Jet City JCA20HV customizing

My setup changed a bit recently. I had been playing a small Bugera V22 combo for two years, but I wanted a bit more low end and I only needed one channel, because all distortion is basically coming from my pedalboard. People have been talking a lot about Jet City amps, which are designed by well known amp designer Mike Soldano. That’s a nice reference, so I checked the Jet City product line and went for a Jet City JCA20HV. This is a single channel 20 Watt all tube amp. It is not so well known as the JCA20H (single channel either) and the JCA22H (two channels).

The JCA20HV is a really nice amp. Built quality is ok, though not perfect. If you open up the amp, you see some improvised layouts and wirings. From what I’ve seen so far, the circuit board is basically a JCA20H, with some modifications concerning the preamp. A serial FX loop has been added (FX return BEFORE Master volume, which really makes sense to me), as well as a depth switch, which adds some fixed low end boost (more on that later). The soldering of some wires could have been done more accurate, but all in all it’s abosultely acceptable. I have the overall impression though, that this amp model has not been designed as a stand-alone product line, but rather as a small “hand-made” (see above: “improvised”) variant of the JCA20H.

The sound of the amp is rather Marshall-like, but not too dark. Jet City says they’ve made it a bit darker sounding, but I can’t really confirm. It is definitely not Fender-clean, but not dark either. I play it into two closed 1×12 cabs, and the amp is a perfect match for my distortion pedals (OCD- and Crunch-Box copies). The preamp breaks up nicely when you turn the gain to about “5”, depending on the guitar/pickups. You can get some crunch when you max the gain, but no high gain sounds at all. Did I say that the amp can get very loud?

So why mod this amp?

  • The fixed depth switch is not effective enough (or maybe only if you have a really bassy sounding cab/speaker)
  • Master volume and FX return are far too sensitive; even when playing with the band I can only turn up the master to “2” – within this small range you cannot really fine tune the volume
  • I decided to add a choke to improve the power supply
  • Preamp tubes are cheap chinese specimen, so try other tubes as replacements
  • It has been designed by Soldano, it sounds like a Soldano – well, then make it look like one…

1. Make depth switch level adjustable

Many owners of the variant JCA amps have added the so-called depth mod. This puts a capacitor in series with the feedback signal from the output stage. By adding this cap the feedback of low frequencies into the driver stage is reduced and the low end gets a boost. By adding a pot in parallel to the cap, the amount of boost is made adjustable. Often recommended values are .0047 μF and 1 MΩ. the JCA20HV has a fixed depth switch with presumably .0047 μF (small cap, could not read labeling) and with 34KΩ in parallel. The boost was audible, but far from sufficient for my needs. The sound was a bit too thin for my taste, especially with distorted sounds – I definitely needed some more low end. Maybe this also relates to the compact closed cabs connected to the amp.

I kept the capacitor, but added a 500KΩ pot in parallel. I placed the pot at the back of the amp, the depth switch (on/off) ist still functional. I generally turn up the pot to about 50%, so 1MΩ would have been to much low end boost for me. The sound gets muddy then.

2. Reduce FX return level and level into master volume

As described above, the master volume is much too sensitive, I can only turn it up to about “2” (no need for “11” here!). If you max out preamp gain, you can’t really handle it anymore. So I added a pot (50KΩ) after the FX loop (see schematic). The level that goes into the master volume pot is now adjustable. the pot is not directly connected to ground, but with a 20K resistor, so that you cannot turn down the volume completely. I placed the pot on the back of the amp also. Master volume is now much more better to handle.

3. Add a choke

See this article for a brief explanation of a choke’s function in a power supply. I had a recognizable hum (though not really problematical) from the power amp before adding the choke. The amp is quiet now. The choke has some more advantages, as lowering the sag when driving the amp hard (you may watch the Youtube series where Mike Soldano himself adds a choke to a Jet City amp); especially in conjunction with the extended depth range, this makes sense to me. Adding the choke is easy, it is more mechanical than electrical work. The chokes wires were a bit too short, so I had to drill a dedicated hole to pass through the wires. I opted for a Hammond 156L choke (5H / 75mA). You can spot the choke on picture #3 on the left side next to the power transformer.

4. Change preamp tubes

Easy. Pull old tubes, put in new tubes. I chose Electro Harmonix EH 12AX7. I did not do extensive comparsions – the amp sounds nice, and putting in new tubes always makes you feel good… 🙂

5. Change face plate

Massive change in sound. Not! But looks nice, at least to me. Jet City say the JCA20HV is a bit more vintage sounding (it isn’t, which is good!), and maybe the wanted it to look a bit vintage, but I like a more modern look. And, you can now see the tubes glow, especially the power tubes.