Jca20H Mod thread and owners club.

[Zozobra]

Large grid stoppers usually reduce the high frequencies by forming a low pass filter with the grids capacitance. If you want to tame some of the higher order harmonics I'd add an anode bypass cap or some negative feedback between the cathode and anode of one of the stages.

[detailman]

New owner of a JCA20H and JCA12S half-stack here! Got it new for $299 + tax from Guitar Center online. It looks and sounds better than any amp I've ever owned, including some I paid twice as much for. I don't plan on modding it as I already like what I hear and was looking for something that sounded good right out of the box. I've gotten caught up with past amps on tube and speaker changes, not to mention pedals to improve the sound. It was never ending, expensive and exhausting. Again, I already love what I hear from my Jet City and will just play it and experiment with various amp settings. One small concern I do have is that it looks to me like the transformers are kind of scratched up on the surface. I haven't actually opened it up to view more closely but just by looking through the back I see surface scratches on the blue paint. Anyone have a similar experience with theirs? Doesn't seem to affect performance or sound, thankfully.

[erikm5150]

Originally Posted by 333

Anymore luck on the brown eye mod? I've done the saturation switch and that works fine. Got all the resistor that were noted, so maybe someone has worked out the cap values. Please let me know. Thanks

Glad to see this thread moving forward.
I have a friend who got one of these BE modded JCA20H with the Sat switch.

I was able to take a peek inside.
Here is what I found:

First stage:
Anode - 220k in series with 100k // 500p -- very interesting!
Cathode - 2.7k // .68 -- standard marshall

then into 2n2 coupling cap then into 68k/68k voltage divider.
then into 33k which is soldered straight to the grid of the next gain stage

Second stage:
anode - 100k
cathode - 2.7k // .68

then into 2n2 coupling cap
then into the standard 470K/470K divider with 470pf treble peaker (it think the actual value in the amp was 500p silver mica)

3rd stage:
anode - 100k
cathode - 820 // 22u

Tone stack values changed to stock marshall -- slope R 33k, treble cap 500p mica

Also, there is a "fixed depth" mod installed -- it looks like a 4n7 cap in series with a 100k resistor.

That's all I remember off the top of my head.
Hopefully that'll get you in the ballpark.

I will be able to take pics over the holidays and i'll post em here

I'm really interested to find out what is hidden in the SAT switch, the stuff that is covered in the black stuff.
It is most likely a jose master, but I have heard that the jose master does not use diodes like everyone thinks.

Enjoy!

[GRB]

Can someone shed some light for me regarding the depth mod? Are there any pictures available? I would really appreciate it. I have the schematic but am not following some of the discussion regarding where to move the stock feedback tap because of too much nfb? Is that just relocation the yellow feedback wire to another tap? I am not following that on the schematic that is why I would really appreciate any pictures of the changes including that as well as the addition of caps. thank you!

[Edge11]

Originally Posted by erikm5150

Glad to see this thread moving forward.
I have a friend who got one of these BE modded JCA20H with the Sat switch.

I was able to take a peek inside.
Here is what I found:

First stage:
Anode - 220k in series with 100k // 500p -- very interesting!
Cathode - 2.7k // .68 -- standard marshall

then into 2n2 coupling cap then into 68k/68k voltage divider.
then into 33k which is soldered straight to the grid of the next gain stage

Second stage:
anode - 100k
cathode - 2.7k // .68

then into 2n2 coupling cap
then into the standard 470K/470K divider with 470pf treble peaker (it think the actual value in the amp was 500p silver mica)

3rd stage:
anode - 100k
cathode - 820 // 22u

Tone stack values changed to stock marshall -- slope R 33k, treble cap 500p mica

Also, there is a "fixed depth" mod installed -- it looks like a 4n7 cap in series with a 100k resistor.

That's all I remember off the top of my head.
Hopefully that'll get you in the ballpark.

I will be able to take pics over the holidays and i'll post em here

I'm really interested to find out what is hidden in the SAT switch, the stuff that is covered in the black stuff.
It is most likely a jose master, but I have heard that the jose master does not use diodes like everyone thinks.

Enjoy!

We'll find out!

[Edge11]

Originally Posted by spy590

Hey guys--new JCA2112 combo owner here. I've been following the thread and wanted to report my findings on my tube experiments. With the gain dimed on the standard preamp tubes, I noticed the tone started to get a tad muddy, so I opted for a 12at7 in V2 in an effort to tighten things up a bit. Others seem to have had a good experience with that tube in V2. I lost some gain overall, but it's worth it for the extra clarity. I can always throw some pedals up front for some metal type gain. I dropped one of the stock tubes in a couple different places, but in the end I went with this:

V1: JJ12AX7 High Gain
V2: JJ12AT7 Standard
V3: JJ12AX7 Standard

I'm still running the stock power tubes, but I'm digging this combo at the moment. The tube change made a pretty big difference to my ears. It also extended the cleans a bit--I can get another notch of clean volume before things start to break up. Thanks for all the great info--maybe a clean channel mod next.

try a 12dw7 in v2

[Zozobra]

 

Originally Posted by GRB

Can someone shed some light for me regarding the depth mod? Are there any pictures available? I would really appreciate it. I have the schematic but am not following some of the discussion regarding where to move the stock feedback tap because of too much nfb? Is that just relocation the yellow feedback wire to another tap? I am not following that on the schematic that is why I would really appreciate any pictures of the changes including that as well as the addition of caps. thank you!

