3 October 2013

Jet City JCA22H Depth Mod

Decided to do a BUNCH of mods to the JCA22 all at once... probably a bad move if I want to know the result of each mod individually but I had a bunch of stuff kicking around from projects I never started and left-overs from other things I've built over the years... so while this thing is in bits I'm going to let loose on it and then see what happens.

Starting with the Depth Mod...

There is a lot of info out there on this mod on different forums so Google is your friend. The Depth or Resonance control is a great little gizmo for pumping up that low end up like a Sir Mix-a-lot music video. This is the way I have decided to do it.

I have added a 1uf capacitor in series with the speaker jacks and the pot. This will keep any DC gremlins from getting up in my pot and making scratchy noises that I don't want when I turn the dial. I mounted the extra cap on some tag strip screwed down under one of the PCB mounts. Check it!

I picked the 1uf because I had a bag of 1uf caps and I've done the same mod with that exact same value before in some other amps, so whatever. The wires to the jacks and to the pot will also be soldered to the tag strip.

Above is the pot. It's actually one of the original gain pots from the amp. I changed those pots for 500K's as part of other mods (which I will write about later) so I decided to recycle one here for the Depth. It also means that the PCB will JUST about squeeze back underneath this pot. A new bigger pot might not be the best fit here and would also be jammed right next to the Standby switch! I'd stick a big pot on the back.

The white wire will be going back to the PCB to feed what is known as the Negative Feedback Resistor. The NFB resistor limits the amount of negative feedback the amp is getting. In this amp it is 47K (If I remember correctly while typing this!). NFB is also variable at different impedance taps on the output transformer secondary, so using the 8ohm or 16ohm output should get you different results. I intend to try both and see which I like best but 8ohm is normally where the NFB comes from in this amp. Using the 16ohm output will increase the amount of NFB going back to the amp.

The Presence and Depth controls both function using Negative Feedback. In this situation the NFB is a signal from the output of your amp, which is fed back into the amp at the Phase Inverter (PI). This signal is actually 'out of phase' with what is going on at the PI (That is why it is called negative and not positive!) so when out of phase frequencies in the NFB meet their counterparts at the PI they tend to subdue each other.

When you turn your Presence control up you actually remove high frequencies from the NFB signal. The means there are now less out of phase high frequencies in the NFB to damp down those same frequencies in the PI. The effect is a perceived high frequency boost. The Depth Control works the same way, but with the low frequencies. Higher frequencies will pass through the 4700pf cap and skip the pot, but the pot will impede lower frequencies more as it is turned up. Thus reducing damping on the low end.

All this also depends on how loud your amp is. The louder the amp the more signal you get at the speaker jacks. Hence the stronger an NFB signal you will get. This is the reason why Depth and Presence (or NFB based controls in general) will not seem to work at low volumes until at higher points on the dial. Once you start to crank the amp up and the NFB is stronger, these controls will become more effective at earlier points on the dials. So when you start gigging and you take your amp out, be sure to reevaluate your use of those dials depending on the volume difference between gig level and practice level. Cranking the Presence at high volume could take peoples heads off with treble and you might sound muddy with a cranked Depth pot!


  1. At what points are you measuring bias on this amp? I've done a few mods to mine and swapped for some Sovtek power & JJ preamp tubes, but I have yet to set the bias.

  2. I usually calculate bias using the plate current method. I measure the resistance at points of the output transformers and the voltage across the points to calculate current drawn at the plate. It's a dangerous method as it means probing some of the highest voltages in the amp. In this amp however I did add 1ohm resistors between the cathode of the EL84's and ground. The voltage drop across those resistors ends up being equal to the current drawn by the plate and screen. It's quicker and less dangerous to measure but less accurate. It also requires breaking traces on the PCB.

    If you aren't sure how to bias the amp, definitely take the amp to a tech. It's not a big task for tech to do.

  3. Do the Depth and Choke mods line up to the 20H as well?

  4. I can't say any component indicators but the circuits themselves would be the same principle.