50W Plexi-Style Circuit Build Completed

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.

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

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

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.

Older Posts:

https://outmodedelectronics.blogspot.com/2019/02/start-of-60s-plexi-50w-inspired-amp.html

https://outmodedelectronics.blogspot.com/2019/04/continuation-of-plexi-inspired-build.html

https://outmodedelectronics.blogspot.com/2019/06/1987-and-5e3-turret-boards.html

https://outmodedelectronics.blogspot.com/2019/07/incremental-progression-on-amp-builds.html

https://outmodedelectronics.blogspot.com/2019/08/plexi-amp-build-chassis-mounting.html

https://outmodedelectronics.blogspot.com/2019/10/finishing-up-plexi-build.html

Finishing Up Plexi Build

I finished soldering all of the components and will begin checking all connections prior to the initial power-up.

I have ordered 3 12AX7s and 2 EL34s for testing of the amp next week.

Chassis Wiring

Front panel without knobs.

Rear panel without knob for NFB.

Top view of chassis.

Edit 10/31/19. I successfully powered up the amp with all tubes installed.

V1: Ruby 12AX7AC5 HG+
V2: Shuguang 12AX7
V3: Sovtek 12AX7LPS
V4, V5: Electro Harmonix EL34 matched pair, average distortion

B+ voltage off the HT fuse is nominally 432V, in line with typical '68 Plexi 50W voltages.

Below are some additional photos of the completed amp.

Front view with cage and knobs installed. I will redo the Midde and Treble Dymo labels.

Output section wiring.

Power wiring.

Input section wiring.

Switchable mod wiring.

Tone control wiring.

Initial observations playing through IRs loaded by Suhr Reactive Load:

• The bright caps increase the noise floor of the amp, especially when the channel has a bright mixer capacitor engaged. Adding gain through the V2A cathode bypass cap does not result in as much noise.
• Engaging a bright cap with the bright mix capacitor off on the active channel does not increase the noise floor as much.
• I have 2 near-identical channels in shared cathode mode, by engaging the bright mixer cap for the active channel, and turning it off the the unused channel. I can set up four different bright cap values this way.
• The 4700pF bright cap (channel 2) immediately distorts the sound as expected.
• 500pF on a clean sound is too bright, even on shared cathode mode.
• The lead channel in split cathode mode is noisy when I turn it up to 10. This is more gain than I need, and I am ok with the noise floor when I dial the amp back to 8 (Fender-scale).

IRs of Jensen P10R in Pro Junior

Download Link of P10R with Seymour Duncan Powerstage 170 Power Amp

I started shooting impulse responses of the Jensen P10R speaker in my Fender Pro Junior IV.

I have used the following microphones to capture the IRs:

• Shure SM57
• Sennheiser E906 (Bass, Flat, and Treble setting)
• 3U Audio Black CM1

The same equipment was used as my previous impulse response captures  (Steinberg UR44, Stam SA73). Each sweep was 10s long.

Jensen P10R speaker mic'd with 3U Audio Black CM1.

Edit - 12/4/24 - I re-shot the P10R speaker with my Behringer A00 power amp.

Download link to latest P10R IRs with Behringer A800 Power Amp.

Plexi Amp Build chassis mounting progress

I found some time to start mounting components onto the Hammond chassis. Most of the front panel is populated, as well as a bit of the backpanel.

One of the on-off-on DPDT switches was a dud, but I had luckily ordered a spare.

Progressing panel component and transformer mounting.

A shot of the front panel from the underside.

Shortening some of the transformer leads.

Testing the fit of the main turret board.

Update 9/4/19: more wire dressing and a few solder joints.

Update 9/10/19: I had half an hour to work: layout heater wiring from V5 to V3.  It’s more time consuming than I had imagined. 

Update 9/13/19: Heaters installed. I tried to match the installation depicted in the MetroAmp builder guide.

Update 9/26/19: Input jack wiring, more grounds, filter cap wiring, bright cap switches.

Update 10/16/19: soldered many of the wires on the main turret board side. 

Update 10/18/19: Connected turret board to front panel controls and input jacks, laid out mod components on the turret strips.

AutoCAD layout used for component placement.

Some Pedal Projects ca. 2012: Triangle Big Muff, Meathead, Woolly Mammoth

My first pedal project was a Triangle Big Muff using a the 2010 version of a Madbean Mudbunny PCB, built into a gutted Behringer VD1 enclosure. I followed the Madbean Triangle BOM: 2N5088 transistors, 1N914 and 1N4001 diodes, all resistor and capacitors values.

I used pre-bonded 24AWG wire from Smallbear in my earliest projects, which I prefer over the stranded wire from Mammoth and Tayda that I now use. The parts used were mostly from Tayda Electronics, except for the electrolytic capacitors I salvaged a component kit from my sophomore year electronics class.

Guts of the Triangle Big Muff Clone

Old video of my pedal through a Vox Pathfinder amp

Pedal through Fender Pro Junior IV

I started to put together fuzzes/overdrives on veroboard after discovering the Tagboard Effects and SabroTone blogs. I discovered Mammoth Electronics as a source of 125B-sized inexpensive enclosures. Two of the pedals I am currently playing again are the Meathead and Woolly Mammoth fuzzes.

