There are five types from 67 to 72 of British Plexi series amps. The changes of circuit are mainly in the first preamp stage and LTP phase inverter circuit.
Preamp and Channels Mod
The first preamp of the two channels of the Plexi uses two individual cathode bias circuits. CH1 is the Normal channel, with its first preamp stage being V1B. The cathode bias circuit consists of R1 (820 ohms) and the cathode bypass capacitor C4 (684pF), which helps make the tone louder and richer. CH2 is the Bright channel, with its first preamp stage being V1A, still using the original cathode bias circuit of the 5F6A.
Changing the R5, R6( 270k ) mixing resistors of two channels to an 470k helps the V2B second preamp stage handle overdrive from the previous two gain stages.
Another considering option is changing the Normal channel's (CH1)C3 0.022uF(223) coupling cap to a 0.0022uF (400v or higher) to reduce some bass tone to tighten up the overdrive.
The Bright channel modification includes a 500pF capacitor (C1) in parallel with R5, forming the coupling circuit of the Bright channel. C22 can be changed from 100pF (Plexi 67) to 500pF (Plexi 68, 69, 70) or 4700pF (Plexi 72), allowing you to achieve different high-frequency tones.
Paralleling the cathode bypass capacitor C2 with the V2B cathode bias circuit will increase the gain of V2B and reduce the amp's noise.
Phase Inverter Modification
The 68 , 69, 70, 72 of Plexi, the foThe 68, 69, 70, and 72 Plexi models have changed the C9 and C11 capacitors from 104 to 223. These two capacitors are coupling capacitors from the LTP to the power amp tube 6L6. This modification allows more high-frequency signals to pass to the final power amp stage.
These modifications to the amp mainly focus on preamp gain and frequency range, as the differences between amps are based on their unique frequency characteristics. If you want to deepen your understanding of electronics and the workings of tube guitar amps, it is worth investing more time and effort in modifying them.