Hi all, noob here. Several years ago I acquired a 1965 WEM Control ER15 amplifier head and found a modification online to convert one of the two channels of this clean amp to a 'Marshall 18w' design. It sounded much better for guitar, which later got me investigating more about the Marshall 18w amplifier.
Having recently found your site and studied the circuits in detail (specifically the original and Lite IIb), I eventually settled on modifying the amp as per the circuit below. In essence I have preserved the WEM's two channel operation with the following principals:
OT retaining the WEM corrective filter (Zobel Network) for more balanced tone.
Output biased for maximum power dissipation with the reduced B+1.
B+3 reduced minimally to run V1 and PI near to original value (280v DC).
PI cathode bias reduced to 470R to increase distortion for lower volume (home) playing. This could be switched to the normal 820R for stage use.
Channel input circuits as per typical Lo/Hi sensitivity single triode design.
To my ears, played through my 2 x 12 G12M Greenback cab this amp sounds phenomenal. However, being of limited capability playing guitar I can't wait to loan it out to hear it in anger in the hands of a decent player.
Anyway, I've attached the original and modded circuits and would welcome any observations and comments.
2-Channel WEMarshall 18w
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2-Channel WEMarshall 18w
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