Tuesday, October 14, 2014

Epiphone Valve Jr Hot-Rod Modification

EVJ Hot-Rod
The Epiphone Valve Jr Hot-Rod is a 5W Class A all valve guitar amplifier with built in spring reverb, and is the less common, and slightly more expensive cousin of the original EVJ.
Although modifications for the original EVJ are found in abundance across the web, the EVJ Hot-Rod seems to have been overlooked by the modding community.

The EVJ Hot-Rod has essentially the same amplifier structure as the original model, with an additional 12AX7 to drive the spring reverb unit, a multi-tap transformer output for different speaker impedance, and a "gain" pot on the front panel (which is just a voltage divider after the first preamp stage). With this in mind, modifications for the EVJ Hot-Rod can be easily copied straight from existing documentation for the original EVJ.

Safety Note
Power supply capacitors in amplifiers retain charge for weeks. Always discharge capacitors before working on any amplifier. Valve amplifiers are especially dangerous, as the working voltage of the power supply capacitors can be in excess of 300V.
A 1K 5W resistor attached to probes can be used to discharge capacitors. Double check with a multimeter before working on the circuit.

Disassembly of the EVJ Hot-Rod head

Remove screws from the back panel, and pry off (The panel may be hard to remove due to traces of glue on the vinyl).
Label the cables to the spring reverb unit for re-assembly, and disconnect.
Remove four plastic caps from the top of the amplifier case.
Remove the four screws, and slide the amplifier chassis out of the case.
Remove valve covers.
Remove valves.
Discharge the power supply capacitors by shorting R14 and R15 to ground through a 1K 5W resistor.
Label all cables for re-assembly, and disconnect.
Remove the input socket retaining screw.
Remove the circuit board support screws.

Remove the circuit board from the chassis.
Thoroughly wash the circuit board with isopropyl alcohol to remove flux and dust.
The circuit can now be worked on.

Clean dust from chassis.

Clean all pots and sockets with de-oxidiser fluid.

I have largely copied the EVJ "marshall mod" (schematics for which can be found elsewhere). After about six hours of changing component values, I found a sound which I was happy with.
The result was a brighter sounding amplifier, with a well defined low end, and higher gain. Adjusting the controls for low gain settings produces a smooth overdrive, and higher gains ramp up to a broad fuzz sound.
A word of caution; the quality of the EVJ circuit board is very poor. Excessive heat will easily lift circuit board pads and traces. Ensure that you use a good heat controlled soldering iron.
EVJ Hot-Rod Original Schematic
Schematic highlighting modified components
Lowering R3 will increase input level to the first triode stage. Lowering R6 increases quiescent current through the second triode stage, while lowering C8 and C9 serves to 'tighten' the bass response.
Increasing C16 improves bass reproduction of the power pentode.
The addition of a 1K 1W resistor at the pentode screen will limit current through the amplifier in the case of valve failure. This modification requires cutting a circuit trace, and drilling holes to mount the additional resistor on the top of the circuit board.
Addition of protection resistor on pentode screen
I found the reverb on the stock unit to be very thin, and so decided to modify the reverb circuit also.
Lowering R22 and R25 increases quiescent current through the triode stages, while removing C21 and C22 produces a more linear frequency response.
Increasing C18 prevents low frequency rolloff (use ceramic cap due to voltage present).
Increasing R27 reduces output of reverb to a more subtle, controllable level.

Details and schematics for the EVJ Hot-Rod can be found here.

Wednesday, October 1, 2014

Designer Focus - Lewis Waters [New Complexity]

Lewis Waters is an experimental instrument designer and luthier based in Melbourne, Australia.

His startup company 'New Complexity' centres on developing instruments with unique sound and tonality, while retaining the familiarity and playability of traditional guitar design.
All devices are available for sale, and each are crafted with acute attention to detail.

