Jet City JCA20 mods

  • Thread starter Thread starter peterc52
  • Start date Start date
The schematic for this model is quite straightforward, and the boards are roomy and easy to work on. Looks like the jca20 is a single channel, 3 gain stages and dc coupled cathode follower. Could quite easily be modded to JCM800 preamp, and all the associated mods people do to JCM800’s would apply.

You can follow other people’s mods but I think the best chance at getting an amp tailored to what you want would be to learn what the various parts of the circuit do, and what changes will do to the sound.

Good idea also to look at schematics of other amps you like the sound of, to see what they did. Did they use large value plate load resistors or small? Cold biased (large value) or warm biased (small value) cathode resistor? (Plate load resistor also matters here). Bypass cap on cathode resistor gives a gain boost and value of the cap will change the cutoff frequency - big cap is wide range, small cap will only boost mids and highs for a tighter sound. Smaller coupling caps will cut bass between stages for a tighter sound. Voltage dividers between stages will dump signal to ground, bright caps over the series resistor in the voltage divider will let treble bleed though (like bright cap on a gain pot).

These calculators are very useful, put in the values from the schematic to see what they do, and change the values to what you want it to do. Then try in circuit to see if it does what you expect.
https://www.ampbooks.com/mobile/amplifier-calculators/
 

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As a reminder, if you’ve never modded an amp before then be warned there are high voltages in an amp, even after it’s turned off, that can kill you.
 
very cool. I am sending my 50 watt to epic next week to get a full blown SLO build including the transformer being replaced.
 
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