Jet City JCA20 Mods?

[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]

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!

[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!

[hasserl]

I can answer *that* one.
Nobody said "reduce high end gain" but "towards removing the ice-picks." which is not the same.
Doubling that cap will leave highest frequencies exactly as before, but will rise mid and mid high frequencies by exactly 6dB.
Very noticeable and less shrill. Try it.(I have).
If you want a quicker check, use Duncan's TSC to simulate it, first with Treble and Bass max. , Mid min. , stock values; then same pot positions but using 1000pF instead of 470pF and you'll see what I mean.
Another side effect is that the midrange dip gets shallower, and sound becomes somewhat less buzzsaw.

"As far as mods go, the biggest complaint I have heard is too much hi-end gain and "frazz"."

[J M Fahey]

How does increasing by 2x the value of a treble cap reduce high end gain? 1000pf treble cap in a Marshall tone stack? Yikes!!! Talk about ice pick.

I can answer *that* one.
Nobody said "reduce high end gain" but "towards removing the ice-picks." which is not the same.
Doubling that cap will leave highest frequencies exactly as before, but will rise mid and mid high frequencies by exactly 6dB.
Very noticeable and less shrill. Try it.(I have).
If you want a quicker check, use Duncan's TSC to simulate it, first with Treble and Bass max. , Mid min. , stock values; then same pot positions but using 1000pF instead of 470pF and you'll see what I mean.
Another side effect is that the midrange dip gets shallower, and sound becomes somewhat less buzzsaw.

[hasserl]

We were Jet City's first service center, so I have the schematics and have played around with mods. As soon as I hear back from Jet City about schematic distribution policies, I will post it here.

As far as mods go, the biggest complaint I have heard is too much hi-end gain and "frazz". You can calm this down, reshape it and thicken the tone by adding a 100K resistor in series with C23 (which forms a HP filter in parallel with R32, and also replacing the 470pF treble cap in the tone stack with 1000pF (.001)@500V. Both of these mods go a long way toward removing the ice-picks. Also, a 47pF cap installed right on the V1 tube socket at Pin 7 (signal input) will reduce some of the garbage noise and increase stability a bit.

Oh yeah, get rid of the Chinese tubes!!!

The Jet City amps, like Soldano, are wide-open gain monsters with tones of high-frequency response that can be unruly at times, but they sound great and are built well. We stock parts for them if necessary. We have complete PCB's for the JCA20H if you want one to mod and keep one original. These are simple amps that readily leend themselves to modding. There's no rocket-science here.

How does increasing by 2x the value of a treble cap reduce high end gain? 1000pf treble cap in a Marshall tone stack? Yikes!!! Talk about ice pick.

I haven't seen the schematic of this amp, but viewing the schematic of the Atomic 16 and making an assumption that this is the same HP filter after the 2nd gain stage that is composed of a 1M resistor/470pf cap, how about just reducing the value of C23, or reduce the value of the resistor? Much simpler/cleaner than adding a resistor in series. Or eliminate it altogether for a nice full range increase in gain, if you like that kind of thing.

[J M Fahey]

another mod I am contemplating is wholesale replacement of the stock metal-film resistors with carbon-comps. Besides giving the amp extra "feel" (I know it will do this), the additional parasitic inductance of the carbon-comps will tame the high-end as well.

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).

[dlscott1973]

BTW- another mod I am contemplating is wholesale replacement of the stock metal-film resistors with carbon-comps. Besides giving the amp extra "feel" (I know it will do this), the additional parasitic inductance of the carbon-comps will tame the high-end as well. Ditto for the caps and tubular replacements. Mr. Soldano takes a hi-fi approach to amp design with component choice.

As with ANY guitar amp, you can realize full potential by springing for NOS tubes. Russian are WAY better than Chinese, but NOS will smoke all of 'em.

