HUDSON: Sidecar Overdrive

I did a layout for the Hudson Sidecar a couple of years ago, and I retraced my steps recently and found a few things that needed to be updated.  The Sidecar is a progression of the 999 Overdrive, which is Hudson's version of a 'green box' overdrive, made for people like me that don't like the green box.  

There's also another variation out there called the Motorcycle, which sounds like the same circuit that's been retuned for a bit more low drive, with the addition of an output transformer, not unlike the Broadcast.  My guess is the 470n / 1k off the inverted input stage has been adjusted, maybe a tweak to the bass side of the active eq???.    

Plenty of great pics on FSB  Schematic on PedalPCB

The layout below should be pretty close to a version of the original from 2017.   0.15" vero, so a tin can IC is a must.

HUDSON SIDECAR  -  0.15" VERO LAYOUT

Note that there are minor variations out there, depending on how Hudson was making them at the time.

FENDER MDX Circuit / AION FX Elite boost / preamp

There's a few variations of this circuit, so I'd recommend reading through the Aion FX build docs or maybe just skip the vero all together and buy the board from them.

The build docs also include LTspice simulations of the frequency response, so that saves me a bit of time plotting them out.

I've labelled a few of the components where variations occur, but not all.  You should be able to work out the ones that I haven't labelled easily enough.

FENDER MDX CIRCUIT / AION FX ELITE BOOST / PREAMP VERO LAYOUT

Vague explainer / demo here

CASTLEDINE ELECTRONICS: Magical Mystery Box

Since I've been on a slight Beatles related run lately, I thought I should tackle the Castledine Electronics Magical Mystery Box.  

This is Stu Casteldine's version of the Brilliant channel from a Vox Conqueror guitar amp.  Castledine included a gain recovery section at the end and the transistor-based distortion switching has been removed.  I ran both the Castledine version and the original in LTspice, and there was no real difference between the two.  

The thing that really sets these apart from similar Vox amps is the mid-range boost (MRB), which is said to be the precursor to the wah pedal.  The MRB has three different options for the mid-range boost frequency - the sound is what we now refer to as a cocked wah.  i.e. turning your wah pedal on a leaving it at a specific frequency.

As this line of Vox amps were transistor-based, the pedal version will be much closer to the sound of the original than a lot of other amp-to-pedal conversions that just replace valves with JFETs.

CASTLEDINE MAGICAL MYSTERY BOX VERO LAYOUT  

I've not done a layout with a Fasel inductor before, so hopefully I have the sizing about right.   The layout is intentionally wide, as this will be in a large box in landscape orientation (three foot switches and five controls along the top).  

Despite the size, you may need to add some space to mount this using your preferred method.  I strongly suggest testing the layout and sizing before starting construction.  The fasel inductor sits quite high compared to other components.  It would be annoying to get to the end of a more expensive than normal project, only to find that it doesn't fit in the box.

VOX CONQUEROR / DEFIANT / SUPREME PRE-AMP

Several Vox transistor amps shared the same pre-amp including the 30W Conqueror, 50W Defiant, and 100W Supreme.  They were experimenting with modular amp designs at the time - probably to save money and streamline production.

The Brilliant channel is the section on the top-left to centre.  If you compare this to the Magical Mystery Box, you can see they are very similar. 

When the distortion was switched on, it provided a bias current to the base of the transistors, allowing sections of the distortion circuit to connect to ground.

This is a redraw of the original schematic by Chris Devine of Back to the Sixties (link below).

CASTLEDINE MAGICAL MYSTERY BOX

ADDITIONAL VOX RESOURCES

Vox Showroom - https://www.voxshowroom.com/uk/amp/conq.html

Back from the Sixties (scroll down for schematics) - https://www.backfromthesixties.co.uk/vox-supreme

SPACEMAN: Equinox Equaliser

The Equinox is Spaceman's trimmed-down version of an old-school studio equaliser - it’s definitely inspired by Pultec, but with a few key differences.  I’m fine with the differences, as it makes it practical to build.

Schematic can be found here on PedalPCB.  