 

A picture how to has been posted on HCAF somewhere. I dont know where it is but there is one.

[GRB]

Yeah I know there is verbage about the depth mod, but a lot of what is posted relates to posts that are no longer there? or not accessible. For example I believe at one time there was pictures or information from VH4 but I have not been able to find anything on the site for the depth mod now except the schematic. I am a noobie, but I have the bug so I am willing to try I just am trying to gather as much info as possible so I do it correctly. thank you everyone for any help!

[Edge11]

Originally Posted by GRB

Yeah I know there is verbage about the depth mod, but a lot of what is posted relates to posts that are no longer there? or not accessible. For example I believe at one time there was pictures or information from VH4 but I have not been able to find anything on the site for the depth mod now except the schematic. I am a noobie, but I have the bug so I am willing to try I just am trying to gather as much info as possible so I do it correctly. thank you everyone for any help!

All of the info has been reposted in thread, look for it.
I'll update the first page after the holidays.

[Jack Tone]

Originally Posted by GRB

Yeah I know there is verbage about the depth mod, but a lot of what is posted relates to posts that are no longer there? or not accessible. For example I believe at one time there was pictures or information from VH4 but I have not been able to find anything on the site for the depth mod now except the schematic. I am a noobie, but I have the bug so I am willing to try I just am trying to gather as much info as possible so I do it correctly. thank you everyone for any help!

Check this thread, page 3 (post #53). I believe the only difference for the 20h is that the feedback wire is yellow.

http://acapella.harmony-central.com/...hp?2671801-NAD

I just finished the mod about an hour ago and the difference is awesome. Thanks again mblooper, ledvedder, shinybulldozer, and especially VH4.

[ZZ007]

Hello,

Somebody knows how to release a footswitch for second channel ???
Best regards
Sorry for my bad english cause i'm nfrench ...

[Edge11]

Originally Posted by ZZ007

Hello,

Somebody knows how to release a footswitch for second channel ???
Best regards
Sorry for my bad english cause i'm nfrench ...

i think weber has a kit for a footswitch.
https://taweber.powweb.com/store/chansword.htm
It would be very cool to see a tutorial on how to use these as i am interested in the one dpdt one and the 4channel one.

[Zozobra]

Footswitching is a pain in the ass. El34world has a good section on relay switching.

[Edge11]

Originally Posted by Zozobra

Footswitching is a pain in the ass. El34world has a good section on relay switching.

http://www.el34world.com/projects/relay_switch.htm
Are you talking about this? I'll give it a look when i have some free time.

[Zozobra]

That's the one. Are the heaters elevated in this amp? Never seen a schematic.

[Edge11]

 

Originally Posted by Zozobra

That's the one. Are the heaters elevated in this amp? Never seen a schematic.

 

I'll p.m. you one since I'm not 100% on that but in the scheme the 100r resitors that for the center tap are grounded and not tied to the cathodes of the power tubes, But like i said before I'm not 100%.

[GRB]

Just received my tubes from eurotubes today for my JCA20H. SWEET!
I got a matched set of jj el84's,
V1 -jj high gain ECC83S
V2 -jj gold pin ECC81
V3 -jj high gain and balanced ECC83S

i get to put them in either tomorrow or Wednesday. I should receive the choke by this weekend also. I will post my findings on the tube compliment and the addition of the choke when I get them put in. I also just ordered the parts for the depth mod. I do have a couple of questions that I was hoping someone could answer. for the depth mod caps, both the .1uf, and .0047uf, does it matter if the caps are 600V or do they need to be 630V? what if any difference would there be? Thanks again for the assistance

[turnituploud]

Hi all. I bought a Jet City Jca2112 in the fall of august, and been with it ever since. While it sounds godly on >4 on master volume, i cant really crank it that loud, since i live in an apartment(3 on master volume tops) and at that setting it just has this fizziness in the top end that i cant quite stand anymore. So i'll be doing this mod to get rid of it

As far as mods go, the biggest complaint I have heard is too much hi-end gain and "frazz". You can calm this down, reshape it and thicken the tone by adding a 100K resistor in series with C23 (which forms a HP filter in parallel with R32, and also replacing the 470pF treble cap in the tone stack with 1000pF (.001)@500V. Both of these mods go a long way toward removing the ice-picks. Also, a 47pF cap installed right on the V1 tube socket at Pin 7 (signal input) will reduce some of the garbage noise and increase stability a bit.