Again, this is an easy circuit to mod. While I cannot post a schematic in agreement with Jet City policy, I CAN tell you that my first schematic for this amp was one that I drew by hand in about a half-hour, just following the traces on the PCB, and also realizing that I was essentially looking at a Marshall preamp circuit with an extra gain stage and a few tweaks. The power amp circuit looks like any other.

John, do you offer a kit for the replacement of the metal film resistors and caps?

[dlscott1973]

We were Jet City's first service center, so I have the schematics and have played around with mods. As soon as I hear back from Jet City about schematic distribution policies, I will post it here.

As far as mods go, the biggest complaint I have heard is too much hi-end gain and "frazz". You can calm this down, reshape it and thicken the tone by adding a 100K resistor in series with C23 (which forms a HP filter in parallel with R32, and also replacing the 470pF treble cap in the tone stack with 1000pF (.001)@500V. Both of these mods go a long way toward removing the ice-picks. Also, a 47pF cap installed right on the V1 tube socket at Pin 7 (signal input) will reduce some of the garbage noise and increase stability a bit.

Oh yeah, get rid of the Chinese tubes!!!

The Jet City amps, like Soldano, are wide-open gain monsters with tones of high-frequency response that can be unruly at times, but they sound great and are built well. We stock parts for them if necessary. We have complete PCB's for the JCA20H if you want one to mod and keep one original. These are simple amps that readily leend themselves to modding. There's no rocket-science here.

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?

[jrfrond]

BTW- another mod I am contemplating is wholesale replacement of the stock metal-film resistors with carbon-comps. Besides giving the amp extra "feel" (I know it will do this), the additional parasitic inductance of the carbon-comps will tame the high-end as well. Ditto for the caps and tubular replacements. Mr. Soldano takes a hi-fi approach to amp design with component choice.

As with ANY guitar amp, you can realize full potential by springing for NOS tubes. Russian are WAY better than Chinese, but NOS will smoke all of 'em.

Again, this is an easy circuit to mod. While I cannot post a schematic in agreement with Jet City policy, I CAN tell you that my first schematic for this amp was one that I drew by hand in about a half-hour, just following the traces on the PCB, and also realizing that I was essentially looking at a Marshall preamp circuit with an extra gain stage and a few tweaks. The power amp circuit looks like any other.

[ledvedder]

We were Jet City's first service center, so I have the schematics and have played around with mods. As soon as I hear back from Jet City about schematic distribution policies, I will post it here.

As far as mods go, the biggest complaint I have heard is too much hi-end gain and "frazz". You can calm this down, reshape it and thicken the tone by adding a 100K resistor in series with C23 (which forms a HP filter in parallel with R32, and also replacing the 470pF treble cap in the tone stack with 1000pF (.001)@500V. Both of these mods go a long way toward removing the ice-picks. Also, a 47pF cap installed right on the V1 tube socket at Pin 7 (signal input) will reduce some of the garbage noise and increase stability a bit.

Oh yeah, get rid of the Chinese tubes!!!

The Jet City amps, like Soldano, are wide-open gain monsters with tones of high-frequency response that can be unruly at times, but they sound great and are built well. We stock parts for them if necessary. We have complete PCB's for the JCA20H if you want one to mod and keep one original. These are simple amps that readily leend themselves to modding. There's no rocket-science here.

Has anyone performed these component mods? I'd like to hear some feedback. I'm thinking about performing them, but I want to be sure the results are good before I go changing things.

[drfunkphd]

OK, so I tried some tube changes and here's the results:

First, I changed the EL84's to JJ's. Nothing special, they were reasonably priced and have been well reviews, so I figured I can't go too wrong. I considered some other makes, but a lot of talk of larger bottle sizes concerned me, although I have measured and they probably would have all fit, but maybe not left too much clearance. Maybe next time...

