Plexi bright cap

Chris-in-LA

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Mids on 10? That will also be very bright. Leave the bright cap alone and work with your mids and treble. Try them both at 3 and work from there especially if you are at high volume.
 

Marcomel79

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Mids on 10? That will also be very bright. Leave the bright cap alone and work with your mids and treble. Try them both at 3 and work from there especially if you are at high volume.
The mids is what give this amp that nice marshall bite... to my ears it sounds a bit too scooped with the mids turned down... here ill post a schematic of the amp so maybe somebody can spot something that can be improved..
5C6DD0A9-8633-4A06-8D81-CA4947A31F97.jpeg D9D4F09F-DF0B-4944-ABBD-26520E562B1E.jpeg
 
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gregr

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Looking at the tone stack it’s a JTM45 (Fender Bassman).

The slope resistor, R16 is 56k. This is why it sounds scooped with more emphasis on the top end. Change it to 33k. Change C6 to 470pF for additional upper midrange. These are the values for a Super Lead Plexi which seems like what you want.

Then worry about the bright cap.
 
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Marcomel79

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Looking at the tone stack it’s a JTM45 (Fender Bassman).

The slope resistor, R16 is 56k. This is why it sounds scooped with more emphasis on the top end. Change it to 33k. Change C6 to 470pF to for additional upper midrange. These are the values for a Super Lead Plexi which seems like what you actually want.

Then worry about the bright cap.
Thank you for your reply! The amp is an 18W version of the plexi. I saw some of the value were different, but i just thought it was because it was a smaller amp...
 

gregr

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Plexi is just the material of the early faceplate Marshall used. It isn’t really any one individual amp. These days I and countless others automatically assume Super Lead Plexi, even the ones with metal faceplates.
 

Marcomel79

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Plexi is just the material of the early faceplate Marshall used. It isn’t really any one individual amp. These days I and countless others automatically assume Super Lead Plexi, even the ones with metal faceplates.
Yeah, i mean they advertised this kit as the 18W version if the 1959 superlead, which basically is the "plexi" most people refer to when they talk about it...
 

gregr

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The front half of the preamp is what you’ll find in a 1959 SLP. Just the tone stack is from the earlier JTM.

No bright cap is pretty dull at lower gain settings. Try 100p, 220p, 470p and 1n each over the full sweep of the gain control. The behavior of the control will change quite dramatically. Pick the value that best suits your style and fine tune from there. Maybe no cap works best, but I fear too many people just cut it out and call it a day.
 
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Emiel

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Although this is a 1959 Super Lead... I found this video on bright caps to be very useful.



My 1987 clone started life with a 500pf bright cap which was just way too bright. I find the 100pf much better but there's definitely something really cool about how the big 4700pf cap affects the tone and gain at higher volume settings... I'll most likely end up with a push/pull volume pot to switch between a 4700pf and 100pf (or no cap at all).
 
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gregr

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This a nice demo of the Friedman Smallbox. The clean channel is basically the bright channel of an SLP. Because it’s being treated like a clean channel the bright cap values are small: 470pF, 220pF and no cap. This will give you an excellent idea of what these smaller values sound like. Compare that with what the larger ones sound like in the previous post.



The one thing I don’t like about the demo is that he splits the coils. To me this sounds brittle, but I think you can still tell what’s coming from the guitar and what’s coming from the amp.

The other thing to take into account is that this amp has a master volume which is set low so all the dirt is being generated by the preamp tubes, which is why it doesn’t roar. Dime the master and you’re there. With the volume turned up you’ll probably want to back off the gain at which point the bright cap can make a difference. How much depends on where you have the gain set.
 
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_Steve

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The other thing to take into account is that this amp has a master volume which is set low so all the dirt is being generated by the preamp tubes, which is why it doesn’t roar. Dime the master and you’re there. With the volume turned up you’ll probably want to back off the gain at which point the bright cap can make a difference. How much depends on where you have the gain set.

They're EL84s - they clip when you breath near them! :p
 

Emiel

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Sad or said? If the latter, can you elaborate?
Autocorrect, my bad! Changed it to:

“there's definitely something really cool about how the big 4700pf cap affects the tone and gain at higher volume settings...”

I think it would be nice to be able to switch between different cap values. No cap or a 100pf cap for lower volume settings (2-5), and the stock 4700pf for anything beyond that.
 

Marcomel79

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Autocorrect, my bad! Changed it to:

“there's definitely something really cool about how the big 4700pf cap affects the tone and gain at higher volume settings...”

I think it would be nice to be able to switch between different cap values. No cap or a 100pf cap for lower volume settings (2-5), and the stock 4700pf for anything beyond that.
I dont ever really go over 5 on the bright volume as it gets too gainy, also doesnt get any louder after that... i just ordered a 2200pf cap but i guess i should have summoned the marshall forum before.....
Seems like im gonna have to place another order...
 

Emiel

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I'd definitely try the Plexi tonestack (33k for R16 and 500pf for C6) first and then tinker with bright caps. Start with no cap at all or a 100pf.

Some other interesting bits:
- it seems that your amp has the 47k NFB resistor (R23) connected to the 16 ohm tap, the Plexis had it wired to the 8 ohm tap. Later it became 100K/4 ohm.
- the Plexis apparently had a log 25K mid pot as opposed to a lineair one as in your schematic. This gives the pot a different sweep.
 

Marcomel79

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I'd definitely try the Plexi tonestack (33k for R16 and 500pf for C6) first and then tinker with bright caps. Start with no cap at all or a 100pf.

Some other interesting bits:
- it seems that your amp has the 47k NFB resistor (R23) connected to the 16 ohm tap, the Plexis had it wired to the 8 ohm tap. Later it became 100K/4 ohm.
- the Plexis apparently had a log 25K mid pot as opposed to a lineair one as in your schematic. This gives the pot a different sweep.
I will!! Im not a very experienced builder, far from it, in fact this is the first amp i built and id never built anything before, not even a pedal. I did notice some differences in the values of my amp from a 100w 1959. But i just thought it was because mine is 18W....also my presence pot is 5k linear...thought about raising the nfb resistor and change tap to see how it behaves...
 

Marcomel79

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@Emiel i ordered a 33k resistor and 500pf cap for the tonestack and 220pf, 500pf and 1nf caps to try as bright caps. Plus a 100k resistor for the nfb. I guess by the end of it i should be 100% satisfied with the result and enjoy my amp fully.
 

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