Greg Robinson wrote:If anyone want's me to explain anything in more detail, or to give a bit of theory of operation, please let me know! I'm happy to share.
increase the anode resistor of the first valve stage close to its limit, use, say, 420 kOhms in order to maximize clean gain of that stage.
decrease it a little bit into the range of 68-82 kOhms in the 2nd stage.
add a switchable 2.2 kOhm resistor in parallell to the cathode resistor of valve stage 2, yielding a cathode resistor of 820 Ohms as found in some Marshall designs.
I did all that in my Epiphone Valve Junior and can overdrive its 2nd stage evene despite the loss of 7 dB of the PI-tone filter after stage 1.
I would also suggest to reduce the values of the coupling capacitors to a lower corner frequency of 70 Hz. You do not need more, even if You play bass over such a small amp (and in the latter case can even run it with more power in the musically important frequency range)
By reducing the kathode resistor, for example, the signal will be amplified slightly more asymmetrical and thus even order harmonics will be increased.
Analogously the value of the anode resistor has a really noticeable effect on nonlinear distortion - it increases if You lower the anode resistor and it decreases if You raise its value. Because the effective (dynamic) anode resistor is always lower than the static one the gain will also be lower - You might expect something between 55-60 für Ra=100k.
Reducing its value will lead to a somewhat more "compressed" tone which is, admittedly, more suitable for bass amps.
With the 1.5k / 100k default (actually a tradition; the optimum for linearity is at somewhat larger values; the german tradition has been 2.2k / 100k) You will always obtain a "fenderish" tone. This is, of course, not the worst to target, but there are other possibilities, and these are applied in actual practise to control the voicing.
Greg Robinson wrote:
Ok, guess I'd best go get some photos and soundclips organised!
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