow to pick up a new one. Oh well.

 

[Primakurtz]

I wonder what a smaller value capacitor would do?

 

[III]

I have a Super Lead Reissue... alot of guys complain about brightness, but once you turn it up the Bright Cap is negligible and it's great to have that bright cutting sound on tap with the Volumes on 3-4. I can always just run the darker Normal channel at low volume. So, yeah... Marshall knew what they were doing.

If anything... I'd put the cap on a switch.

 

[alivegy]

I wonder what a smaller value capacitor would do?

I was wondering the same thing. I'm not knowledgeable enough to go messing around and I don't want to screw up my favorite amp, so I will probably just put it back the way it was.

 

[Boytbpc]

Since you clipped it, you effectively set the voltage that the cap lets through at 0, so anything smaller than the existing cap should be safe. (Somebody check my logic, I'm a tinkerer, not an E.E.)

 

[blackba]

I would try to add a switch if you can find a place not to drill into the chassis to add one. The lower value the cap, the less the effect the bright cap will have. 120pF is a typical value in BF fender. Try a silver mica 68pF if the stock one is 120pF.

 

[Gnarlly]

I clipped the bright cap on the volume pot on my JCM800, thinking that I would like it more without it. I ended up not liking it as much, and putting it back to stock. I agree, Marshall put it there for a reason.

 

[just_one_more]

You can use any value you want. The bigger the cap value the further down the frequency response goes for a set pot value. There comes a point where the cap would be bypassing the pot completely. So there is no "danger" in doing it, just unwanted results as long as the voltage spec is followed. You can put a smaller value in there and it will push the corner frequency higher. All you are doing is making a variable high pass filter. The result is that the frequecies shunted around the pot for a specific pot setting will be higher.

 

[DavidLM]

It's not just brightness, with the bright cap in place there is more gain.

And more gain in a Marshall is usually a good thing IMO.

 

[FrankieSixxxgun]

Lol, I just removed all the mods I did to my Metro Plexi because, like you said, the circuit is just right the first time. I did replace the 500pf bright cap with a 100pf bright cap though, and it made a huge difference. 500pf was icepicky, but no bright cap made the amp sound kinda sterile and dark.

 

[alivegy]

I believe the cap is a 1000pf in a JCM.

 

[FrankieSixxxgun]

I believe the cap is a 1000pf in a JCM.

Should be per the schematic. If it's too bright, try something smaller like a 750 or a 500.

 

[Roe]

if it's too bright, try half the value or even lower. actually the bright cap works together with the vol settings. here's a some combinations which tend to work well:
vol 2-3:100pf
vol4-6:500pf
vol6-9:5000pf

 

[FrankieSixxxgun]

if it's too bright, try half the value or even lower. actually the bright cap works together with the vol settings. here's a some combinations which tend to work well:
vol 2-3:100pf
vol4-6:500pf
vol6-9:5000pf

I disagree with that rule of thumb. I followed that advice originally when building my amp and put a 500pf cap in... it was icepicky to death. I normally play around 6 on the dial, and 500pf was completely unusable. 100pf sounds much, much better.

 

[Roe]

the idea is to get noticable brightness and cut. if you dont want that you obviously need a smaller cap

 

[FrankieSixxxgun]

the idea is to get noticable brightness and cut. if you dont want that you obviously need a smaller cap

I know what the idea is, and the 500pf wasn't it. Even my neck pickups were ice picky. With the treble off, presence off, and tone on the guitar off, the bridge pickup was still a face ripper. There's cut and then there's uncontrollable treble. With the 100pf in there it's nice and chimey, like you'd expect it to sound. Neck is nice and clear, treble and presence can come up to usable levels, and I can use my bridge pickup all day with just a little tone roll off like my other amps.

I usually play into an AC30 with brilliance enabled AND Top Boost linked and/or a Super Reverb in the vib channel with the bright switch on. The 500pf in the Plexi put both of those to shame if we're talking about the amount of treble they put out. Good lord was it bright!

 

[alivegy]

Cool, thanks for the info guys. I think I'll start with 500pf and go from there.

 

[Roe]

I know what the idea is, and the 500pf wasn't it. Even my neck pickups were ice picky. With the treble off, presence off, and tone on the guitar off, the bridge pickup was still a face ripper. There's cut and then there's uncontrollable treble. With the 100pf in there it's nice and chimey, like you'd expect it to sound. Neck is nice and clear, treble and presence can come up to usable levels, and I can use my bridge pickup all day with just a little tone roll off like my other amps.

I usually play into an AC30 with brilliance enabled AND Top Boost linked and/or a Super Reverb in the vib channel with the bright switch on. The 500pf in the Plexi put both of those to shame if we're talking about the amount of treble they put out. Good lord was it bright!

sounds very strange to me. did you measure the pot? pots tend to vary a lot

 

[skoora]

I like 500pF in my Metro 69. 4700 was a little more aggressive (more '72) and I found 500 to be the best all around value. I did for quite a while run it shared cathode and no bypass cap on V2A but it's now split cathode and still no bypass cap on V2A. I much prefer how the EQ works without the V2A bypass cap.

 

[FrankieSixxxgun]

sounds very strange to me. did you measure the pot? pots tend to vary a lot

Nah, but it's not that strange. In another thread I was linked to Marstran's database of Marshalls, and the bright cap values are all over the place for late 60's models. I had that MFer cranked up to 8, plugged into input 1 of the lead channel, through my 425A cabinet and it was unholy last night. Best it's ever sounded.