If you don't have a power supply that kicks out 18v, Dirtbox layouts have a vero layout with a charge pump included.  One could be tacked on the bottom of this board - which is how I started doing the layout until I noticed that the Spaceman version doesn't appear to have this, and my power supply does 18 volts.

This is probably a good candidate for an additional loop to be added, so another pedal can be switched on with this at the same time.  There's certainly going to be enough room for a couple more jacks in the enclosure, and this could really tailor the sound of your favourite fuzz / drive / distortion pedal.

I may even make a point-to-point version of this to keep the vintage vibe of the Pultec.

SPACEMAN EQUINOX EQ - VERO LAYOUT

Clearly, this is not for the faint-hearted in terms of off-board wiring - I would not recommend trying this unless you are quite confident with your wiring.  

ENCLOSURE SIZE & LAYOUT

A sizeable box will be needed and you will need to do some planning before getting started.   Spaceman uses a 1590XX which has internal dimensions of 139mm x 115mm.  You may even want to go up to a 1590DD, which has internal dimensions of 179mm x 112mm, which is a lot of real estate on a pedalboard,  but it will be comfortable for the build and will allow better knob placement.  As a reference, the vero layout above is 71mm x 54mm.  

At this size, the vero will have to sit on top of the pots.  Rotary switches are usually too tall to sit boards on top of them, apart from the height, there are a lot of wires going on straight off the top.  With this in mind, the rotary switches will have to be the top row of knobs on the enclosure.  Could probably sneak the level control in next to them on one side.

A big vero board in a large box with a lot of wire should be well secured.   Some kind of mounting is a must.  There's some space on the layout for mounting, but you may need to add a little more, depending on your preferred method.  I often use adhesive stand-offs.  These have a 13mm clearance, which gets the board just above pot height, which is usually around 10mm.  

I'd suggest doing a mockup of the internals using the actual parts you plan on using, on a piece of cardboard before you start drilling an enclosure (including the mount positions).  I often use cardboard templates to pre-wire circuits before I finish them off in the enclosure.  It tends to be a bit neater and access is easier as you don't have to worry about the sides of the enclosure getting in the way.

EQUINOX VIDEO DEMO

ARBITER: Treble & Bass Face

The Arbiter Treble and Bass Face is the lesser-known cousin of the Fuzz Face from the mid to late sixties and was probably overshadowed by the Colorsound Power Booster, which had more gain on tap, and perhaps a nicer frequency response.  Speaking of which, it's easy enough to mod this to suit your tastes.  I might add some suggestions for this later.

It's not a fuzz, it's more of a preamp really - practically no dirt to speak of (or at least very minor at best).

ARBITER TREBLE & BASS FACE - VERO LAYOUT

ORIGINAL SCHEMATIC

LTSPICE

Frequency response

As you can see, these waves are very clean 

TREBLE AND BASS FACE VIDEO DEMO (ALONG WITH THE TREM FACE)

HOFNER: Germanium Pre-amp

I only came across the Hofner Preamp quite recently, despite seeing some other builders making point-to-point style germanium preamps based on this circuit - the prime example being the R2R Electric preamp.

There's a good write up here, but in summary: Back in the day before guitar amps were affordable, and TV was a thing, pretty much everyone had a big valve radio in the house.  Many had an input for turntables, but most people didn't have a turntable as they were expensive.  So Hofner made a preamp to allow early electric guitars to be plugged into a valve radio, via this additional and often unused input.   

The preamp boosts signal and helped fix the massive impedance difference, turning the family radio into a primitive guitar amp.   

Worth noting that old record players had a much higher output than the current ones, hence the need for a boost.  The output was so high, that when you look at old schematics for valve PA systems, the P.U. input skips the first 12AX7 gain stage, and goes straight to a mixer, so guitars really needed an extra boost to make it work.

HOFNER PREAMP SCHEMATIC

It kicks out a good 20dB of boost and has a relatively flat response

  

I'd be very surprised if the boost is as clean as it looks on LTspice - I'll have to breadboard this and check it out on the scope.  I'm sure there would be some dirt.

