Modifying a JMP1c to sound brighter. Noob questions

[bruzanhd]

Well I've made a rough schematic of the preamp. At this point I'm more interested in learning about things than necessarily making big changes. I may be interested in increasing presence/air and also changing the first gain stage a touch. Maybe change the boost to have a jcm800 cold clipper feel but that stuff is all pretty straight forward. I'm interested to know everyone's thoughts on the function of C9, C15. I believe they are 100pF. Are they tonally cutting highs or likely more just preventing noise and oscillation? Also wanted to see if anyone thought there were obvious mistakes in my schematic.
Imgur linked
https://imgur.com/a/7JVMDKq

[Chuck H]

If they are 100p then they aren't affecting audible frequencies significantly (barely over 1dB @10kHz). So they must be there to mitigate instability.

[Mick Bailey]

+1

They're snubber capacitors, usually to reduce HF instability or oscillation. They're not always needed, but the factory must have encountered some issues when prototyping the amp and decided to fit them.

[Helmholtz]

Found a full schematic here:https://www.tube-town.de/ttforum/ind...ch=51584;image

[Helmholtz]

Low pass filters using small caps also serve RFI rejection, which seems to have become more important over the years.

[bruzanhd]

Found a full schematic here:https://www.tube-town.de/ttforum/ind...ch=51584;image

Any chance you could pm me or make a post with the schematic? I don’t speak the language required to make an account over on that site haha.

[bruzanhd]

If they are 100p then they aren't affecting audible frequencies significantly (barely over 1dB @10kHz). So they must be there to mitigate instability.

Okay, so then the values would need to be more in the roughly 1nF or higher range to be having a tonal effect? TBH the markings on some of them are rubbed off that why I said “I think 100pf” they look very similar to another cap that was 100pf. Either way thanks for clearing this up a bit for me.

[Helmholtz]

Does this work?

[bruzanhd]

Does this work?

VERY helpful. thank you sir!

[John G]

Hi,
I modified the post-eq as per the diagram to my clone that I made. C20 will adjust the treble level .
John.

[bruzanhd]

I ordered a bunch of caps and resistors to make some changes and I will report my findings once I get the install completed and I settle on values.
Currently thinking the following:
-move first gain stage treble peaker to after the volume pot
-add a 470k resistor to ground after the new treble peaker location to simulate the 470k mix resistor from the unused channel of a true superlead
Those two changes should make the amp react more like the high treble input of a superlead (any opinions on this btw? it makes sense according to my limited electronics understanding but that doesn't mean a whole lot haha)
Then
-add 5n bright cap since that was correct for the era I'm going after (The amp actually has a dedicated spot on the PCB for a bright cap but oddly enough they dont actually come with one from the factory.)
-allow slightly more presence to come through at the last stage of the amp

I still plan on modifying the gain boost function to allow for a more typical modded plexi type of sound either using a JCM800 style cold clipper stage or maybe more like an SLO cold clipper with the bigger cathode resistor. Either way I suspect the main mods I'm doing will affect the way the stock gain boost works so I may end up not wanting to change it.

Hi,
I modified the post-eq as per the diagram to my clone that I made. C20 will adjust the treble level .
John.

Thank you for the confirmation. I actually have a quote that I got from the original amp designer steve dawson of marshall who explained that the cap was "tonally setting the presence". I plan on changing it to a 1n instead of the 1n5 for a subtle increase in the top end.

[bruzanhd]

I have a question for people who know more about grounding etiquette than me, I'm intending on adding some ground impedance to r7 in my picture, r6 goes to ground and is also the cathode resistor for the following gain stage. By using a 470k resistor to bridge from r7 to the PCB pad of r6 which goes to ground will i be creating some kind of ground loop that will cause unwanted side effects? I've uploaded a quick schematic showing what i mean. fwiw all the grounds on that side of the board go to the same reference point. I could always create a bridge on the bottom of the board but i feel like that's messier and has its own issues.

[Jazz P Bass]

"I'm intending on adding some ground impedance to r7"
Isn't 'V1 IN' doing just that?

[Chuck H]

Without being able to see the ground plane and tracing on the other side of the board it's impossible to say, BUT... Ideally you want that added 470k resistor to ground with that circuits volume pot. Also ideally all the preamp grounds would be on the same ground system as they occur in circuit. What you propose should work fine but the only way to know is to try it.

[Chuck H]

"I'm intending on adding some ground impedance to r7"
Isn't 'V1 IN' doing just that?

I think what bruzanhd intends is a voltage divider to simulate the channel mix circuit in a plexi type design.