I started looking into building a bass-spec Plexi circuit started when I first came across the Metro Amp 50W build guide. I read about amps that switched between different Marshall eras, like the Heritage Colonial, Aiken Intruder, Germino Club 40, and Fargen Mini-Plex.
The form factor was definitely inspired by a one-off JTM50 build on the Lil Dawg Amps blog. It showed me that the space spacing layout of a Marshall circuit in a 17” wide head was possible. My lunchbox 5F1/5F2A head was also influenced by his offerings.
The kitchen sink approach to the switching options was mainly based on a Johan Segeborn video demonstrating a Marshall 1987X that covered several Marshall circuits. I can get close to Bass Spec, Organ Spec, and Lead Spec ('67, '68, '69, '70).
I ordered the following tubes for the amp from Doug's Tubes:
- V1 - Ruby 12AX7AC5 HG+
- V2 - Shuguang 12AX7
- V3 - Sovtek 12AX7LPS
- V4, V5 - Electro Harmonix EL34
I will explore more fancy tubes in the future, but purchased these to make sure the amp would fire up.
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| Front panel of the amp. |
Front Panel Option Summary:
- Switchable 470pF mixer capacitors on each channel
- Either Channel 1 or 2 can be set as the bright channel
- Channel 1 Bright Cap Select: 500pF, none, 100pF
- Channel 2 Bright Cap Select: 4700pF, none, 250pF
- With the Channel 2 mixer cap engaged, I have a shared cathode bright channel with more bright cap options
- PI Coupling Cap Value Select: 0.022uF, 0.09uF
- Switchable between Lead and almost Bass spec
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| Rear panel of the amp. |
Rear Panel Option Summary:
- V1 split/shared cathode switch
- V1A Cathode Bypass Cap Value Select: 221uF, 1uF, 5.7uF
- V2A Cathode Bypass Select: 330uF, none, 0.68uF
- Lead/Bass Tonestack Switch
- Variable Negative Feedback: 47k to 147k on 8 ohm tap
- Bias Test Points
Transformers:
- Power - Classictone 40-18095
- Output - Classictone 40-18025
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| Modifications shown on Metro Amp's 50W Plexi Schematic. |
Bass Spec: changing through different bright caps.
Lead Spec: Trying out the different switch options.
Lead Spec: All controls at maximum.
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