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