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

[Tortuga]

V1 closest to input jack, V5 nearest power section

V1 & V2 are the gain tubes. Most sensitive to tube changes. Try your 5751 in V2, too.

V5 is the Phase Inverter. I think lots of people have used 5751s & AT7s there. AU7 is fairly low gain, and may not give great results in this amp (I tried and old one, and thought it made the amp fairly lifeless).

V3 & 4 are (I think) EQ and effects loop buffer.

[Nproctor]

Thank you for the info! I was planning on running jj12ax7's for the first 3, a jj12au7 in the cathode follower, and a Mullard 12ax7 in the PI.

[Shinigami75]

Can anyone point my to how to adjust the high freq on just the lead channel or just the rhythm channel. I can't handle the diff between the two it renders one channel or the other useless for me. I need to shave off some high end off the lead. Or I could add some to the rhythm channel then roll both channels down with the treble knob. I am not a able to read amp circuits and interpret. I only read and learn from you guys. Thanks.

I'm done the depth, low end boost and slope mods already.

I wish this 33 page post was its own forum. This awesome amp deserves it!

"So, finally I've got new tubes, all JJ.

changing the valves made a diference in sound, but biggest diference was when I did the BIAS on the amp (21/22ma) and lifted a leg on the C6 (c6 acts as a bright cap over the gain knob, so taking it out would create less brightness).

I can tell you, it's a new amp!! Great sound

I can now use the treble and the presence pot a lot. On my gig it went well, i used the volume on 3 and it was a bit less fizz, but the changes I did yesterday on the BIAS and de C6 Cap where immense."

Have you used a BIAS probe to bias the amp? What method have you used to measure the plate voltage?

[Shinigami75]

Hi Folks,

I used to have an Egnater Tweaker few years ago and one cool feature was the "Tight/Deep" switch which works this way: The TIGHT setting works by cutting the deep bass at the input stages of the preamp. This helps prevent your overdrive sound from getting too “muddy”. The DEEP position creates a full, rich bass that sounds great for clean sounds and also help “fatten” up single coil pickups.

The schematic is here:

http://el34world.com/charts/Schematics/ ... ker_15.pdf

Do you think this kind of mod could be done on the JCA models? How hard would it be to make it?

Cheers

[Nproctor]

Anyone know the voltage of the led inside the head? It's not 9 volts, I found out the hard way.

[Shinigami75]

Hi Folks,

I'm planning to have a go with the Depth Mod on my Custom 22 but since its layout is different from that of the 22H I was wondering if somebody would be able to confirm that the 2 points I have to work with are those highlighted in yellow in this picture:

http://i1356.photobucket.com/albums/q73 ... qn2abx.jpg

Thank you in advance

[Tortuga]

I don't see the highlighted yellow in that pic

BTW - since you're on photobucket, it's real easy to post pics here - just click the "IMG" box, and it'll copy that text to your clipboard. Then, paste it into your posting box here, and it'll work fine.

Just like this...

[Tortuga]

And, now I see the highlighted circles... I don't think there's a resistor involved. On my JCA50H board, there is a red wire that's soldered to the board, which I cut and re-routed to the depth pot on the back panel. I'm looking at my pics, and will provide more help...

[Tortuga]

On the first page of this thread, I have a set of references, from which I found my Depth Mod thread. There's a picture on that posting, which shows my 50H board pretty clearly, and where my connections are made...

Depth Mod thread: http://www.guitarampboard.com/forum/viewtopic.php?f=53&t=19992#p391970



I cut the red wire from the output jacks, leaving it connected to the PCB, so I wouldn't have to resolder on the board. I then re-routed that wire to the new pot (actually, to the in-line cap connected to the pot - see thread for info), and connected a new wire (grey) from the output side of the depth pot back to the output jacks.

[Shinigami75]

Hi Grime thanks a lot for the picture I had highlighted that resistor only because I thought it was the NFB resistor so I had thought it would be enough to solder a wire on one of its legs and back to the pot for the depth mod.

