18Watt.com

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.

Nice job for sure.

The only comment I have is that you have an opportunity to perhaps do something more radical at the input. I always found that the hi vs lo input on a Marshall type amp to be kind of a waste of an input. I never use the low input since you can achieve the same thing by adjusting the volume control on the amp or on your pickup (depending on type).

The version of Trinity plexi that i built had a hi value cathode bypass cap on its normal channel which didn’t sound good with humbuckers. As a result i kind of ignored that channel and always used the TMB side. Only recently did I do the frequency half boost calcs and replace that cap with a much smaller value and wow .. a new day dawned.

You have an opportunity to do the same thing here since you have 2 sides to work with but you could always put a fat cap option on a switch on one side and could do something totally different on the other, possibly a different variant of tone stack, or even a cascade option.

You're right about the inputs. That type of circuit uses a second ECC83 so I abandoned the two channels and went with the single/parallel circuit of the original 18w normal channel.

I removed the corrective filter from the OT as this was WEM's early way of keeping the output linear (less guitary and later abandoned). I also added the diode protection to the EZ81 as per Ceriatone's circuit.

I tried the updated tone circuit but this was way too bright for my ears so reverted to the original.

Whilst digging around on this site I noted some interesting suggestions about PI cathode and tail values so incorporated them using rotary switches in redundant positions on the amp panel. This allows selectable PI/OP distortion and is very useful in practice.

I had the opportunity to road test the amp at the weekend and it sounded monstrous with my Les Paul in the Hi channel, volume and tone at 5, PI tail at 56k and cathode at 1k2. The new Lo channel now works well for cleaner tones.

I note your suggestions about bypass cap values. As I have one spare position on the amp panel* I can incorporate another switch so would be interested in any suggestions.

Updated circuit diagram attached.

* Amp panel L to R as follows:
Hi in, Vol, Tone, spare, Lo in, PI tail, PI cathode, On/Off