JCA OWNERS!!! (RE)JOIN THE JET CITY LOUNGE!!!

[ironman28]

Just picked up a used JCA50. Been lurking here for the last couple of months watching things progress.
Fired it up last night into a 2-12 celestion cab. Not sounding too good at this point, still has the original Chinese pres in it. The previous owner installed a set of JJs for the PA. Gonna start swapping the pres out to see what kind of improvement I get.

Some observations:

The OT is huge compared to most 50W amps. I don't know if this reflects a design philosophy with a full band width OT or was it a mfg decision.
The FX loop is not very flexible. Kind of a 80's design
Seems to be a well constructed PCB design with a sturdy chassis that lends itself to modding (that's why I'm here)

Any way once the tubes are upgraded I will start to tweak it. In no particular order;
Clean up the crunch channel
Move the FX to post MV (like the 22)
Add send and return controls for the loop
Maybe change up the tone stack slope and mid shift
redo the presence circuit to be a "cut/boost" like a HIWatt

This should be fun!
BTW anyone have the JCA50 PA schematic?

Hiya - welcome to the club!

Anxious to hear what you come up with on the mods, although tubes, bias, and depth mod will give you a huge head start. The effects loop stuff is of major interest to me - I've wanted to try out impulses for recording with the send, but was disappointed to find that it's pre-EQ. That said, I'm a little fuzzy on how you'd go about implementing the effects loop post-master volume - wouldn't that potentially overpower even line-level effects?

I actually created a 50H documentation thread over in DIY some time ago, but just realized that I never posted the schematics. That's been fixed as of a couple minutes ago. I've made links to relevant stuff (like my 50H documentation thread) on the 3rd post in this thread, but here's the shortcut anyway: JCA50H Documentation

Thanks for the welcome!

If you look at the loop placement in the JCA22 schematic you will see how a post MV loop circuit would work. I think the JCA22 loop works better for most players because of the signal losses in the tone stack and MV attenuation. There also seems to be some send attenuation through R29/R30.
So in essence the tone stack would be driven by V3. V4 would be freed up to perform both the send and return functions for the loop. 1/2 of V4 would be a send buffer using a cathode follower and the other 1/2 of V4 is the return buffer. From there it is easy to add send/return controls for each section. I would not use the exact circuit from the 22 as there are better loop designs. There are several kits available to add tube FX loops but that's not required here . All the basic elements can be created using V4 and the associated parts. The end result will be a transparent tube loop with send/return controls, switchable series/parallel and switchable -20/0db return.

Thanks for posting the schematic. I'll keep everyone updated as things progress. I would eventually like to create some DIY step by step instructions for the other members.

[Tortuga]

Sounds like a great plan - definitely above my pay grade . Perhaps Jay, käsebrot, or one of the other regulars here can offer up some input that may be of use.

If this works as well as you say, it'd certainly be of interest to me, and something I'd probably do on my own amp. I wish you the best of luck

[Agradeleous]

As marshmallow from annoying orange would say Yaaaaaaaaaaaaay I got my jca22h head today and ups followed the note I left on the fence as to leave it on grass in the rain but instead put on the small covered steps so UPS did put on the steps the part not covered by the small roof (add words of choice here ) luck was with me we missed the rain so all good .

I mean really UPS 12 inches more next you 'll end up like Fedex and leave it while were home on the fence so anyone can drive up and steel it LMAO no wait that an't funny !

K so enough of the rant .... Man it looks great ! foot switch is well ... a foot switch . all metal looks like it's well built and has a nice clicky to it LOL . I also got a nice power cord Yaaaaaaay . I put the cab and the head together and was surprised because some say it's ugly in looks.

Well the blue on the web looks a lot lighter in color but not the case looks good to me . What I did not get was a speaker cord holy no cord batman ! Really I thought I'd get a speaker cord with one of the two items LOL

The cab is the JCA12s+ I got it because it's versatile not knowing the sound quality is pit fall in my head at this point. The perks are the convertibility of the cabinet venting/porting options and most important to me is the speaker emulated out via XLR . I wish it was on the head but tit's for tat I guess.

Now I have but can't play it well I can and will but only have guitar cords. I can make one up with 12g audio wire err.... yeah or I could out and buy something you all recommend. I get a vid and sound bits together as soon as I can .

I have one question and that is the JCA22H says the BIOS is fixed ! Umm .... no toying with electric stimulation devices even if I replace tubes ? Bummer ! .... This is the part I need your all expertise . So BIOS info and Speaker cable info gauge type , strand or solid ... does anyone use solid cable speaker wire ? hahahaha .... I sure some have .

