jca2212c jet city mods

[shep]

i will be documenting my jca2212c modifications. i notice there aren't a lot of current mod posts on this topic. hopefully this will save some tinkering for people.

problems i noticed with the combo amp. flabby bass, lack of clarity with high gain. the worst is the high end and high end fizz. there is no clean channel. over all the amp sounds pretty sweet and has lots of potential. by no means are any of these flaws major.

choke mod: hammond 156m replace r52 according to jca22 schematic. this mod tightens up the tone. helps reduce hum and bring clarity to the low end.

speaker change: replaced the jet city eminence speaker with a celestion vintage 30. tightens up the bass and provides more clarity with the overdrive. IMO, it is a must.

added channel toggle switch to front end. just connects foot switch lead to ground. a jack that grounds when not plugged in will allow the foot switch to bypass the toggle switch, if wired as such.

V1 biasing mod: beginning of preamp stage. replaced plate resistor r6 from 220k to 100k. this allows for slightly clearer and more usable lead channel. it adds some really nice harmonic content as well. IMO, clearing the lead as well as adding more tone is by far one of the best things i've done to the lead. it squeals with ease even on the neck pickup. it really just helps the tone over all. the downside to this mod is that its not as quiet as with the 220k. while on the crunch/clean channel i hear a little bit of scratching from time to time. it does not get louder as the volume is turned up.

clean mod: this one is a work in progress and will be updated as i make more discoveries. the clean mods that are posted on the internet do not actually make the amp clean. it cleans it up a bit but there is a hint of high end clipping. you can literally achieve the same tone if you just roll the gain way back. while on crunch, the mod i have tried is to bypass v3 (cathode follower) after r10 and introduce the signal back at the tone stack at r26. this gives an actual clean tone that is usable. this mod is with r6 at 100k. similar to a vox kind of tone. you can get clean at 12'o'clock and get a little dirt after that. the downside is that it is way more quiet than the other channels. i will be experimenting with the biasing of the second half of v1 to obtain more volume gain. switching from this to other channels may require a relay by the time i'm done. any comments or help with this would be greatly appreciated.

one thing i will eventually try is playing with cathode bypass values. there is not a lot going on in this design as far as thats concerned and doin so may yield really fine results.

new mod based off of mesa rectifier red channel:

values to be changed on jca22h schematic:
c5 .002uf "coupling/filtering"
c14 500pf "tone stack"
c6 .001 "gain bright cap"
r9 680k "coupling/filtering"
r25 220k "plate resistor"
r19 omit

in accordance to the rectifier schematic add:
c12 20pf "filtering"

i have installed this on my jca22, sounds pretty awesome.

sound clip: https://youtu.be/EDXRtOOam_4

[shep]

i've played with some cathode bypass caps. changing the first cathode bypass cap from 1uf to 10uf. according to the jca22 schem the cap is c1. it seems to have amplified the mid-range bass frequences substantially. it brings the "low frequency gain" level with the "high frequency gain". giving a more even and more lush sound. its kinda like a fat switch.

note: some people have said that this would make the sound too muddy and/or flabby. they're are actual amps that have been designed with a fat switch similar to this. maybe a good idea to try a lower value? 4.7uf?

[BWB2632]

I am a beginner when it comes to modding amps. I saw a recommendation on a different forum to lift one leg of C6, R9, and C20. Lifting a leg of C6 made a HUGE improvement in the overall sound, removing that annoying "ice pick" sound. I can now use the treble and presence knobs instead of keeping them all the way down.

I don't actually know what lifting a leg of R9 and C20 did. I can't really tell a difference for better or worse there. Could you shed some light on what effect these have?

Also, while removing C6 made the OD channel much less bright, the Crunch channel is still overly bright. Is there a bright cap for that channel that I can lift a leg on?

Thanks!

[shep]

I am a beginner when it comes to modding amps. I saw a recommendation on a different forum to lift one leg of C6, R9, and C20. Lifting a leg of C6 made a HUGE improvement in the overall sound, removing that annoying "ice pick" sound. I can now use the treble and presence knobs instead of keeping them all the way down.

i

I don't actually know what lifting a leg of R9 and C20 did. I can't really tell a difference for better or worse there. Could you shed some light on what effect these have?

Also, while removing C6 made the OD channel much less bright, the Crunch channel is still overly bright. Is there a bright cap for that channel that I can lift a leg on?



Thanks!

c6 acts as a bright cap over the gain knob. so taking it out would create less brightness. for the crunch channel you can reduce c7. this will probably create a loss of gain though. try reducing it down to 250pf. you can also put a cap across r7. doing this is common practice for reducing high end just as c10 (.001uf) is doing. i wouldnt mess with r9.

hopefully that helps

[mesa3077boogie]

clean mod: this one is a work in progress and will be updated as i make more discoveries. the clean mods that are posted on the internet do not actually make the amp clean. it cleans it up a bit but there is a hint of high end clipping. you can literally achieve the same tone if you just roll the gain way back. while on crunch, the mod i have tried is to bypass v3 (cathode follower) after r10 and introduce the signal back at the tone stack at r26. this gives an actual clean tone that is usable. this mod is with r6 at 100k. similar to a vox kind of tone. you can get clean at 12'o'clock and get a little dirt after that. the downside is that it is way more quiet than the other channels. i will be experimenting with the biasing of the second half of v1 to obtain more volume gain. switching from this to other channels may require a relay by the time i'm done. any comments or help with this would be greatly appreciated.

one thing i will eventually try is playing with cathode bypass values. there is not a lot going on in this design as far as thats concerned and doin so may yield really fine results.