MODS

I guess the first and most obvious is adding a volume pot.  A 220n output cap is pretty healthy, so a standard A100K volume pot will work just fine.   Note the R2R electric method, which is similar to a rangemaster.  This is probably a better way to do it. 

Reducing R1 increases the input level, as it normally forms a voltage divider with R2.  Things will get a little fuzzy and overdriven.

C1 can be reduced to treble booster territory. 

C4 can be reduced for a bit of low-end roll-off.

TAGBOARD & VERO LAYOUTS

The original was constructed in a very similar fashion to the Rangemaster - a simple point-to-point circuit in a box.  

JONNY RECKLESS: Reckless Abandon

A recent request.  This is a monster of a pre-amp from Jonny Reckless.  Best to read the details on DIY stomp boxes, as there's a lot to this.

I've not built this, and there's a bit going on with it.  I'd suggest checking it against the schematic before starting.  I've looked over it a few times, but I probably won't be building this one any time soon to confirm it.

Link to thread on DIY Stompboxes

JONNY RECKLESS - RECKLESS ABANDON  VERO LAYOUT

SCHEMATIC BY JONNY RECKLESS

VIDEO DEMO, ALSO BY JONNY

SPACEMAN EFFECTS: Redstone Germanium Preamp

Yet to build this myself, I quite like the idea behind this - I am partial to anything with bass / mid/high controls for guitar and a germanium transistor.  Bonus points for also including a discrete op-amp, which I've been playing with lately.

Before I looked at the schematic for this, I was playing around with modding a Boss Blues Driver to make the tone stack a real tone stack with external pots and removing the last stage, just leaving the two discrete op-amps.  This is a refined version of where I was heading, so I think I'll park that project for now. 

BOOST STAGE

The boost stage uses a LND150 MOSFET which is common on several Spaceman Effects designs.  It could just as easily have been a JFET-based boost - this is a clean boost to make up for the gain loss at the EQ stage.  A little bit of high-end is also filtered off at this stage, but nothing substantial.  You may not even hear it.

EQ 

It's a fairly standard bass / mid/high tone stack with some tweaked values.  This could be modded to typical values for Marshall or Fender, but there's a decent chance Spaceman went this way for a reason - it looks pretty balanced to me.

DISCRETE OP-AMP

It's a Boss-style discreet non-inverting op amp using two JFETS and a PNP transistor (as found in the Blues driver).  There are a couple of resistor value changes, and the PNP transistor is, of course, germanium instead of silicon - hence the germanium preamp name.  The GE transistor will add some extra flavour, but maybe not much more. 

According to the Dead End FX build docs, a 1T308B transistor is used - it makes sense to use Soviets for a pedal with a large production run.  If you don't have a 308 - try something around 70-80 hfe and low leakage (most Soviet-era transistors have low leakage)  Japanese transistors might also be a good choice.

Potential mod:  As this is just another type of op-amp - diodes can be added across the output of the op-amp and inverting input for some soft clipping.  It works on LTspice, so it should work in practice. I'm definitely going to try this.    

SPACEMAN REDSTONE GERMANIUM PREAMP - VERO LAYOUT

The layout below is missing the soft switching to save on size.  If you don't have ferrite beads, jumper it on the input, or use a low-value resistor in the power section (100ohm).  You could quite happily not use the MOSFET in the power supply section, but I had the room, so may as well keep it in.

The same layout as above, but this one is for a Soviet 1T308 transistor, which uses the less common BCE pinout, as many Soviet-era transistors do - you could always use the one above and bend the legs. Still, I find that annoying, so I made another layout to suit.

SCHEMATIC

I redrew part of the Deadend FZ schematic, as I found it difficult to picture the discreet op-amp.  I have not included the soft switching or fancy power supply in the drawing below.   

VIDEO DEMO

EAE: Model feT Preamp

I saw that DirtBox Layouts has done a layout for this from the schematic on PedalPCB - it's one that I've been wondering about for a while after playing with the CODA FX Black Hole Doom Machine.  I had an idea of what was happening, but I was slightly off in some parts around the transistors replicating the power amp stage.  

