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

[frankencat]

Hey all, just got a JCA50 combo yesterday. I have replaced tubes, rebiased, swapped the speaker etc. and it sounds very good. The only thing is that the eq of the two channels is too far off for me. Has anyone come across a mod that brings the channel eq's closer together? Thanks -Frank

[glepko]

RE the fizzle: The primary frequencies of nuisance for me are between 3-4k. A judicial cut with EQ in the loop reduces the fizz. However - opening up the volume does the same thing, much to my surprise. The low end grows into too much when letting the master section work - that's a good thing. Very pleasant surprise with that. Gorgeous with mid gain on crunch right through ripping high gain on overdrive channel.

There is an known problem with master volume controls that connect the wiper of the master volume pot to the input of the PA section. This is common on all the JCA amps. The master volume becomes a high pass filter at low settings on the master. This gives a thin, fizzy sound with the master set below 3-4. You open up the master and the low end jumps up. The fix is easy, one 100k resistor in series with the MV wiper restores the low end below 3-4 on the master. I find this far more effective than the standard "depth" mod to restore lows and it only takes one resistor.

Whoa - any potential drawbacks to this mod? Will it reduce the max volume potential of the amp if it's cranked? Should this be done to both masters (on the dual channel amps)?

With a 1M master and a 100k series resistance you lose 10% of the signal at full volume but by that time your power amp is well into clipping. Its all relative anyway like knobs that go to 11. if you need that extra 10% to push you over the cliff then you probably don't set your master to 2. JCA amps only go to 9 anyways If you have a 500k master try a 47k. I have used as high as 220k on some amps. If you routinely run the master above 5 this mod will not be noticeable. You only need to install one resistor at the PA input in series with the input cap for dual masters.

This is really cool. Definitely on my list of things to try next time I open the chassis.

I could use a serious hand holding on this fizz fix. Confident in soldering, confident in following directions and photo instruction recipes and such, heck I can even reason how and why this is going to work - but have no idea where the PA input (power amp input?) and input cap are looking at the schematics.

[jhall]

Somebody out by me is selling two new unopened box JCA 2212Cs
for $275.00 ea. Sounds like a good deal. I bought a barely used one from him a few months
ago for $350.00. Great amps. He has a high end audio business and gets promotional stuff all the time. He
had like five of these amps at one point.

[ironman28]

RE the fizzle: The primary frequencies of nuisance for me are between 3-4k. A judicial cut with EQ in the loop reduces the fizz. However - opening up the volume does the same thing, much to my surprise. The low end grows into too much when letting the master section work - that's a good thing. Very pleasant surprise with that. Gorgeous with mid gain on crunch right through ripping high gain on overdrive channel.

There is an known problem with master volume controls that connect the wiper of the master volume pot to the input of the PA section. This is common on all the JCA amps. The master volume becomes a high pass filter at low settings on the master. This gives a thin, fizzy sound with the master set below 3-4. You open up the master and the low end jumps up. The fix is easy, one 100k resistor in series with the MV wiper restores the low end below 3-4 on the master. I find this far more effective than the standard "depth" mod to restore lows and it only takes one resistor.

Whoa - any potential drawbacks to this mod? Will it reduce the max volume potential of the amp if it's cranked? Should this be done to both masters (on the dual channel amps)?

With a 1M master and a 100k series resistance you lose 10% of the signal at full volume but by that time your power amp is well into clipping. Its all relative anyway like knobs that go to 11. if you need that extra 10% to push you over the cliff then you probably don't set your master to 2. JCA amps only go to 9 anyways If you have a 500k master try a 47k. I have used as high as 220k on some amps. If you routinely run the master above 5 this mod will not be noticeable. You only need to install one resistor at the PA input in series with the input cap for dual masters.

This is really cool. Definitely on my list of things to try next time I open the chassis.

I could use a serious hand holding on this fizz fix. Confident in soldering, confident in following directions and photo instruction recipes and such, heck I can even reason how and why this is going to work - but have no idea where the PA input (power amp input?) and input cap are looking at the schematics.

