To get a little more gain out of this JCM800 style 18 watt amp, what would you guys suggest ?
Thank you very much
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JCM800 18W - More gain ?
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RyanBianchi
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Re: JCM800 18W - More gain ?
Add a .68uf cathode bypass cap across the 820ohm cathode resistor on V2a. Can try other values too, fully bypass it with 22uF if you like the sound of it, but that's where I'd start.
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Crabman
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Re: JCM800 18W - More gain ?
Take a look at what Headfirst amps are doing with mods to plexis and 800's. Jason Tong has many videos on hot rodding these circuits with plenty of videos and also support documentation:
-> headfirstamps.com
-> select PCBs and Mods
-> scroll to bottom and select MODS SHARE
There are a number of schematics there that demo mods to existing plexi/800 type circuits. Most of these mods are not highly invasive and intended to leave the amp in a stock format so that things can be reversed easily.
If you want to keep it at 3 gain stages plus CF, then you can look at the Jake E Lee (JEL) schematic. This converts the 800 second stage cold clipper to a 2.7K/0.68uF combo at the cathode. This means that some signal from the first stage needs to be dumped through a 220K resistor in parallel with the 1M gain pot. The 3rd stage has a 22uF fat cap on the 820 ohm cathode resistor (the so called EVH mod) and the 100K cathode resistor on the CF has a cap on that as well (watch his videos for explanation .. this approach is used on many Friedman circuits). Also, a depth control on the power section.
If you want a more extreme approach then look at his videos that discuss adding a gain stage at the front end.
I'm not sure how well these mods will mesh with an EL84 power section though. You may have to experiment and perhaps restrict signal at various points if it's too much.
-> headfirstamps.com
-> select PCBs and Mods
-> scroll to bottom and select MODS SHARE
There are a number of schematics there that demo mods to existing plexi/800 type circuits. Most of these mods are not highly invasive and intended to leave the amp in a stock format so that things can be reversed easily.
If you want to keep it at 3 gain stages plus CF, then you can look at the Jake E Lee (JEL) schematic. This converts the 800 second stage cold clipper to a 2.7K/0.68uF combo at the cathode. This means that some signal from the first stage needs to be dumped through a 220K resistor in parallel with the 1M gain pot. The 3rd stage has a 22uF fat cap on the 820 ohm cathode resistor (the so called EVH mod) and the 100K cathode resistor on the CF has a cap on that as well (watch his videos for explanation .. this approach is used on many Friedman circuits). Also, a depth control on the power section.
If you want a more extreme approach then look at his videos that discuss adding a gain stage at the front end.
I'm not sure how well these mods will mesh with an EL84 power section though. You may have to experiment and perhaps restrict signal at various points if it's too much.
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