There are a few small variances here and there, but the bones of it are a JFET version of the Sunn Model T Preamp.  What EAE have done differently is to approximate the output/power amp stage after the preamp to give the preamp something to actually push.  

I tried a straight Model T preamp section emulation with JFETS, and it's not all that exciting - this looks like a different beast.

EAE MODEL FET SCHEMATIC

Fairly standard response for this type of EQ - all volume/gain @ 100% and tone controls @ 50%.  I haven't bothered modelling every option, as this has a lot of knobs...  

This is the power amp sim stage, with clean signal in @ 440hz 100mv

The frequency response is pretty flat, with a little roll-off on highs and lows, which is generally desirable in a guitar pedal.

It needs a little push before it starts to clip - and the output level is quite high.  You can see the lower side of the wave has clipped on the output.  Being a spice sim, this will not capture some other distortion that might be present simply due to part tolerances or slightly mismatched transistors.

Same again, but at 50% gain for both channels

The single channel running at 25% - but this is a LTspice model, so it may not actually run this clean.  Depending on how you bias it, it may well be able to run clean - but why would anyone want to run this thing clean?

EAE MODEL FET VIDEO DEMO

GREER AMPS: Soma 63 Vintage Preamp

The Soma 63 is an amp in a box style pedal, with the usual array of J201s with trimmers replacing valves, an amp-style tone stack, and a transformer at the end of the signal chain thrown in for good measure.  

I added some mounting points on this, as you don't want a board with a transformer on it floating around inside an enclosure - just be careful with traces around the mounting points, depending on your preferred mounting type. 

GREER AMPS SOMA 63 VINTAGE PREAMP - VERO LAYOUT

GREER AMPS SOME 63 PREAMP - VIDEO DEMO

BENSON: Preamp, Mojo Vero Layout (with 1m bass pot mod)

The Benson Preamp is pretty popular, but 2M pots are hard to find.  This is a modded vero layout using a 1M bass pot.  To make this change and keep the same frequency response, resistances are halved, capacitance is doubled.  Note that the Drive pot value also had to be halved.

I'll add a regular size layout with normal 1/4 resistor later - just felt like a larger layout for this one.

BENSON PREAMP - MOJO VERO LAYOUT

BENSON PREAMP - MODDED SCHEMATIC

As you can see, the response is nearly identical between the stock and modded values.  This will change a little using stock cap and resistor values, but it will still be pretty close.

BENSON PREAMP - VIDEO DEMO

RCA: BA-31C Preamp, Vero Layout (The Balmoral Broadcast)

The RCA BA-31C would have been found in broadcast consoles of the day (late sixties), as a microphone preamp module.  

I think it may have been the starting point for the Hudson Broadcast, as they look quite similar and I've heard some references made to RCA.  If it is what they used, I can see why they did - it's very tweakable, with some nice options for setting gain as well as high and low-end roll-off.   

The original runs at +/-30V.   I was aiming for a 9v single rail supply to keep it simple, and of course without expensive transformers.

RCA BA-31C MICROPHONE PREAMP - ORIGINAL SCHEMATIC

RCA BA-31C - MODDED FOR GUITAR

I tried to keep things simple, while still maintaining the structure of the circuit.  

Note:   This is still a work in progress, still some fine tuning and changes in progress.

GAIN

Like a Hudson Broadcast, it runs from clean boost to an overdriven fuzz.  It's not super fuzzy like a fuzz face, but you can hear it.

After experimenting on the breadboard, I decided to use an A1K pot for R6, and set R10 to 22k - 68k (anything a bit bigger than the original 6.8k resistor is fine, so long as it still has some feedback).  Decreasing R5 also provides a bit more dirt.  

FREQUENCY RESPONSE

With a few capacitor value changes the frequency response is easily adjusted.  

Increase C5 to roll-off high frequencies, although this is only needed at the highest gain settings, on lower gain settings, the tops are already reduced.

Decrease C3 to roll-off lows after Q1.  1u is pretty flat, can go down to relatively small values (22n as an example).  