Ok the following is based on the schematics here: viewtopic.php?f=53&t=12372
I am assuming that the 22h/2212 share the same component numbering. If not you will need to trace the circuit to find the correct components. If you have internal pics of the 2212 we can refer to that would help.
First off the 22H (and 2212?) have an improved FX loop that places the loop send and return after the MV controls. This is different than the 50/100 models where the master volume connects directly to the power amp input. If you look at the 22h output stage schematic you will see the "from preamp" going to C19. This is the power amp or phase inverter input. C19 couples the preamp signal to V5. Because the fx loop is between the master volumes and the input to the power amp this is not where the mod would be done on a 22h. The master volume circuit is on the preamp schematic. C17 couples the master volume wipers to the first half of V4 which drives the fx send. Unsolder the end of C17 opposite r27. Solder one end of your series resistor (47k to 100k) into the hole where the C17 lead was. Connect the free end of the resistor to the free end of C17. Solder the connection of the new resistor and C17 together.

Here is a picture of the mod done to a JCA50. The power amp input capacitor is C13 on the JCA50.

[63628100]

Hi all, I got a jca20h head and already make some mods on it, finally would like to install the slope resistor switch and choke mod, but I really get confused on which resistor on board is stand for the slope resistor, is it R20, R21 or R19, also, what values of resistor should be use?
Also, for the choke mod, can I use other than Hammond 156m as this one is expensive in my city as it must be shipped from other countries.

Thanks all

[Axe]

Ordered a JCA22H. It's scheduled to land in a week or so.

I've already ordered some white chicken head knobs but I went to the Amp Factory to see about ordering a white tolex baffle and they want $50 for shipping to Canada! So much for that idea.

[Elessar [Sly]]

Bought a JCA22H head the other day. Should arrive next week. First Jet City amp!

[Tortuga]

Welcome to the three of you! What are you planning to do with them? Tubes & bias?(!) Mods?

Ace - here's a couple of places to check for the knobs:

http://www.mammothelectronics.com/Searc ... icken+head

http://www.taydaelectronics.com/catalog ... ?q=chicken

Have you thought about re-tolexing the blue baffle?

[Tortuga]

Hi all, I got a jca20h head and already make some mods on it, finally would like to install the slope resistor switch and choke mod, but I really get confused on which resistor on board is stand for the slope resistor, is it R20, R21 or R19, also, what values of resistor should be use?
Also, for the choke mod, can I use other than Hammond 156m as this one is expensive in my city as it must be shipped from other countries.

Thanks all

Hiya - I just posted a documentation thread for the JCA20H, which includes schematics. There is a compilation of JCA-related links on the front page of this thread.

According to that schematic, the slope resistor is R21 and the stock value should be 47k. I put in a 33k (along with the original 47k that I can switch back and forth). I've also recall some advising a 39k. Again, I've posted links on the first page of this thread for easy reference.

...and here's my standard disclaimer. Make sure you know how to properly discharge your filter caps before you even THINK about opening the chassis. Hit the wrong thing inside and that shit can blow yer ass into next week. Not kidding - we want you to be around to tell us all the cool stuff you wind up doing with your amp

[Axe]

Welcome to the three of you! What are you planning to do with them? Tubes & bias?(!) Mods?

Ace - here's a couple of places to check for the knobs:

http://www.mammothelectronics.com/Searc ... icken+head

http://www.taydaelectronics.com/catalog ... ?q=chicken

Have you thought about re-tolexing the blue baffle?

thanks . I've already ordered some from Pedal Parts Plus. I was hoping to avoid retolexing the baffle but I'm not paying $50 for shipping . I might do white with black piping from Mojo Tone.

[63628100]

Thanks for the advice on the slope resistor, it really help, thanks a lot

[Elessar [Sly]]

I plan on using it how it arrives. I want something a bit different than my IRT's but can still compete with them when needed for recording. In vids the Jet City seems to have a different voiced clean sound, but with my playing and gear we'll have to see.

[Bengerm77]

I too have a Jet City amp. My JCA 100 sounds alright, but it hums like nobody's business. Frequently, I'll get tired of hearing it at practice and will switch it to standby when we're not playing just so I can talk. What gives?

[Elessar [Sly]]

I too have a Jet City amp. My JCA 100 sounds alright, but it hums like nobody's business. Frequently, I'll get tired of hearing it at practice and will switch it to standby when we're not playing just so I can talk. What gives?

The city full of jets living in your amp.

[marnold]

I too have a Jet City amp. My JCA 100 sounds alright, but it hums like nobody's business. Frequently, I'll get tired of hearing it at practice and will switch it to standby when we're not playing just so I can talk. What gives?

Does it do it on every outlet? I assume you tried swapping out guitar cables and guitars for that matter?

