JCA 2112 Mods

These amps are and with a few mods they sound fantastic. I started with the mods on valvesphere and then graduated my own to make it less flabby in the distortion. The only mods that are mine are the 100nF cathode bypass caps and the mesa tone stack the rest are from valvesphere.  I removed some of the valvsphere mods to suit my tastes.

Preamp MODs

C27, 100nF. Removed the 1uF and replaced it with a 100nF.
R28, 220k. Fit an additional 220K in parallel to make 110K.
C26, 20nF. Fit a 22nF 400V cap in parallel to make 42nF in total(optional).
R25, 2K2 fit a 100nF 25V cap  in parallel.
R53 1Meg. short with a spst switch
Gain Pot 1Meg Log. Fit a 62K or 68K resistor across the outer track connections.
R53 to Wiper of Gain pot. 1.8Meg and 120pF in series
C23, 470pF short with a spst switch.
R20 100k. Fit an additional 100K in parallel to make 50K
I also fitted a neon between pins 2 and 3 as protection(optional).

Tone Stack

Modified the tone stack ala mesa style

R21 changed to 100k
C8 changed to 250pF
C10 changed to 100nF
c9 changed to 47nF


Power Section Mods
R12 47k (NEG Feedback Resistor, Power Amp section) Fit 330pF in parallel to quash harshness.
added depth mod which is a 1MegA pot shorting a 4.7nF cap


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