MODDED RCA BA-31C ON THE BREADBOARD / SCOPE

So it works....  this is using a B5k pot for R6, and I think a 100n on the output as it was sitting right in front of me at the time.  Will change over to 1u.

PNP is a germanium 2SB457, NPN is a BC108.   It really isn't at all fussy about what germanium is used for Q2. 

 

caution: this starts at low volume and increases - watch your ears / speakers.

VERO LAYOUT - THE BALMORAL BROADCAST

Initially I thought I would end up with a few switches for bass and treble - but for now I decided to keep it simple.  Think I will most likely land on gain, volume, and a fat switch (which will change the coupling cap value between Q1 and 2).  For me it doesn't get bright enough to need control on the treble - if anything it's a bit low on treble on cleaner settings.

At low gain settings, it has a slightly rolled off high-end, with is nice and bluesy, maybe a bit of a jazz tone.  I did try it with a boost in front, which works pretty well, although it can use a bright cap across the boost level control.

This layout is a little different to the schematic above, as I decided to change the output cap.  I've called it the Balmoral Broadcast - due to some similarities to the Hudson, and I live in Balmoral.

 

Note:  not final versions

REVOX: A77 Input Amplifier Clean Boost, Overdrive / Light Fuzz

This is the input amplifier from a Revox A77 reel to reel (circa ~ 1967) - which also happens to be almost identical to the mic preamp in Trident A-Range mixing consoles from the early seventies.

So far out of the preamps that I've been working on, which is the Neve, RCA and this, I'm happiest with this one.  Apart from sounding cool, it also retains a pretty flat response, and could be used as an "always on" sweetener.  I was probably just hearing very light distortion, but I did prefer the tone of my guitar with this on running clean, rather than off.  It felt a bit more forward and bigger sounding.

It’s very very loud - tons of output level available.  This could happily be described as a clean boost as well as an overdrive / light fuzz.  

Due to the insanely high output, I think this is a good candidate for some form of High or low pass filter for some tone shaping.  If you’re using it to smash another pedal or front end of an amp, some bass cut might be handy.   Note that on the highest gain setting (which is fuzzy), it does roll-off high and low frequencies.

The original uses silicon transistors, I've used Japanese 2SD128 germanium transistors, which are usually around 40hfe.    

REVOX A77 INPUT AMPLIFIER SCHEMATIC

Note the switching used to set impedance and recording (gain) levels.  As I was aiming for boost and fuzz, the resistor switch on the input has been removed on my layout.  The fixed resistors controlling gain have been replaced with a pot.

TRIDENT MIC PREAMP SECTION FOR COMPARISION

REVOX A77 INPUT AMPLIFIER FREQUENCY RESPONSE

On the highest gain setting, high and low frequencies roll-off.  It's very flat elsewhere.

The output is very high, and the overdrive / fuzz does not occur until the last couple of hundred ohms when resistance to ground is heading towards zero (hence the reverse log pot)

my notes from spice re the schematic above:   

• input cap can be 1u without effect response
• 1u output cap only has slight low frequency roll-off, even with a 5k volume pot
• decrease C4 to 10u to roll-off low frequencies
• increase C2 to increase high frequencies
• decrease R8 to reduce fuzz
• C6 can be smaller (down to 10u)
• C5 can be removed, or increased to boost treble from 8k / 10k
• R5 can be removed for a very slight gain boost

There are so many variables, I haven't show the LTspice results here, or it would be a very lengthy post.  

REVOX A77 INPUT AMPLIFIER - CLEAN BOOST / OVERDRIVE / LIGHT FUZZ

Work in progress on the bench - as you can see, it's the mojo layout (also know as the unnecessarily large version).   Approx 65mm x 40mm on 0.15" vero with 1w carbon film resistors

And it’s boxing day - using my ‘spare no expense’ cardboard box wiring rig to get the spacing right before finishing it off in the enclosure.  

It’s not the greatest looking, but I do like the sound.   Really really loud by the way.

ON THE SCOPE / FFT

caution re audio levels - start low.  This is bench test of my build.