but then i will also do the whole mod(depth, clean/crunch, gain) by VH4(thanks a lot btw) and the fx loop, so i have 3 questions so far:
1) Will the HP filter mod stack with VH4's mod?
2) Is the choke or the other component mods responsible for the increase in gain?(cause i cant really get a choke here in Latvia, sending from overseas would be expensive) And if that wont be enough gain, then, besides replacing my bad monkey with a ts-7, i will retube it. So the question is, what preamp tubes for more gain? I'm guessing 3 jj high gain 12ax7's including a balanced one in v3.
3) I will probably do the fx loop by either the attached schemes but what tube should i use: 12at7, 12ax7, 12au7?
Oh, and dont buy the bad monkey, especially with this amp, it really kills the sound.
[ATTACH=CONFIG]329639[/ATTACH] [ATTACH=CONFIG]329638[/ATTACH]

[Edge11]

Originally Posted by GRB

Just received my tubes from eurotubes today for my JCA20H. SWEET!
I got a matched set of jj el84's,
V1 -jj high gain ECC83S
V2 -jj gold pin ECC81
V3 -jj high gain and balanced ECC83S

i get to put them in either tomorrow or Wednesday. I should receive the choke by this weekend also. I will post my findings on the tube compliment and the addition of the choke when I get them put in. I also just ordered the parts for the depth mod. I do have a couple of questions that I was hoping someone could answer. for the depth mod caps, both the .1uf, and .0047uf, does it matter if the caps are 600V or do they need to be 630V? what if any difference would there be? Thanks again for the assistance

no difference. I would go with 630 just to be on the safe side.

Originally Posted by turnituploud

Hi all. I bought a Jet City Jca2112 in the fall of august, and been with it ever since. While it sounds godly on >4 on master volume, i cant really crank it that loud, since i live in an apartment(3 on master volume tops) and at that setting it just has this fizziness in the top end that i cant quite stand anymore. So i'll be doing this mod to get rid of it

but then i will also do the whole mod(depth, clean/crunch, gain) by VH4(thanks a lot btw) and the fx loop, so i have 3 questions so far:
1) Will the HP filter mod stack with VH4's mod?
2) Is the choke or the other component mods responsible for the increase in gain?(cause i cant really get a choke here in Latvia, sending from overseas would be expensive) And if that wont be enough gain, then, besides replacing my bad monkey with a ts-7, i will retube it. So the question is, what preamp tubes for more gain? I'm guessing 3 jj high gain 12ax7's including a balanced one in v3.
3) I will probably do the fx loop by either the attached schemes but what tube should i use: 12at7, 12ax7, 12au7?
Oh, and dont buy the bad monkey, especially with this amp, it really kills the sound.
[ATTACH=CONFIG]329639[/ATTACH] [ATTACH=CONFIG]329638[/ATTACH]

1. I don't know compare schemes and you tell me.
2.The choke makes the amp sound tighter especially at high voulumes. What gives you more gain for example in deus' mods are the cathode and anode resistors on the preamp tubes. The stock chinese tubes have alot of gain but also cause frizz and fuzz.
I like jj short plate in v1
Sovtek lps in v2 and
any in v3 ( Phase inverters in guitar amps aren't balanced any ways, no point in a balanced tube.)
3. I don't know what schematic your referring to.

[SciHi]

R11 and R12 (artificial center tap for heaters) just went up in flames on my combo while I was testing different nfb resistor values. I was hoping to add some life to the lower gain settings by going to a higher value nfb resistor, 47k on 16 ohm speaker jack is a lot of negative feedback. So anyway, anybody else ever had this happen? I don't know whether a tube failed or what? I will take it slow and test each tube individually, pretty crazy those are 1 watt 100R resistors so they had some serious current to flash!!! I have carbon film 1 watter to replace them but need to figure out what went south...

[StudioPlsCherry]

Has anyone had any problems with getting the back plate off your JCA20H? I've loosened all 6 screws, but the darn thing is jammed in there pretty seriously... I really don't want to try forcing it out, but I'm not sure what else to do.

[SciHi]

 

Originally Posted by StudioPlsCherry

Has anyone had any problems with getting the back plate off your JCA20H? I've loosened all 6 screws, but the darn thing is jammed in there pretty seriously... I really don't want to try forcing it out, but I'm not sure what else to do.

 

Yea, I think they put the back on when the glue is still wet and tacky. Putty knife needed sometimes but be careful it is easy to tear the tolex and scratch the paint off the chassis (done both before!). Took both hands to pull the back off my combo, pry up one corner then pull like H---

[StudioPlsCherry]

You were spot on...I got one of the edges to lift with a putty knife and then managed to wiggle the panel until it finally popped. Also, the amp sounds pretty awesome with V1 replaced with a 12AY7 and V3 with a 12AT7.

[turnituploud]

 

Originally Posted by mblooper45

3. I don't know what schematic your referring to.

 

the attached ones.

[Cannibalbritney]

hey i just got a weber tube fx loop... anybody have a schematic on how to install ? also everyone keeps referring to some guy on here, who has all these schematics and DIY info but i dont see anything... can someone link me? thanks guys.

so far did the choke mod, Big Difference. loving this little monster.


also if anyone has a schematic on adding a tube gain stage id like to take a look at it.

thanks again