Jet City JCA20 Mods?

[dlscott1973]

JCA20H mods

When I recieved my JCA20h the first thing I noticed was that It was a really nice amp for the price. The aquality of materials and craftmanship really appeared to be top notch at first, but while I was playing around with the settings I found that the amp would go into ocillation when at high volume with the presence cranked. I contacted JCA, and they were more than happy to issue me a RMA to get the problem solved. Well I sat and thought about it, and for the price of shipping I could just replace all of the components on the board, and perhaps have a better amp in the long run, so I decided to figure it out on my own. Drew up a list of component values and placed my order. I didn't really care for what I saw once the amp was apart and I could really see how the assembly was put together. I found that I had a bunch of cold solder points, and some poorly made connections. Wire terminations to the board looked as though they had not been pretinned, and were not the cleanest. I removed each connection point, retrimmed the wire, tinned, and resoldered. I inspected the pots, and found that they weren't of the greatest quality, so I replaced each pot with a nice Alpha, and swapped all the poly caps with Sprague Orange Drops, replaced some of the resistors in the signal path with Carbon Comp resistors, and added a 3H choke. Threw it back together and dropped my JJ's back into it. I let it burn in for a bit before I plugged in, but once I did I was astonished! The difference in sound was thirilling. It went from a nice sounding amp to a great sounding amp. The addition of quality components and removing the cold solder points just really brouight the amp to life. The difference in sound quality is like night and day. I have since replaced some more resisters and changed a couple of cap values with nice results. I am integrating reverb this weekend, and just ordered up a Mercury Magnetics Axiom output tranny. The transformer cost more than the whole amp did, but when I heard the tone of this monster I just couldn't resist bringing it to full potential. I am really looking forward to that mod! Oh yeah, also the ocillation (squealling) is now gone. I can crank it to full max with the presance at full bore now with no excessive noise, or feedback. What a GREAT amp!

[jrfrond]

Is it a typo?.
Carbon composition resistors, being a straight "stick" of graphite+clay paste have less inductance than others.
And (big) noise and stability problems make them foes of high gain preamplifiers.
Maybe you meant carbon-deposited resistors?
The most common today, made out of a ceramic body where a thin layer of graphite is deposited, then cut in a spiral groove as a final adjustment .
That "spiral" can become inductive,it does have 2 to 4 turns, although the effect is minimum at audio frequencies; even more so compared to the resistive part of their impedance.
Scratch the paint in a regular resistor and you'll clearly see that spiral.
Can't talk about the "feel", being an undefined entity, although I very much doubt anybody can discern with any certainty between both "sounds" (leaving hiss and popcorn noise aside).

No, you read it right. The thicker leads of the carbon comps themselves lends the circuit more parasitic inductance. The response of the resistors, which has a slower rise time and tends to soften transients, are what give the amp better feel vs. carbon or worse, metal film. It's a subtle tonal thing, but really more of a FEEL thing.

Of course I am aware of thermal and shot noise that can happen with CC resistors. It is, after all, common knowledge. But I've taken the time to mod my own JCA20 with CC resistors, and it actually tamed the amp a bit, not to mention that I've already performed some noise reduction mods.

Yes, I should have worded the treble cap mod different by stating that you gain more emphasis on the mids in that respect. Oops!

Having said all that, I am not a big fan of high-gain amp designs like this. Cascaded stages always mean noise, which I am ALSO not a fan of!

[J M Fahey]

Well, coming from you I'll have to accept your point of view.
Anyway, out of the top of my head, I would imagine that the inductance of a couple inches straight wire would be noticed above, say, 50 or 100MHz.
I'm not exactly getting younger and my hearing doesn't reach such heights anymore.
As a side note, I didn't imagine you playing High Gain Metal.
A nice surprise every day
Now we only need enzo chiming in to talk about the skull he just had tatooed in his biceps, just above the image of his nitromethane supercharged Harley 1947.

[jrfrond]

Well, coming from you I'll have to accept your point of view.
Anyway, out of the top of my head, I would imagine that the inductance of a couple inches straight wire would be noticed above, say, 50 or 100MHz.
I'm not exactly getting younger and my hearing doesn't reach such heights anymore.
As a side note, I didn't imagine you playing High Gain Metal.
A nice surprise every day
Now we only need enzo chiming in to talk about the skull he just had tatooed in his biceps, just above the image of his nitromethane supercharged Harley 1947.

Actually, high-gain metal is definitely NOT my thing. I'm a Classic Rock guy myself, though being born in '59, it was just called "rock". No it's Classic. I guess I'm classic too!

I have a Jet City because my 15 year-old son demo'ed/endorsed them at Winter NAMM when they came out. He's not a metal-head either, so I modded the amp more to his liking. However, his main amp is still one of my own designs/builds. Fatherhood is good!

Seriously, I don't get overly technical with this stuff, even if it sounds like it occasionally, 'cuz it ain't rocket-science. I'd rather be more like a Paul Prudhomme than Food Network's Alton Brown when it comes to amp mods and design. After 30+ years, I have my recipes and know what works and what doesn't. Empirical design is a valid art form. Software, books and numbers tend to sterilize the process, somewhat analogous to reading Masters & Johnson for sex tips.

