Here are the mods I did to my JCA20

  • Thread starter Thread starter psychodave
  • Start date Start date
One of the head guys from JCA had the transformers for sale on FB so I bought two after reading some really positive reviews. I preferred the sound over the original transformer, but the original wasn’t bad.
 
psychodave":2w34sye3 said:
One of the head guys from JCA had the transformers for sale on FB so I bought two after reading some really positive reviews. I preferred the sound over the original transformer, but the original wasn’t bad.
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Yeah, I remember them having a few upgrade transformer options. I agree listening with stock transformer, it sounds good to me. Mercury Magnetics has some, but of course they are WAYYY overpriced as usual.
 
scottosan":3oy9fgyo said:
FourT6and2":3oy9fgyo said:
I havne't double-checked the photos, but are you sure that 68k/68k divider is in the right order? Shouldn't the 68k and cap to ground be before the 68k going to the grid and not after it?
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No that would be an incorrectly implemented voltage divider and equate to and and act as a grid stopper.



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Is the 500k in the image on the left even a grid stopper? The way I see it, left image is a voltage divider (no grid stopper) and is what a 1M pot would look like turned up to 50%. Right image is a 500k grid stopper and I'm not sure what you call the 500k to ground (both 500k in series is the grid leak resistance on right). The AC load seen by the previous stage is 1M in one and 500k in the other (in parallel with plate resistor and tube plate resistance IIRC). 500k grid stopper like in right image will chop off highs and the lower AC load on previous stage will slightly reduce lows compared to voltage divider on left.
 
Shask said:
Very cool. Is this a Jose inspired Mod?

I plan on modding my JCA100HDM one day. I am first gonna fix that crap of a loop. Not sure about actual tone mods. I thought I might go small to start, like reducing some coupling caps to tighten up the low end, and a snubber cap to reduce the fizz. Go from there. I keep putting it off because I am not looking forward to pulling the PCB out, lol.
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Hey just saw you have a 100hdm, I did a ton of mods on mine and I have to mention if you haven’t seen it there is a channel bleed issue with the Vactrols for the channel switching circuit and can be easily fixed. Not sure if you need it but this was a major problem and made the amp sound horrible on the OD channel almost like a crappy fuzz pedal was turned on. Glad to see another HDM owner in here! Let me know if you ever want to sell it, cheers!
 
CornNutz said:
Alright here's the preamp schemo, its missing the bright switch, but that easy to figure out. If anyone can solve the r36 mystery let us know.

UPDATED
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I know this whole thing is pretty old but if possible I was wondering if you might have an idea why the electrolytic input cap would have been bypassed? They have them on the inputs and fx return jacks of all the jet city amps including mine and I was just curious if there’s a tonal benefit to removing them? Thanks!
 
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