[63628100]

Can l ask again other than Hammond 156m for the choke mod, what choke can I use for that mod? Because I really can't find that one in my city and if I order from ebay it is expensive for shipping cost

[thesockmonster]

Two quick questions:

Are the 5012c's just noisy by design?

Are the loops on these extremely picky, or is it just me? Picked up a Vox Delaylab, and it seems to HATE it...overloads the signal.

[Tortuga]

Can l ask again other than Hammond 156m for the choke mod, what choke can I use for that mod? Because I really can't find that one in my city and if I order from ebay it is expensive for shipping cost

Wish I could help, but no experience with choke mod on the 20's - my 50 already has one.

Two quick questions:

Are the 5012c's just noisy by design?

Are the loops on these extremely picky, or is it just me? Picked up a Vox Delaylab, and it seems to HATE it...overloads the signal.

What kind of noise? Stock or upgraded tubes?

The JCA Loop design has been discussed in great detail. See first page of this thread. Bottom line is that the loop is designed for rack gear that can handle a line level input. Some pedals can, but many do not.

[thesockmonster]

What kind of noise? Stock or upgraded tubes?

The JCA Loop design has been discussed in great detail. See first page of this thread. Bottom line is that the loop is designed for rack gear that can handle a line level input. Some pedals can, but many do not.

Stock tubes. Bought it on a whim...thinking I should have gone with the 50h and an American 2x12.

[Elessar [Sly]]

Seems like the place to post this. Vid I did of my new JCA22H the other day - literally turned it on and started recording haha.

Featuring a Tubey McScreamer.

[video]https://www.youtube.com/watch?v=JUmqwCpAP4A[/video]

[Jay]

Some of, if not all of the hum seems to have been traced back to the preamp tube heaters from the stock PT. I have heard of some amps having the heaters to he preamp disconnected and a separate filament PT being used and changing the heater to DC with excellent, noiseless results. Seems like the hum is part of the beast in these amps.

[Tortuga]

Would love to get some detailed info on exactly how to do that - is it simply removing an existing lead from the PT & replacing it with another? Would I need to build up another rectifier / voltage regulator circuit and apply that, or is there an existing DC source that I can use?

Now that I'm finally able to crank the amp, I am definitely wanting to do something about the hiss & hum.

...and, by the way, welcome back, Jay thought we completely lost you to all the Marshall guys

[stump]

Just joined the site today after finding it yesterday. I spent yesterday modding my JCA20 head and went through a bunch of variations, finally culminating with the BE mod, no Sat switch yet. I'll post some pics as soon as I finish up a few things. As of now, I must say that this is killer tone. The only issue I have is oscillation when I turn the master volume up above 5 with the gain maxed. At that point the amp is WAY LOUD, much louder than it was stock on 10. Cutting the gain back gets rid of it. I still need to change the slope resistor to 33k and will probably replace most of the caps with either Mallory, Sozo or some no name caps that I got from my parts distributor, might be made by Illinois. These caps are dirt cheap and sound similar to the Mallory, but smoother. Gonna do a choke too. The first mod I did to this amp, when I got it a few years ago, was the Depth Mod. I put the pot in one of the 8 ohm speaker spots and feel that this is a must with these amps.

Another cool mod was converting it to Marshall 2204 specs, it sounded really, really close to my real 2204. I would recommend this mod to anyone looking to get that sound. It's just a few component changes and all soldering can be done from the top of the board with no traces being cut. I don't use the Presence control so I've done nothing to it at this point.

[63628100]

How to convert to 2204, any details that I could follow?

[stump]

Here are the 2204 conversion details:
I use 1 watt carbon film resistors and Polypropylene 630Volt capacitors.

Change the following caps and resistors.

C27- .68mF
R29- 2.7k
R28- 100k
R25- 10k
R53- 470k with a 500pF Silver Mica cap across it
R32- 470k
R33- 470k
R36- remove and install jumper
R43- 820 ohms
R21- 33k
R13- remove and install jumper
R14- remove and install jumper
C15- remove

These changes match the Marshall 2204 schematics and sounds very close to my 4104, which is the 212 combo version of this classic circuit. Good luck and please be sure to work safely. If you are unsure how to do so, please do not risk your life, ask or learn by any means necessary, other than getting zapped. Lol. If you have any questions, don't hesitate to ask.

With that being said, let me know how you like your amp with these changes in place. Also, I encourage you to make one change at a time to see what it does. It takes more time, but is a great way to learn, if you don't already know. Enjoy.