Jet City JCA20 Mods?

[Matt B]

Hi Billyz! I know this is an old thread, but is their any way you can send me the thumbnail pics of your mod? I can't see them on the thread for some reason. Thx!

[g1]

I think this is the schematic in question:

[J M Fahey]

This is an ***OOOOOLLLLDDDDD*** thread and John Frondelli hasnīt been seen here for years now, but if sombody here updates the schematic showing "pin 7" and on which tube, I can suggest what to do with a 47pF cap there.

Pin 7 on a 12A*7 is a grid, so in principle thereīs 2 basic possibilities:

a) grid to ground .... which I donīt think will do much,but letīs check the schematic first, and

b) grid to plate, which should cut top end.

Again, letīs see that schematic.

EDIT: many old files were lost when server crashed, so old images and PDF not opening are common.

[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

[Jack1962]

pdf files don't open

[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... )

[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 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]

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 Iīve already seen more than a few guys trying to mod $4.99 First Act MA104 amps

[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

[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

[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.

[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.

[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.

[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.