Hello shep, can you clarify what kind of vox tone you are talking about? A vox "normal" channel has no tone stack , BUT the top boost channel does. Given the context of what you have mentioned above, I believe you are talking about the Vox normal channel.

Also, have you found a way to get that vox like tone on the crunch channel but match the volume levels?

Thanks,

[mesa3077boogie]

so im actually looking for more chimey brightness in the clean channel with less bass. I got the clean and scoop mods on the way but I fear the scoop will boost the bass too much..

Any chance putting an additional 470K resistor with a 470pf cap in parallel with eachother (to create a treble peaking circuit) after R5, 7 , 8 or 9 would yield less bass without reducing too much signal?

[mesa3077boogie]

bump

[Zozobra]

Maybe reduce a coupling cap somewhere to cut bass? I haven't got a schematic to hand so i can't be more useful.

[mesa3077boogie]

Maybe reduce a coupling cap somewhere to cut bass? I haven't got a schematic to hand so i can't be more useful.

Here is a schematic:



I found a few ways of reducing the bass, now I am more after answering my first question if SHEP ever found a way to get the voxey clean tone at adequate levels.. Heres his post:

clean mod: this one is a work in progress and will be updated as i make more discoveries. the clean mods that are posted on the internet do not actually make the amp clean. it cleans it up a bit but there is a hint of high end clipping. you can literally achieve the same tone if you just roll the gain way back. while on crunch, the mod i have tried is to bypass v3 (cathode follower) after r10 and introduce the signal back at the tone stack at r26. this gives an actual clean tone that is usable. this mod is with r6 at 100k. similar to a vox kind of tone. you can get clean at 12'o'clock and get a little dirt after that. The downside is that it is way more quiet than the other channels. i will be experimenting with the biasing of the second half of v1 to obtain more volume gain.
one thing i will eventually try is playing with cathode bypass values. there is not a lot going on in this design as far as thats concerned and doin so may yield really fine results.

Thanks,

mesa3077boogie

[hendrixdun]

Some advance??

[shep]

It's been a long time since ive posted. i forget exactly what i did to the clean channel at this point. from what i remember, i reduced the plate resistor to 100k and added a 10uf bypass cap. as well, i bypassed the other gain stages, and reintroduced the signal at the tone stack. reducing the plate resistor to 100k, i think, would be unnecessary, after reviewing the ac30 schematic. ac30 has the plate resistor at 220k. the two defining things i think for making a good clean is a large cathode bypass cap and bypassing the other gain stages.

i'm using the saying "vox clean"
loosely

"""Generally speaking clean channels are normally way more quiet than over driven channels."""
i dont think it will be possible to reach the same volumes completely. that's the purpose of the two masters, so that you can match the channel volumes. I never did try to get a larger volume out of the clean, Just turned the master volume up louder.

i am no expert, by the way.

[shep]

my thoughts so far on jca22/recto mod:

values to be changed on jca22h schematic:
c5 .002uf "coupling/filtering"
c14 680pf "tone stack"
c6 .001 "gain bright cap"
r9 680k "coupling/filtering"
r25 220k "plate resistor"
r19 omit

note: other rectifier schematics (three channel version) have c14 500p and have 250kA gain pots

in accordance to the rectifier schematic add:
c12 20pf "filtering"
c55 .005uf "filtering" note: c55 is probably not needed.

this is what i have so far, any input would be awesome.
rectifier schematic:
http://www.thetubestore.com/lib/thetube ... ematic.pdf

[shep]

so im actually looking for more chimey brightness in the clean channel with less bass. I got the clean and scoop mods on the way but I fear the scoop will boost the bass too much..

Any chance putting an additional 470K resistor with a 470pf cap in parallel with eachother (to create a treble peaking circuit) after R5, 7 , 8 or 9 would yield less bass without reducing too much signal?

i know its been years, but i will try to reply for people to reference the information. here would be my first approach to just about anything similar to what you're asking. putting a small value cathode cap on v1b, such as .68uf, would be considered the same as some amp designs apply a bright switch. this will make the amp brighter but will create a bit more over all gain.

another way to apply a bright switch is by running another cap in parallel with c14(250p, or 150p) somewhere in that range. this effects the treble knob in the tone stack. i think this would be more accurately a treble shift?

and lastly, lower the coupling cap c4 to as low as .001uf. this will act to block bass frequencies. this will also lower the over all gain.

i also forget something pretty simple. just put a bright cap over the gain pot of the crunch channel exactly as there is one on the lead gain pot.

i am no expert, by the way.

[shep]

jca/rectifier mod parts list (mouser bom):

273-680K-RC
71-CCF02-F-220K
594-2222-375-26222
594-2222-375-14102
75-561RU2JQA102EC200
594-S501K29X5FN63L6R


update: completed the recto mod and it sounds awesome.