Below are some modifications you might choose to implement in order to change the tone and response of your 18:
1. On the 2 cathodes of V1, the R-C filter is set to 1uF X 820 ohms. This will roll the bass off slightly. Increase the capacitance up to 25uF to increase bass response of the front-end pre amplifier. Try the following combinations for the pair for the TMB channel connected to V1a.
2.7K/68uF; 1.5K/25uf; 2.2K/2.2uf;
The lower the capacitor value, the less bass response.
2. TMB channel input impedance
To change TMB channel input impedance, replace the 1M resistor with 820K; 560K or 470K resistors. This may be necessary for particular instruments.
3. V3 Bias – Dirty Sound
Change the 56K cathode resistor on V3 to 47K. This changes the bias, and operates the tube in a different part of its operating curve and it will not sound as clean.
4. Master Volume Range
To change the range of the Master Volume, reduce the MV potentiometer from 1M Log/Audio, to 500K.
5. Increased Treble Control
For modified Treble control, try the following modification to the R-C filter around the Treble control potentiometer.
33K /250pf– more gain and treble; 47K/330pf - middle of the road; 56K/500pf – bassier slope, less gain
6. Gain and Biasing of the Inverter
To change the gain and biasing of the inverting stage, reduce the resistors in the chain to the following:
Change the two 100K resistor to 82K or 68K (add a 100pf cap between the plate resistors). Change the 470K resistors to 330K or 220K
7. Low Frequency Response of the Output Stage
Output tubes, cathode bypass filter: The original Marshall 18Ws used 500uF, and the Watkins Dominators they derived from used 50uF, there is no right value.
Larger values will give more bass response. What is used depends on the type of speakers, cab, OT and even guitar. The -3db low frequency cutoff point is approximately 1/(2 x pi x R x C), so for example, 125 ohms and 50uF has a lower cutoff of 25 Hz.
Try either a 50 or 100uF. Some people find that lower values keep the low end from getting muddy or boomy.
This is not like a conventional high pass filter in that there is still some gain/band pass at Low Frequencies. Instead of a continuous cutoff there is a step in the gain response, lower gain below the crossover point, and higher above.
8. Bass and Midrange Control Filter If you want to modify the frequency of the control, change the 22n capacitors. Try 10n for the Bass and 47n for the Middle.
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