JVM C83 Mod...

[RedB4Black]

Has anyone done this? Did you like the results?

I have a JVM with Mercury OT and Choke. Sounds awesome, but there is way too much gain. I generally have it at 9:00 on the orange channel of OD1 and that's about all I can take.

This mod looks simplistic enough and its supposed to take the gain down a little. Or would I be better off using lower gain pre-amp tubes to get the same effect?

 

[Digital Jams]

Most people that I have jammed with run the gain around 12:00 w/ od1 orange, plenty of seperation and cleans up with the fiddle volume.

C83 does take some off from I heard but it is not drastic, you can just remove one leg and try it.

 

[Gainzilla]

I aslo heard that Choke mod does wonders for the feel as well, anyone heard this?

 

[ericsabbath]

are you talkin bout removing the C83 capacitor?
cause I wouldn't do that
it's an essential part of its hot rodded Marshall tone
you'll lose a lot of upper mids without it

there are many other ways to remove the excessive gain without changing the voicing
the JVM has a ton of useless gain

 

[Digital Jams]

od2 yeah has a ton of gain but od1 orange is hot rod marshall all the way.

My cali mod in full stun has more gain than my jvm in od1 orange.

 

[ericsabbath]

I'd swap R72 and R75 for 100k (1/2 watt resistors)
and replace R117 and R93 with 68k (1/4 watt resistors)

gain will be more usable, less fizzy and the midrange should get more open and crunchy instead of processed

there are many other possible minor mods, but these should be enough
you could tweak the tonestack/gain board to fix how the bright caps are wired
the bright cap bypasses a big resistor AND the pot, so you still have a lot of gain even with the pot off, but reducing the plate resistances is a better move, imo

 

[RedB4Black]

I aslo heard that Choke mod does wonders for the feel as well, anyone heard this?

I have the choke mod, but I can't really say how it compares to stock since, aside from 2 minutes at a GC, I've never really played. Killlertone played mine and he has one in stock form, perhaps he could talk about the differences.

 

[RedB4Black]

I'd swap R72 and R75 for 100k (1/2 watt resistors)
and replace R117 and R93 with 68k (1/4 watt resistors)

gain will be more usable, less fizzy and the midrange should get more open and crunchy instead of processed

there are many other possible minor mods, but these should be enough
you could tweak the tonestack/gain board to fix how the bright caps are wired
the bright cap bypasses a big resistor AND the pot, so you still have a lot of gain even with the pot off, but reducing the plate resistances is a better move, imo

That's a little more advanced than I'd get into myself. I just read about he C83 on another forum and figured I'd ask about it here. From what you are saying, it sounds like it wouldn't be a good thing to do. I guess I'll try some lower-gain pre-amp tubes.

 

[blackba]

I'd swap R72 and R75 for 100k (1/2 watt resistors)
and replace R117 and R93 with 68k (1/4 watt resistors)

gain will be more usable, less fizzy and the midrange should get more open and crunchy instead of processed

there are many other possible minor mods, but these should be enough
you could tweak the tonestack/gain board to fix how the bright caps are wired
the bright cap bypasses a big resistor AND the pot, so you still have a lot of gain even with the pot off, but reducing the plate resistances is a better move, imo

Sweet, may have to give those a try. Here is a link to a site that has the schematics for those that are looking for them. http://forum.metroamp.com/viewtopic.php?f=25&t=21932

 

[ericsabbath]

That's a little more advanced than I'd get into myself.

just take it to anyone who can handle a soldering iron
it's a really simple mod, actually, and about $0.60 on quality parts
the hard part is taking the board out with all those mounted jacks and sockets

you could wire piggybacked resistors in parallel without removing the board or replacing stock resistors (like most Bogner factory mods), so you'll get the same effect without swapping parts
in this case the resistor values would be:
- 180k or 220k for R72 and R75
- 68k for R117
- 100k for R93

 

[jcj]

I'd swap R72 and R75 for 100k (1/2 watt resistors)
and replace R117 and R93 with 68k (1/4 watt resistors)

gain will be more usable, less fizzy and the midrange should get more open and crunchy instead of processed

there are many other possible minor mods, but these should be enough
you could tweak the tonestack/gain board to fix how the bright caps are wired
the bright cap bypasses a big resistor AND the pot, so you still have a lot of gain even with the pot off, but reducing the plate resistances is a better move, imo

That's a little more advanced than I'd get into myself. I just read about he C83 on another forum and figured I'd ask about it here. From what you are saying, it sounds like it wouldn't be a good thing to do. I guess I'll try some lower-gain pre-amp tubes.

I hear you, buddy. I leave that shit to those who understand it

Throw a 12AT7 in there, see how that works....

 

[killertone]

I aslo heard that Choke mod does wonders for the feel as well, anyone heard this?

I have the choke mod, but I can't really say how it compares to stock since, aside from 2 minutes at a GC, I've never really played. Killlertone played mine and he has one in stock form, perhaps he could talk about the differences.

Didn't notice a ton of difference on feel but it was probably because I didn't have mine there to compare side by side. What I did notice was that it held together way better than mine does at really high volume and was way less noisy.

FWIW, I have seen a lot of people that do the C83 mod undo it because they don't like it.

 

[Bloodrock]

I use a 5751 in place of the V1 12ax7. If that doesn't do the trick you could try a 12aT7, or another 5751 in V2.