Harmonic Master #1
Harmonic Master #2
Harmonic Master Designs
The 'Harmonic Master' is a twelve string guitar which emits chiming overtones through use of an amplified third bridge. The bridge pieces can be tuned to different ratios, and the third bridge pickup can be routed through effects separate to the primary dual course strings.

Harmonic Isolator
Onboard Preamp
The 'Harmonic Isolator' takes third bridge sympathetic resonance to an extreme, with 4:3 string field ratio. The neck features a microtonal fretboard to more closely approximate a "just intonation" harmonic scale, and both the primary and complementary courses have accurate intonation adjustment.
The instrument is also available with a sustaniac - all of the droning harmonics you could ever wish for.


For the faint of heart, Lewis also produces more traditional designs.
The 'Contra' will provide you with vintage Teisco styling with the superior build quality and reliability of a modern guitar.

Tuesday, March 11, 2014

Japanese Toolbox

Not sound related, but I love the design of this toolbox. It was published on the MAKE website quite a while ago, and was recently included in a MAKE Magazine publication. My toolbox was chosen to appear in the article.

The toolbox is simple to construct, and is very sturdy. I use this design to transport my tools all the time for work as an AV tech. The box should only take a few hours to complete, and can be made on a small budget; a single sheet of plywood will be enough for a large box; first iteration (800x400x400).

Second iteration (500x250x250).

Monday, December 9, 2013

FAB Pedal Multi Effects

This analogue multi effect unit can be made from Danelectro FAB pedals sourced cheaply on ebay. Other parts include press button switches, LEDs, 6.5mm mono jacks, and some hot melt glue.
Aluminium sheet was cut to size and squared, then marked and drilled for each of the existing pots on the circuit boards. Press button switches, jacks and power was also drilled.
Each circuit board was fixed to the face plate with hot melt glue, then each pedal wired in a chain (output to input of next...).
Tinned copper wire was used for the ground connections, to give some rigidity to the rear circuit boards.
Plywood box was made to suit.

Sunday, September 22, 2013

True Bypass / AB Box Circuit

Back to basics; here is a super simple true bypass pedal. It can be used to bypass a chain of effect pedals with one switch, to switch between two sources (eg, two guitars), or to switch between two outputs (eg, an amp and a tuner).
The pedal uses a DPDT foot switch, 4* 6.5mm jacks, and the project should cost ~$25.

Saturday, September 7, 2013

Silicon Heaven

I have started an apprenticeship as an electronics technician at Silicon Heaven, a repair shop in Sydney's north western suburb of Baulkham Hills.

Work includes the repair and maintenance of high power amplifiers, mixing consoles, speakers and microphones. I will be building on my previous knowledge of sound and electronics as I study.

Silicon Heaven is proud to be Australia's one and only Bosch Arc Service Agent! . Midas, Electrovoice Pioneer, Bosch, Dynacord, JBL, RCF, Fender, Blackstar, NAS, Presonus, Telex, Yamaha. We service all Pro-Audio Equipment.
We are now Sydney's Warranty service agent for NAS and AMS. Enabling warranty repairs for Bosch, Electrovoice, Klark Teknik, Dynacord, Telex, Midas and Blackstar and many others. More importantly most repairs take less than a week so no expensive delays.

Sunday, April 28, 2013

Make MonoBox - Trash speaker

Many mix engineers will cross reference a mix through an alternative set of speakers. The concept is that no one out in the real world has a set of studio monitors. This is part of the reason that NS10s and Aurotones are such sought after speakers.

4" powered speaker
Make magazine has published a small amplifier circuit based on the LM386 op-amp, which is well suited to use in small form powered speakers.
Unbalanced line level feeds the amplifier, which is powered by a 9V wall adapter. I have re-drawn the circuit layout for veroboard.
Veroboard Layout
 R1 is 10K for line level source. Change to 100R for headphone level source. If using a wall adapter, remember to filter your psu with a 100uF electro cap.
Original circuit from Make Magazine