The output speaker looks different but that won't be an issue. My thoughts is that everyone specifies 8ohm jack but I use the 16ohm 95% of the times so I may have to use that instead.

I'll keeo researching and gather more info and knowledge

[Shinigami75]

Also, I thought it was just the same as it can be seen in this video at 0:06 where the depth mod pot is in between that FB resistor and the FB point on the board where the output jacks are

https://www.youtube.com/watch?v=nhvgnCy9QPs

[maynardo]

Hi Folks,

I used to have an Egnater Tweaker few years ago and one cool feature was the "Tight/Deep" switch which works this way: The TIGHT setting works by cutting the deep bass at the input stages of the preamp. This helps prevent your overdrive sound from getting too “muddy”. The DEEP position creates a full, rich bass that sounds great for clean sounds and also help “fatten” up single coil pickups.

The schematic is here:

http://el34world.com/charts/Schematics/ ... ker_15.pdf

Do you think this kind of mod could be done on the JCA models? How hard would it be to make it?

Cheers

Whoa, that schematic looks weird, a lot transistor action going on there.

About what you're asking the simplest solution it would be to make the first coupling cap variable, for if you have the JCA22, that would be C2... the "tight" part could be anywhere from 470p to 2n2 and then with a switch would you add something like 10n, 22n or more in parallel to the first one and that would be your "deep".

[Shinigami75]

Hi Folks,

I used to have an Egnater Tweaker few years ago and one cool feature was the "Tight/Deep" switch which works this way: The TIGHT setting works by cutting the deep bass at the input stages of the preamp. This helps prevent your overdrive sound from getting too “muddy”. The DEEP position creates a full, rich bass that sounds great for clean sounds and also help “fatten” up single coil pickups.

The schematic is here:

http://el34world.com/charts/Schematics/ ... ker_15.pdf

Do you think this kind of mod could be done on the JCA models? How hard would it be to make it?

Cheers

Whoa, that schematic looks weird, a lot transistor action going on there.

About what you're asking the simplest solution it would be to make the first coupling cap variable, for if you have the JCA22, that would be C2... the "tight" part could be anywhere from 470p to 2n2 and then with a switch would you add something like 10n, 22n or more in parallel to the first one and that would be your "deep".

Hi there

Thanks a lot for your reply, I've got a Custom 22 which is little bit different from the 22h but I'll have a look at the latter so that I should be able to spot the equivalent of C2 on my amp (some of the components are still numbered the same though).

Thanks again and keep on rockin'

[maynardo]

Hi Folks,

I used to have an Egnater Tweaker few years ago and one cool feature was the "Tight/Deep" switch which works this way: The TIGHT setting works by cutting the deep bass at the input stages of the preamp. This helps prevent your overdrive sound from getting too “muddy”. The DEEP position creates a full, rich bass that sounds great for clean sounds and also help “fatten” up single coil pickups.

The schematic is here:

http://el34world.com/charts/Schematics/ ... ker_15.pdf

Do you think this kind of mod could be done on the JCA models? How hard would it be to make it?

Cheers

Whoa, that schematic looks weird, a lot transistor action going on there.

About what you're asking the simplest solution it would be to make the first coupling cap variable, for if you have the JCA22, that would be C2... the "tight" part could be anywhere from 470p to 2n2 and then with a switch would you add something like 10n, 22n or more in parallel to the first one and that would be your "deep".

Hi there

Thanks a lot for your reply, I've got a Custom 22 which is little bit different from the 22h but I'll have a look at the latter so that I should be able to spot the equivalent of C2 on my amp (some of the components are still numbered the same though).

Thanks again and keep on rockin'

Try emailing jet city, maybe they'll send you the schematic for "repairs".

[Nproctor]

I am in the midst of doing a simple clean mod on my jca100hdm. I've got dpdt switch with a 33k resister adding to the one that's there off the leg just before the pot. The question is, where can I ground the other end of the resister? Please help!