[ironman28]

Swapped V1 to a JJECC83 much improved now. That's the worst tube I've heard since I pulled the original peavey tubes out of a 92 rockmaster preamp
Now another problem with loud pops when switching channels...fun

[Agradeleous]

Hey Ironman happy to see you and the other members here so tech skilled. I have the skill not the knowledge. LOL

I just fired up the new amp and I have to say WOW ! I see were the channel one is pretty hot by itself but with my Ibanez pickups I get real good blues sound and lead is great to though some of you heavier rockers might not agree LOL


It's a hell of lot quieter the my old Peavey triumph 120 in fact to quite what gives LOL my wife says it hurts her ears turn it down i said hun I only have on 2 hahaha . The speaker cab is tight which is ok but I will get to the open back and see how that sounds other wise for now the stock tubes sound pretty good though I have been out of touch for a while .

Take care and play well

[Jay]

Just picked up a used JCA50. Been lurking here for the last couple of months watching things progress.
Fired it up last night into a 2-12 celestion cab. Not sounding too good at this point, still has the original Chinese pres in it. The previous owner installed a set of JJs for the PA. Gonna start swapping the pres out to see what kind of improvement I get.

Some observations:

The OT is huge compared to most 50W amps. I don't know if this reflects a design philosophy with a full band width OT or was it a mfg decision.
The FX loop is not very flexible. Kind of a 80's design
Seems to be a well constructed PCB design with a sturdy chassis that lends itself to modding (that's why I'm here)

Any way once the tubes are upgraded I will start to tweak it. In no particular order;
Clean up the crunch channel
Move the FX to post MV (like the 22)
Add send and return controls for the loop
Maybe change up the tone stack slope and mid shift
redo the presence circuit to be a "cut/boost" like a HIWatt

This should be fun!
BTW anyone have the JCA50 PA schematic?

Hiya - welcome to the club!

Anxious to hear what you come up with on the mods, although tubes, bias, and depth mod will give you a huge head start. The effects loop stuff is of major interest to me - I've wanted to try out impulses for recording with the send, but was disappointed to find that it's pre-EQ. That said, I'm a little fuzzy on how you'd go about implementing the effects loop post-master volume - wouldn't that potentially overpower even line-level effects?

I actually created a 50H documentation thread over in DIY some time ago, but just realized that I never posted the schematics. That's been fixed as of a couple minutes ago. I've made links to relevant stuff (like my 50H documentation thread) on the 3rd post in this thread, but here's the shortcut anyway: JCA50H Documentation

Thanks for the welcome!

If you look at the loop placement in the JCA22 schematic you will see how a post MV loop circuit would work. I think the JCA22 loop works better for most players because of the signal losses in the tone stack and MV attenuation. There also seems to be some send attenuation through R29/R30.
So in essence the tone stack would be driven by V3. V4 would be freed up to perform both the send and return functions for the loop. 1/2 of V4 would be a send buffer using a cathode follower and the other 1/2 of V4 is the return buffer. From there it is easy to add send/return controls for each section. I would not use the exact circuit from the 22 as there are better loop designs. There are several kits available to add tube FX loops but that's not required here . All the basic elements can be created using V4 and the associated parts. The end result will be a transparent tube loop with send/return controls, switchable series/parallel and switchable -20/0db return.

Thanks for posting the schematic. I'll keep everyone updated as things progress. I would eventually like to create some DIY step by step instructions for the other members.

You could just cut some PCB traces and re-route the circuit. If you want to go crazy, just configure the loop to use a 12AT7 and bootstrap that bitch. Heck, if you want to go simple for now, the 50H I modded for a friend, I did the SLO mod and the FX loop mod and he says there is no loss of tone, or very little loss when the loop is used. I did not try it with the stock JCA circuit, but I have heard numerous times that it changes the tone/feel of the amp. With the SLO circuit, I think the change is negligible.

[mjtripper]

Just picked up a used JCA50. Been lurking here for the last couple of months watching things progress.
Fired it up last night into a 2-12 celestion cab. Not sounding too good at this point, still has the original Chinese pres in it. The previous owner installed a set of JJs for the PA. Gonna start swapping the pres out to see what kind of improvement I get.

Some observations:

The OT is huge compared to most 50W amps. I don't know if this reflects a design philosophy with a full band width OT or was it a mfg decision.
The FX loop is not very flexible. Kind of a 80's design
Seems to be a well constructed PCB design with a sturdy chassis that lends itself to modding (that's why I'm here)

Any way once the tubes are upgraded I will start to tweak it. In no particular order;
Clean up the crunch channel
Move the FX to post MV (like the 22)
Add send and return controls for the loop
Maybe change up the tone stack slope and mid shift
redo the presence circuit to be a "cut/boost" like a HIWatt

This should be fun!
BTW anyone have the JCA50 PA schematic?

Hiya - welcome to the club!

