How to modify KLD Bassman25 hand wired amplifier to British Plexi amp
Like Fender 5F6A, it is easy to convert KLD Bassman 25 to British Plexi series amps. If you want to build Plexi amp dirctly by our amp kits of Bassman 25 or after you assembled Bassman 25 amp kits successful, you want to modify it to Plexi style amp, You can reference below instruction.
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.
KLD 5F6A preamp MOD
Bassman25 Preamp V1,2 Plexi modification

Preamp and Channels Mod
The first preamp of two channels of Plexi used two individ cathode bias circuits. CH1 is Normal channel, its first preamp is V1B, R-1, 820 ohm and cathode bypass capacitor C-4 684 consisted its cathode bias circuit, which let tone louder and richer;CH2 is the Bright channel, which first preamp is V1A, still using original cathode bias circuit of 5F6A.
Changing the R5, R6( 270k ) mixing resistor of two channels to an 470k help the V2B second preamp stage to 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's modification included: C-1 500 pf capacitor paralleling with R5 is consisted of coupling circuit of the Bright channel. C22 could be changed from 100 pf(Plexi 67) , to 500 pf(Plexi, 68, 69,70) or 4700pf(Plexi 72), so you can have different high freq tone.
Paralleling cathode bypass capacitor C-2 into V2B cathode bias circuit, it will increase the gain of V2B and reduce the noise of amp.

Bassman25 Phaser Inverter Plexi modification

Phase Inverter Modification
The 68 , 69, 70, 72 of Plexi, the four types amp changed the C9 and C11 from 104 to 223. The two capacitors are coupling capacitors from LPT to power amp tube 6L6. The modification will couple more high Freq signal to final power amp stage.

Tips:This modifications of the amp are mainly focus on the preamp gain, frequency range. Because the differences between amps are based on the their special freq range. If you want to grasp electronic knowledge and working of tube guitar amp more deeply, so it is worth of investing more time and effort to try modify it .