So first up, I tried an EH 12AY7 in V1, Svetlana 12AX7 in V2 and an unbranded 12AT7 (NOS) in V3. Wow, what a difference from the stock tubes. I'm sure the JJ EL84's made a part of that difference, but I really noticed the pre-amp tubes when I dropped the Master and cranked the Pre-amp. The tone was better when I started all the tone stack at 12:00 and it didn't take a lot of movement to get a good tone. Even the presence, which was like evil icepicks tearing into my eardrums at anything above 3 was smoother. I still wouldn't turn it up past 6, but at least it can be used. A lot more headroom and a much better clean tone when the preamp volume is down and the master is up. As to be expected, you lose a bit of the modern high-gain sound of the top of the gain dial, but turned up to 9, it was still plenty of gain and worthy of the Soldano name. Everything dimed (or nine'd, I guess...the controls max out at 9) and it gives you as much gain as you should need without putting a pedal in front of it.

Next, I tried subbing a JAN Phillips 5751 in V1 and kept everything else the same. I didn't expect much as I have found these tubes to be a bit sterile sounding when pushed. They sound great clean and in a lower gain setting (I have used these in a SF Bassman and on the clean channel of a Vox AC100 CPH) they are great, but require a little tone control tweaking. In this amp though, it really was hard to get it sounding right and there wasn't really any difference compared to having a 12AX7 in V1.

Next up, I put the 12 AY7 back in V1 and put the 5751 in the PI. This combo gives the amp a little more gain at the top of the preamp dial and a little extra bump when dimed (less one penny...).

So there you go...I might try some different combos, but I think this one is the winner. It doesn't break up too much until between 3-4 then it gets into plexi crunch between 4-6 1/2, then gets into a nice high-gain sound the rest of the way. Pushing the EL84's gets you a little more bite and unless I start playing some serious nu-metal, it's as much gain as I would need...and I can throw a pedal in front of it if it's not.

BTW, this tube combination also takes a couple decibels off the max volume...which should be fairly obvious given that V1's gain has been reduced by more than half and V3 by 20%. But if you're planning on using this amp for playing anything more than a small club show with a "quiet" drummer (although, I have never met one), losing a couple decibels is the least of your worries.

[drfunkphd]

Definitely a high gain little amp and easy to push the power tubes into overdrive.

I agree with John about the hi-end gain (or "frazz"...that ones new to me, but somehow it really fits). The other things that I am hoping to try to address is the overall tone stack balance and that painfully bright tone when the presence is turned up past about 3:00. It is a really mid-high focused amp, at least stock. I am going to be doing some tube substitutions in the next few days from the stock tubes and hopefully we will see if that addresses any issues. I am going to try some different 12AX7's, changing V1 to a 12AY7 and 5751, putting 5751's in V1 and V2, trying a 12AT7 in V3 and the power tubes are going to get changed too.

I will report back with any good or bad news on the tube subs before I crack anything open to start modding the board (because I really don't like working on PCB's...)

[defaced]

A mod that gets it sounding like a low power SLO would be pretty sweet though...

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.

[jrfrond]

We were Jet City's first service center, so I have the schematics and have played around with mods. As soon as I hear back from Jet City about schematic distribution policies, I will post it here.

As far as mods go, the biggest complaint I have heard is too much hi-end gain and "frazz". You can calm this down, reshape it and thicken the tone by adding a 100K resistor in series with C23 (which forms a HP filter in parallel with R32, and also replacing the 470pF treble cap in the tone stack with 1000pF (.001)@500V. Both of these mods go a long way toward removing the ice-picks. Also, a 47pF cap installed right on the V1 tube socket at Pin 7 (signal input) will reduce some of the garbage noise and increase stability a bit.

Oh yeah, get rid of the Chinese tubes!!!

The Jet City amps, like Soldano, are wide-open gain monsters with tones of high-frequency response that can be unruly at times, but they sound great and are built well. We stock parts for them if necessary. We have complete PCB's for the JCA20H if you want one to mod and keep one original. These are simple amps that readily leend themselves to modding. There's no rocket-science here.