Anxious to hear what you come up with on the mods, although tubes, bias, and depth mod will give you a huge head start. The effects loop stuff is of major interest to me - I've wanted to try out impulses for recording with the send, but was disappointed to find that it's pre-EQ. That said, I'm a little fuzzy on how you'd go about implementing the effects loop post-master volume - wouldn't that potentially overpower even line-level effects?

I actually created a 50H documentation thread over in DIY some time ago, but just realized that I never posted the schematics. That's been fixed as of a couple minutes ago. I've made links to relevant stuff (like my 50H documentation thread) on the 3rd post in this thread, but here's the shortcut anyway: JCA50H Documentation

Thanks for the welcome!

If you look at the loop placement in the JCA22 schematic you will see how a post MV loop circuit would work. I think the JCA22 loop works better for most players because of the signal losses in the tone stack and MV attenuation. There also seems to be some send attenuation through R29/R30.
So in essence the tone stack would be driven by V3. V4 would be freed up to perform both the send and return functions for the loop. 1/2 of V4 would be a send buffer using a cathode follower and the other 1/2 of V4 is the return buffer. From there it is easy to add send/return controls for each section. I would not use the exact circuit from the 22 as there are better loop designs. There are several kits available to add tube FX loops but that's not required here . All the basic elements can be created using V4 and the associated parts. The end result will be a transparent tube loop with send/return controls, switchable series/parallel and switchable -20/0db return.

Thanks for posting the schematic. I'll keep everyone updated as things progress. I would eventually like to create some DIY step by step instructions for the other members.

Do you have a link or schematic for a transparent tube loop you would recommend? I'd be interested in playing around with that.

[ironman28]

Check out this thread:
http://www.el34world.com/Forum/index.php?topic=10208.0
Lots of good examples for tube based loops. The one I prefer is here:
http://www.el34world.com/Forum/index.ph ... 6557;image
This is an exact copy of the design from KOC TUT volume 1. It is designed to minimize any distortion or coloring of the signal passed through the loop. If you are inclined you can purchase a kit from London Power. It requires a new tube to be added to the chassis.

[ironman28]

I thought I should mention a few caveats if anyone wants to try reassigning the V4 stage to a post MV type loop

1. You may have to cut traces on the PCB to build the circuit ( I have not confirmed this but I am pretty sure)
2. V4 in the soldano design does provide some gain. I don't believe it creates distortion but the amp may "feel" different without it
3. I would test by removing V4 temporarily and connecting the tone stack to V3 before committing V4 to a new function
4. Some of the gain lost in the stock V4 stage can be recovered in the loop return stage
5. The loop design linked to above is a 0db/-20db level. It is not designed for +4db operation

[Jay]

I wonder how you would be able to utilize that loop design on the JCA board. Moving the loop post-EQ is a good idea, but here's how I might do it:

Use a cathode-biased, bootstrapped design rather than the fixed-bias design you posted. The design in the 22H is really good, but with a few changes, could be better. The channel volumes would be used as the send level, and you could install another pot just before the return stage as a return level/master volume control. The return stage won't offer "distortion" gain, but just the drive to bring the signal back up to level. Also, I didn't take a good look at the 50 I modded, but it might be easier as well to retrofit a cathode bias loop rather than the fixed-bias loop. In this way also, you don't need the -20dbv switch, the channel volumes will do that for you. Use a tube like the 12AT7, and you will have a very low output impedance and a rather transparent loop.

[ironman28]

Some mods for the JCA20/22:
http://valvesphere.blogspot.com/2012/01 ... -high.html
http://valvesphere.blogspot.com/2012/01 ... hness.html
http://valvesphere.blogspot.com/2012/01 ... ca20h.html

[ironman28]

To remove the PCB:
1. Remove the knobs and tubes
2. Remove all pot nuts and input nut
3. remove all jack nuts on the speaker and footswitch jacks. Move the jacks out of the way
4. remove the PCB screws, there are 12
5. disconnect the two brown leads on the standby switch. the connectors pull off
6. unsolder the red wire connected to the NFB point on the PCB
7. Slide the board toward the back of the chassis. The pots should be almost all the way out of their mounting holes
8. Lift up on the rear of the board until the pot shafts are free of the chassis

Stand the PCB on edge with a light behind to help trace connections, like this, pots are facing up





Upon close inspection I found some cold solder joints on the speaker jacks and FS jacks. I am going to check each joint



There are two jumpers JP2 and JP3 that connect pin 8 of V7 and V8 to ground. I removed these and replaced with cathode sense resistors.


There are some dodgy solder joints around V2 that will be redone, and the flux cleaned up.


Installed 1k 5W screen resistors. You can see the cathode sense resistors here.


more to follow...

[Jay]

Check the filter cap on the switching supply, also. The 50H I repaired had a bad joint/bad replacement/something weird and the end result was a blown PT. Just a heads up. I hoping it's only an isolated issue and not widespread.

