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Mod ManiaEven if your kit’s documentation doesn’t cover mods, you can still research and incorporate them. Here are a few popular mods for 18-watt amps.
The EF86 mod. You can substitute an EF86 preamp tubes for one of the 18-watt’s 12AX7/ECC83 tubes. It’s not a direct swap—the EF86 is a pentode, not a triode, so it requires a different socket and different wiring. The EF86 appeared in some early Vox models and in their boutique descendants. Seeking something fresh, I tried the EF86 a few years ago in my first 18-watt build, Ceriatone’s 18W TMB EF86 kit. It’s a fine kit that turned out well, and the EF86 did indeed lend an effervescent, Vox-like quality. But I wouldn't wish that godforsaken tube on my worst enemy.
I should have believed what I read online: the EF86 is almost comically microphonic. I spent a small fortune on replacement tubes and tube-damping gizmos to get a remotely studio-worthy amp, and it’s still too noisy for many applications. (An EF86 might work better in a head-only model, where it’s not inches from a loud, vibrating speaker.) Proceed with caution.
The TMB mod. The original 18-watt amps had only a single tone control per channel: a low-pass filter to roll off highs. One popular option is to replace the single-band tone control with treble/middle/bass (TMB) controls voiced like those on the larger Marshall amps of the ’60s. I went that route on my Ceriatone build, and the tone section sounded fine—but did I need the extra controls? Probably not. While the ability to prune highs is essential, the 18-watt doesn’t exactly suffer from an excess of bass, so I found little reason to manipulate the lows. The mids tend to sound fab as-is. The single tone control is simpler—and more fun.
Master volume mods. Original Marshall 18-watt amps have no master volume or gain controls—just a single volume control per channel. A master volume can be a nice addition, especially if installed after the phase invertor, as described in Tube Depot’s 18-watt assembly manual. That way, distorted tones lose relatively little impact as you dial down your volume.
Rectifier replacements. A tube rectifier is “the vintage way,” yet many players prefer the faster, punchier response of a solid-state rectifier. You can also split the difference by replacing the default EZ81 with a GZ34, which tends to exhibit less tube “sag,” though it requires different wiring and an octal tube socket. Tube Depot’s 18-watt documentation covers such options, including a tube/solid-state toggle.
Dual tremolo-speed mod. Mojotone’s 18-watt kit includes a two-button footswitch for activating tremolo and toggling between two modulation rates. You can add this dual-rate mod to any 18-watt build following the directions in Tube Depot’s 18-watt documentation.Tone-cut control. You can add a pot/cap pair after the phase invertor as a low-pass filter, siphoning off highs. This can be helpful for high-gain tones that get nasty on top due to particularly hot or bright pickups.