[Tortuga]

I am in the midst of doing a simple clean mod on my jca100hdm. I've got dpdt switch with a 33k resister adding to the one that's there off the leg just before the pot. The question is, where can I ground the other end of the resister? Please help!

I believe that I connected my clean (blue) switch to ground by soldering to the grounded leg of R49, as shown in the picture below. You'll need to verify this is same on your HDM (mine is a 50H), AND that this is an actual ground point (measure resistance / continuity to chassis ground). I won't have time before tomorrow night, but if I can, I'll open up my amp and get some better pics showing these mods.



You are taking proper precautions (ie: discharging filter caps) before working on the amp, right? I really don't want to have to read about how you killed yourself working on your amp

[jhall]

I just rejoined X2!
I got a JCA 20h (the original series) but this one has a tube buffered FX loop installed by a pro tech . It is toggle switchable and really great. If I jumper the amp it sounds amazing.
I just today got a JCA 50h (the one with CRUNCH and OD with a loop).
It has a ballsy rock tone. The thing LOVES my Gibson Les Paul Classic with Duncan Antiquitues. I paid $225.00 used mint with a decent cover.
It seems they can be had for the price of a decent dirt pedal these days. This will be another "abuse amp" so I can spare my Marshalls the agony of being run dimed into the hotplate all the time.

[mbranford]

I have been waiting to post here until I can get the mods done that I have been planning for several years. I have the JCA2112RC, which is pretty much like the JCA20, except with a tube-buffered reverb unit. Of all the Jet City models, this one seems to be one of the least popular, at least for modding. I basically had to take the who combo apart to complete it, making me jealous of everyone else who has a head.

Here is the schematic:
https://drive.google.com/file/d/0B77889FOL1MnSUNTSXoxbDEzME1tQjVzaDhzYWU0UXJtelZB/view?usp=sharing

These are the mods that I completed:
156M Hammond Choke
Depth Pot
Effects loop

I will also change the tolex on the front baffle from blue to black in the next few days, add new piping in the perimeter of the cabinet and at the bottom of the front baffle.

I will not waste time on the choke and the depth mod, as these have been explained and document previously except where the choke comes into play with the B+ voltage on the effects loop.

I decided on a kit for simplicity: the Granger Kit seemed easy, and is adjustable via internal screws inside the jacks. Because I do not want to open the amp for a while once I completed the mod, this seemed like an obvious solution.
Here is the link for the Granger kit: http://grangeramp.com/ufxloop.php
I looked at a tutorial posted on the Metro Amp site (and also considered the Zero Loss effects loop, but it was more money and had an internally set level). The tutorial is the last post from dustplumb. http://forum.metroamp.com/viewtopic.php?f=7&t=32582
There are many other acceptable kits from other companies, including Iron Sounds, and also a tube buffered loop kit from London Power (a Canadian company).

Here is the upshot: the loop needs to go between the tone stack and the master volume. On both the JCA20H and the JCA2112RC, the best spot is at R14. I lifted the easiest end to work with, as I didn't take the board completely out, so I think that I ended up with the loop before R14 instead of after. If I had to do it again, I would take the resistor off the board completely, to get it out of the way, the re-solder the connection. I don't think this issue will affect the sound, but if it doesn't work, I will re-attempt it so the resistor is before the loop in the circuit.

Here is the holes drilled in the back panel of the chassis:


Please ignore the extra hole. (I will have to fill it with Grimespaces favorite mod, the slope resistor switch)

Here is the loop installed in the back panel of the chassis:



To the right is the depth mod.

Here is the choke where the loop draws from the B+ current:



The loop is laid out extremely easy, and the directions are clear. It is a good thing that I was working on the choke anyway, as this saved me a step.

Here is the wires to the in and out of the effects loop board, where I soldered it on to the pad where I lifted R14:



Again, I believe I placed the loop before R14. We will see how this goes....

Well it is late. I will bias the amp tomorrow if there isn't any issues and get it fired up. Hopefully I can finish the baffle and the piping as well.

