There have been a few threads on how to get a full high gain no-holds-barred design from the Jet City JCA20H amplifier.
I find this topology does a good job of going clean-ish when the volume from the guitar is backed off nearly all the way but has more than enough overdrive to use with my strat. I have a bleed cap fitted on the guitars volume BTW.
Compared with the original first three gain stages setup of x69 x23.6 x59 the individual gains are now x55.7 x47.6 and x54.6 so no valve is pushed too hard to spec. What has happened is a attenuation between stages has been removed and the treble voicing moved to after the tone stack. If we had infinite supply voltage and valves that didn't arc then the original cascaded gain to the start of the tone stack was around a factor of 12.5 thousand, with the modifications it would be around 118 thousand.
The mods are presented as achievable without cutting tracks or removing the PCB from the chassis. Simple to try and simple to put back. From the official schematic here are the changes made.
C27, 1uF. Fit an additional 1uF in parallel to make it 2uF.
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.
R25, 2K2 fit a 1uF 25V cap in parallel.
R53 1Meg. Sort circuit this part
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 (was100k and 1nF)
C23, 470pF Short circuit this part.
R14 470K Wire 220pF and 220K in series to restore some of the treble lost shorting out C23.
R12 47k (NEG Feedback Resistor, Power Amp section) Fit 330pF in parallel to quash harshness.
And two mods from The Valve Wizard Pages
R20 100k. Fit an additional 100K in parallel to make 50K
I also fitted a neon between pins 2 and 3 as protection.
The amp also has a depth or resonance modification fitted which warms the amp in the correct place.It also has a triode switch which mellows the tone and an unbalanced phase splitter to allow 2nd Harmonic distortion in the output stage.I'll detail the output stage modifications in another post.
I found that the treble bleed at the gain pot helped establish a usable clean sound lifting the mid to treble but not the bass which often pushes the stage into grit. For now I have fitted R-3 as 100k and C-3 as 1nF. However this may change as it is still work in progress. (UPDATE: Fitted R-3 = 1.8Meg and C-3 = 120pF as final value)
Below is the response at the cathode follower with the gain pot set to about 2. The lowest plot is without a bleed modification. The green upper plot is 100k and 1nF which gives a 12dB lift beginning at the lower mid frequency. The intermediate red trace is 100k and 220pf.