[Agradeleous]

Ok you guys are gunna laugh at me but it's ok hahahahaha my playing sucks when i do stuff like this . I'm just a guy who works in the dirt comes home and grabs a beer and play something most of late I have not been motivated by anything anyway if you wanna laugh you can my poor mans demo of the amp here so you don't have to down load it LMAO
http://www.ttmbandroom.com/index.php?t= ... ng_id=2071

I gave this forum a plug at my place of hang because after what I have been reading here the amp will need some modding so you folks will be my support just can't do right away . you can read my post here if interested.

http://www.twotrackmusic.com/forums/ind ... 4#msg50824

Thanks for all your info and a great thread play well all and happy recording

[thesockmonster]

Wild question, but would anyone be willing to mod my amp to BE specs? I'm terrible with an iron, and have killed enough pedals to leave well enough alone.

[ironman28]

Do you have the BE specs? component values etc?
If you do I would suggest finding a local tech to make the changes for you.
On the other hand you may find a forum member able to do the work that is close to you. I like to do mods where the customer can test the amp before it leaves the shop. Shipping a 40 lb amp back and forth is a PITA
Also I would encourage you to try some simple mods to build your confidence. Tube amps are much easier to work on compared to FX pedals. The components are physically larger and the pc board is more robust so traces dont lift.
Just be sure you understand and follow all safety precautions before attempting any work on a tube amp.

[maynardo]

I have the schematic and even pictures of a jca20 with a BE mod done by friedman's people, is nothing but a Jose Arredondo mod with a fixed depth and first gain pot dimed a.k.a. taken out of the circuit , I would not recommend the mod for a begginner since you have to cut two traces and throw jumper under the pcb .

[Tortuga]

I have the schematic and even pictures of a jca20 with a BE mod done by friedman's people, is nothing but a Jose Arredondo mod with a fixed depth and first gain pot dimed a.k.a. taken out of the circuit , I would not recommend the mod for a begginner since you have to cut two traces and throw jumper under the pcb .

Care to share? Would love to see what you got!

[maynardo]

I have the schematic and even pictures of a jca20 with a BE mod done by friedman's people, is nothing but a Jose Arredondo mod with a fixed depth and first gain pot dimed a.k.a. taken out of the circuit , I would not recommend the mod for a begginner since you have to cut two traces and throw jumper under the pcb .

Care to share? Would love to see what you got!

sure PM me with your email & I'll send you an schematic and a picture

[ironman28]

Did a quick google on the friedman brown eye and found this:

[maynardo]

yup second schematic is correct, I have it in a more 'pro' presentation but its the same thing, didn't wanted to post it here cause it is a USA forum and one of friedman's people might see them & ask to take the pics down from the server and what not. The last two pics are from a spanish forum so they fly under the radar

[Dunlop]

Hello guys, I will be a new (hopefully proud) owner of JC amps (jca22h)

For first I wanted to ask, is there any way how to install depth mod without drilling a new hole into chasis?
My amp will be new with 3years warranty, and it is chinese amps so I think that warranty may be good way how to fix it if something went wrong :-)

Thanks

[ironman28]

No disrespect to Mr. Friedman and his peeps but there is nothing infringing here that they can complain about. The first schematic is from the HCA forum and is EDGE11's take on the fmbe mod. It is almost identical to the second schematic as far as the component values but no saturation switch. The other two pics are of the posters personal amp.
I would be interested to see your pictures of the work done by friedman compared to the picture I posted above. I dont know if that is representative of his work. It looks more like a DIY job than a professional mod.

[maynardo]

No disrespect to Mr. Friedman and his peeps but there is nothing infringing here that they can complain about. The first schematic is from the HCA forum and is EDGE11's take on the fmbe mod. It is almost identical to the second schematic as far as the component values but no saturation switch. The other two pics are of the posters personal amp.
I would be interested to see your pictures of the work done by friedman compared to the picture I posted above. I dont know if that is representative of his work. It looks more like a DIY job than a professional mod.

That picture IS from friedman's work, I have another one where the guy went with sozos everywhere . If you register in sloclone forums you can see many more pics from marshall's he or his people modded and they're as messy or more than that one you posted

btw edge11's schematic has a couple of small errors a grid stopper and a bypass cap, that's what I saw at first glance plus it doesn't include the fb loop.

Best regards

[Tortuga]

Hello guys, I will be a new (hopefully proud) owner of JC ampps (jca22h)

For first I wanted to ask, is there any way how to install depth mod without drilling a new hole into chasis?
My amp will be new with 3years warranty, and it is chinese amps so I think that warranty may be good way how to fix it if something went wrong :-)

Thanks

Welcome!

You could probably look into using one of the holes for speaker output (assuming you don't use at least one of them) - I think some people have done that. Alternatively, I suppose you could hard-wire a resistor / capacitor inline if you determined a value you like - I'm pretty sure at least one of the Soldano amps does that.