50W Plexi-inspired build in a lunchbox head

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bikescene
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Posts: 1
Joined: Thu Oct 09, 2008 10:12 am

50W Plexi-inspired build in a lunchbox head

Post by bikescene » Tue Nov 12, 2019 1:25 pm

I want to share my build here since the Metro 50W build guide and this forum’s posts were my main sources of information.

Basically switchable from 68 Lead to Bass Spec:
-Shared/split cathode
-V1A Cathode Bypass: 1uF, 5.7uF (1+4.7uF), 221uF (1+220uF)
-V1B Coupling Cap: 0.0022uF, 0.024uF (0.0022+0.022uF)
-V2A Cathode Bypass: none, 0.68uF, 330uF
-Lead/Bass Tonestack
-PI Coupling Caps: 0.022uF, 0.09uF (0.022+0.068uF)
-470pF Mixer Caps switchable for each channel
-Channel 1 Bright Caps: 100pF, none, 500pF
-Channel 2 Bright Caps: 250pF, none, 4700pF
-NFB adjustable pot from 47k to 147k

I built it in a 17”x10”x3” Hammond 1444-32 chassis that isn’t the strongest. I will only be using the amp at home through a Suhr Reactive Load or attenuator. The chassis is much deeper than the typical Marshall chassis. I sourced the parts from Valvestorm and Mouser. I used a mix of metal film and carbon film resistors. I used dual 32uF filter caps throughout.

I have a 50k bias pot installed on top of the chassis but may change it out for a 25k pot since I currently have it set at 4.4k between lugs 1 and 2 for ~30mA biased on the colder side.

Using the 312.-0-312 tap of the Classictone 40-18095 PT, I got about 460V B+ at the HT fuse.

The wiring got a bit messy once I no longer had a 1:1 reference to relate to.

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Video Clip
https://www.youtube.com/watch?v=NZ1cM0oGBWw

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