[capota]

I don't think that anyone has the Custom 22 Schematics ))

who can help me to tame down the gain on the Lead channel?? I've Already changed the R6 from 220k to 110k (switchable) and changed the Slope resister (R26) from 47K to 33k (Marshall feel, also switchable). I want to make it a bit more Marshall flavor.

Also cut the C6 but it made it too much of a dark amp. What value for the C6 (Bright cap) do you guys recommend??

Thank You!!!



Here it is after the mods

[Shinigami75]

It looks very nice and tidy I think if it sounds too dark for you with the 470p at C6 you could try to experiment with different values such as 560p or 680p.

Also, but I may be wrong, lowering the slope resistor to 33k in Marshall style would increase the bass response and make it sound darker.

Moreover, a typical Marshall at input stage not only has 100K plate resistors but also 820 (2.7K at the bright channel input stage) cathod resistors where our Custom 22 has 1.8K (R3).

Try this:

http://www.duncanamps.com/tsc/

You can see how the amp would sound as you alter values in the tone stack.

I personally would like to try to use the same values I've got in my Mesa DC-3 such as a 500p cap instead of the 470p and the slope resistor at 82k

[Hipster Salad]

Not sure if anyone here is aware, but looks like the Earhart coming to full mass production

First prototype (official name yet to be determined, but I can confirm it will NOT be called the 'Earhart'):



Compared to my Earhart 50:





Likely coming out in the summer, and will most likely be LESS than the HDM.

I haven't seen schematics, but I've been talking with Doug about this for a few months (had to keep my lips sealed until he released the first photo!) and I can tell you it's the Earhart in mass produced form.

If you guys want to know what it's based on, it's laid out like the Cornford MK50HII, except the Jet City version only has 1 effects loop and not 2, and this one is only EL34 and not switchable to 6L6.

[Tortuga]

Wow - that sounds like killer news. That company really needs a shot in the arm.

[maynardo]

I don't think that anyone has the Custom 22 Schematics ))

Ho can help me to tame down the gain on the Lead channel?? I've Already changed the R6 from 220k to 110k (switchable) and changed the Slope resister (R26) from 47K to 33k (Marshall feel, also switchable). I want to make it a bit more Marshall flavor.

Also cut the C6 but it made it too much of a dark amp. What value for the C6 (Bright cap) do you guys recommend??

Thank You!!!



Here it is after the mods

Nice! Do you have the power section schematics?

About the gain question you have, if you don't want to change the tone too much, change R18 from 1M to 470k, or any value under the 1M really, the lower you go the more gain you drop .

You can also play with the cold clipper value (R16) , in this case is 15k. If you make it a little bit bigger you will loose some gain and make the the channel brighter. Don't go beyond 39k.

[Hipster Salad]

Wow - that sounds like killer news. That company really needs a shot in the arm.

A little changing as far as their catalog goes.

More than likely kill off the 50H (already discontinued the 100H) in favor for the HDM which is basically the exact same amp. The way I see it, you're competing with yourself if you have 3 of the same amps for sale.

Name redesign. Likely will follow suit of the new Custom 22. That prototype may very well be named Custom 45, but isn't final.

20 watt offerings paired down.

[capota]

It looks very nice and tidy I think if it sounds too dark for you with the 470p at C6 you could try to experiment with different values such as 560p or 680p.

Also, but I may be wrong, lowering the slope resistor to 33k in Marshall style would increase the bass response and make it sound darker.

Moreover, a typical Marshall at input stage not only has 100K plate resistors but also 820 (2.7K at the bright channel input stage) cathod resistors where our Custom 22 has 1.8K (R3).

Try this:

http://www.duncanamps.com/tsc/

You can see how the amp would sound as you alter values in the tone stack.

I personally would like to try to use the same values I've got in my Mesa DC-3 such as a 500p cap instead of the 470p and the slope resistor at 82k

It's too dark with C6 out of the way (I cut it out) that's why I asked for a different cap value that it's no so bright as with the 470p

The Mesa sounds great, but I'm after a more vintage marshall tone.

I'll try the cathode resistor at 820k

Thanks