[Modern Saint]

The difference in sound quality is like night and day. I have since replaced some more resisters and changed a couple of cap values with nice results. I am integrating reverb this weekend, and just ordered up a Mercury Magnetics Axiom output tranny. The transformer cost more than the whole amp did, but when I heard the tone of this monster I just couldn't resist bringing it to full potential. I am really looking forward to that mod! Oh yeah, also the ocillation (squealling) is now gone. I can crank it to full max with the presance at full bore now with no excessive noise, or feedback.

I would love to hear your feedback when you swap out the OT in your unit.

[diagrammatiks]

Assuming it's the same as the Atomic 16... It's missing a stage, so I don't know how close you'll be able to get it to sounding like the SLO. I imagine you can get the voicing easily enough, but I don't know if you'll be able to get similar gain levels. But you can see the similarities between the two amps big time.

it's actually missing two stages.

I wouldn't change the ot or the pt in the amp. They are actually pretty decent units.

Easiest thing to do given the price of the amp is to rip out the board and start over.

The OT and PT will easily handle 6v6s and another 12ax7. 6v6s just because I like them better then el84s.

However, the amp will handle another 12ax7 easy. From there it's easy to do the slo's crunch channel or the low input of the avenger.

[billyz]

Here is my mod for the Jet City 20. Makes it a much more useable amp all around. Still distorts well but much fatter and does clean up. That 220k plate resistor on V1 makes it very difficult to get any clean. The parallel V1a/b really fattens the tone and lower noise.

[tankonedog]

jet city jca20 add a reverb?

There was a mention of adding a reverb to this amp. Any luck/ info or any more info on other mods?

peace

[Steve A.]

Over at TGP in the long thread on mods for the Crate V18 (800+ posts the last time I checked) one of the main contributors, scihibmxer@juno, mentions the XTC preamp mods that his friend Rock Mumbles did to his JCA20. Anybody interested in mods for the JCA20 might want to send a PM to him.

Crate V18: Mods and Conversions - Page 53 - The Gear Page

Steve Ahola

[J M Fahey]

800 + posts? Wow !
The apparition of $99 tube amps causes much excitement among would-be modders.
Fine with me, let them enjoy the solder fumes experience too.
It might be worse: on other Forums I´ve already seen more than a few guys trying to mod $4.99 First Act MA104 amps

[Steve Conner]

That's actually quite a good idea for newbies wanting to get into DIY electronics. If you destroy it, you're only out $4.99.

[J M Fahey]

Agree.
I fact, I tell tem to buy 4 or 5, as many as possible, so they can get as wild as they want.
Worst case, it provides a ready made and working power amp, and a simple chassis, great for taping a protoboard inside and test building *all* fuzz or whatever pedals they fancy.
It works with a 12 to 16V wall wart, which can easily supply regulated 9V, great for 99% of pedals.
The original nes had a TDA2003; newer ones seem to have a TDA2030 , great testbed.

*I* would buy a dozen at that price.

[Steve A.]

The Crate V18 had been selling for about $329 at GC when they first came out in ~2005. They really sounded like crap and some of them were exploding on stage. In any case MF was having so much trouble with people returning these amps that they were clearing them out for under $150 and were going to drop Crate completely. I figured it would be fun to play around with one and see what I could come up with. After looking at the schematic (which had many errors that I corrected) I decided that it needed a lot more than substituting a few parts- it was in need of major surgery.

I decided to reconfigure the printed circuit board to recreate several different really cool amps (SLO, ODS, XTC and some Dr Z and Trainwreck amps) . The most popular one over at TGP was my Soldano SLO mod which was fairly easy because the architectures were very similar. Those amps really rocked, and if you used lower gain preamp tubes it had some really good clean sounds, too. I kept buying up more V18's to rewire as other amps- one of them that worked really well for me was the Trainwreck Rocket mod, not for the faint-hearted because it did require cutting some traces on the pcb. My last redesign project was the Dumble ODS, with an added channel switching relay and extensive cutting on both sides of the PCB. While trying to decide whether to convert my last V18 to the ODS or the Trainwreck Liverpool I dropped something on the printed circuit board and broke off the pots, said forget it and moved on to my next OCD target. That was 3 years ago and I haven't looked back except to answer questions about those mods. This link goes to the color-coded drawings which showed which parts to remove, which parts to replace and which parts to add:

Index of /new/schem/misc_mod

For me it was more of an intellectual exercise, figuring out how to reconfigure an existing printed circuit board for a variety of different circuits with different architectures. IMO a lot more interesting and rewarding than Sudoku.

Steve Ahola

P.S. I guess I still have that in my bloodstream- right now I am almost finished rewiring my JCA PicoValve as a 5 watt Trainwreck Rocket. (GC was blowing out the PicoValves last summer- for $179 or less. I guess I am a sucker for sale prices... )

[Jack1962]

pdf files don't open

[Valvehead]

John, Can you elaborate on the placement of the 47pf capacitor that you are placing on pin 7? Do you break the connection to pin 7 and solder the cap in as a bridge?

i have that same question