Meathead Clone with switchable input/output coupling capacitors

Guts of Meathead Clone

Video of Input Capacitor Effect in Meathead

The effect of the large values that I chose for the output caps were inaudible. I may switch out for smaller caps in the pF and low nF range, as shown in the alternate layout on the Tagboard Effects post. The effect of changing the input cap is most prominent when using the bridge pickup for me. The 43nF setting is much more audibly different to the 10nF setting than the 20nF setting in terms of bass response.

I tend to keep the Dirt knob low, because my build turned out to be noisy past 12:00. It is also much past unity gain at that point.

Woolly Mammoth clone

Guts of Woolly Mammoth Clone

Incremental Progression on Amp Builds

I finally got all the components for the Marshall-inspired build. I had to reorder and redrill the chassis due to a mistake in the layout. 

Populated 1987 Turret Board

Populated 5E3 Turret Board

Hammond Chassis Laid Out for Drilling

Top View of Drilled Chassis

Fitting the cage to drill cage mounting holes

Power Transformer Mounting Holes

Output Transformer Mounting Holes

Buzzaround and Naga Viper Pedal Builds

Left - Buzzaround; Right - Naga Viper

I finished vero board builds of a Buzzaround and a Naga Viper based on the layouts from the Tagboard Effects blog (Buzzaround Link and Naga Viper Link). Passive components are mostly sourced from Tayda Electronics. Enclosures are from Mammoth Electronics.

Buzzaround Clone Guts.

The Buzzaround is wired as a -9V effect. I added a 100KA pot for a volume control.

Smallbear PNP Transistors.

I bought a PNP transistor set from Smallbear Electronics. Q1 and Q2 are Motorola-branded transistors of an unknown make, with markings of “9270 336” on the transistors. Q3 is a Texas Instruments 2N404A with a HFE of 140, mislabeled on the bag, but consistent with typical Buzzaround values. I also ordered the 1N270 germanium diode from Smallbear.

Video of Buzzaround Build

Naga Viper clone guts.

I had trouble fitting the vero board into the enclosure due to the height of the electrolytic capacitors I used, so I installed the board at the bottom of the enclosure. Despite the long connections to the pots, the pedal is fairly quiet for much of its gain range. A 2N2222A transistor was used.

Video of Naga Viper Clone through Fender Pro Junior and 5F1 Clone

My next pedal projects are Son of Screamer and Honey Bee pedals using the respective Tagboard Effects vero layouts.

Another Distraction - GGG Green Ringer Kit

Enclosure and Bag of Components

Front of Enclosure

Completed Wiring

I bought a Green Ringer pedal kit during the General Guitar Gadgets recent sale. I had attempted 2 unsuccessful builds of a Green Ringer on vero board in the past.

The kit came with 2 2N5089 transistors (Q1, Q3) and a MPSA92 PNP transistor (Q2), with 1N4148 diodes.

I had originally tested the pedal by reamping a recorded guitar signal, and came across a persistent hum. I rechecked continuity through all of the solder joints.

However, the effect worked properly when interfaced with an electric guitar’s pickup. The effect seems sensitive to impedance. With a pickup simulator in front of the pedal, the hum was not present.

The pickup simulator is an older build, based on the project on the AMZ website (Link). I used the same 42TL019 10KCT:600CT transformer, with switchable capacitor values based on the AMZ (330pF) and DIYRe (820pF) versions. The transformer secondary is switchable between using the full windings and the center tap to simulate Humbucker and Single Coil pickups respectively.

Pickup Simulator

Pickup Simulator Wiring

Clips of the Green Ringer to come.

Plexi and 5E3 Turret Board Progress

As I upgraded some of the 2W metal film resistors to 3W on the 1987 turret board, I started populating a board for a future 5E3 head build that I have been putting off.

All passive components are now ordered except for the filter capacitors and a diode for the 1987 board. I am now considering installing a hi/low B+ voltage switch on the Plexi. I plan on using the 625V tap on the ClassicTone 40-18095 power transformer, but am thinking about utilizing the 690V or 560V taps.

The 5E3 will likely  be a straightforward build, with little or no modifications. Parts will be Vishay metal film resistors, Vishay MKT1813 and Cornell Dubilier 150 film capacitors, F&T filter caps, Cornell Dubilier silver Mica capacitors, and ClassicTone transformers.

Budget Dirt Pedalboard

Signal Flow
DOD Carcosa
Way Huge Havalina
Way Huge Saucy Box
BBE Green Screamer
DOD 250
DIY SHO Clone
DIY Fetto Clone
DIY Red Llama Clone
DOD Boneshaker
DIY Distortron Clone

The pedals were mounted with Velcro on a piece of 22”x12” lexan. A Roadrunner All-In-One Pedalboard Gig Bag houses the pedals. A 10-pedal daisy chain is used for power connections. EBS flat patch cables and pancake patch cables were used to interconnect pedals.

All pedals are either DIY or impulse sale/clearance buys.

Video of Green Screamer, Saucy Box, 250, SHO Clone, SD-1 (not on board) boosting a Fender Pro Junior

Video of Jet City JCA22H boosted by the same pedals plus a DOD Boneshaker