The Oh My Darling(ton)

A "'60s Convertible" Rangemaster

2012 By Small Bear Electronics LLC

Here is my first "build-it-this-way" tutorial using  my Bare Box #1 construction platform. The Oh My Darling Rangemaster incorporates a number of Bearishly-clever construction techniques and technical tweaks that make it unusually fun-to-build and useful.

This article describes a perfboard version and includes technical detail meant for intermediate-to-advanced hobbyists. For those who want an easier build, a kit with a ready-to-solder PC board is available. The instruction manual files (which include numerous high-resolution photos) can be found here:

Whether on perf or PCB, the build can be configured with jumpers to be:

Because the devices are NPN, no separate power supply is needed when setting up the pedal on a board. Better yet, the darlington versions produce perfect Rangemaster biasing and tone using transistors that used to be considered discards. Here is a look at the two-NPN circuit:

Cascading two relatively low-gain devices to get a "composite" higher-gain part was fairly common back when making single high-gain germanium parts was much more difficult. My implementation plays into the present transistor market, in which there is a lot of inexpensive low-gain/low-leakage NPN germanium sitting unused because "everyone knows that you need a minimum gain of 70 to get the biasing right." By selecting the transistors and adjusting Rc and Rd, you can make the effective gain of the pair whatever you need it to be within very broad limits. Low-gain parts are usually also low-leakage, so temp stability is reasonable. Tone? I will gladly stack up one of my builds against any one-device, "boutique" Rangemaster clone. The Oh My Darling is all-gain/no-pain, and it can be built with a wide variety of inexpensive parts, both silicon and germanium.

The Hungarian engineer who came up with complementary configurations gave his name to the Sziklai darlington pair. Here is the NPN version, with the appropriate bias resistors to create an "Oh My Sziklai" or "Really Szik" Rangemaster. "Szik" in a good way, of course!:

As you can see, the hybrid Q1-Q2 gain block easily drops into the same circuit position as the regular darlington. Its Q2 position offers a great way to make use of your drawer-full of 2N404s that were not hot enough to use in Tone Benders.

How To Build One

Since I'm starting with the pre-drilled Bare Box # 1, let's take a look at that and get it prepared for the build by adding holes for the two potentiometers. I am basing this description on using Alpha's RD901-40 9mm pots with right-angle PCB pins. You don't have to use those for a two- or three-knob build, but they are very convenient to work with. Similarly, you can locate the hole centers by creating a paper template, but it is very easy to get exact registration by using the 5-Pot board that I had made for this enclosure.

The second pic from left shows the 5-Pot board lying in place in the "well." Each pot outline has a drilled hole marking the center of the control, so I used a straight pick to mark the points where I wanted to drill. That was pretty easy, right?

Because the LED is installed on the circuit board, I left drilling the hole for it till the board is done.

Choices For Building The Board

Here is a complete layout drawing and parts list:

The above layout presumes that you want to build the full "protoboard" arrangement that allows re-configuring the transistors, so it includes all of the connection points needed to do that. I will show the jumpering arrangements later. If you only want to build a single configuration, you can reduce the wiring and the number of socket pins accordingly:

The perfboard shown in the following pics is my SKU 0355. It's 60 x 60mm pad-per-hole, sized exactly for the Bare Box # 1. You can, of course, cut to size a larger piece of stock and drill mounting holes. The layout presumes the use of Molex headers to terminate input, output, power and stomp switch. Again, you don't have to go this route. If you do, the first step is to tool the cutouts in the board for the headers. The layout drawing and the pics below show the areas that need to be cut away. Mark these carefully with a Sharpie or similar marker.

Use any combination you want of cutting, scoring or grinding to create the cutouts. (IMORTANT NOTE: Machining epoxy-glass circuit board creates fiberglass dust, which is noxious stuff! When doing any tooling of the board, wear disposable gloves and a face mask! If you use a cutoff wheel, eye protection is an absolute must!

I found that soldering the headers in place first gave me good reference points for doing the wiring. They are positioned identically in all of the layouts. Follow the left-hand pic and the layout drawing for the build you choose, and pay rigorous attention to where the header pins go.

I began wiring with the connection from the negative power input to the Ring contact of the input jack. Notice that I used the soldering iron to heat and remove a few pads around the mounting hole? This is important for reducing the potential for accidental shorts.

I continued by installing power-protection diode D1. Its anode goes to the positive power input pin. Then I laid the path from its cathode (bar side) to one side of where the connection to R4 will be made. I have colored in the drawing the pads I removed that might short to adjacent locations. Continue to do this as you wire. As always, I mark off what I have done with a highlighter.

If you have had some experience working with perfboard, you should be able to finish the wiring with these few suggestions:

The pic below left is the completed board prior to soldering in place the connections to the pots and the LED.

I love these Molex "SPOX" plugs and headers that I have used to make the off-board connections. I am sure the reason more people don't use them is that crimping the terminals manually is hard without a very expensive tool. Pre-assembled connectors for power, input and output, and pre-terminated wires in a variety of colors are posted in my Stock List, so you can pick colors and create your own plugs for the pots and stomp switch.

The resistors that are plugged in for Rb, Rc, Rd and Re are the nominal values that I worked out when I came up with the idea for the Oh My Darling. This article describes how I got to them and gives some ideas for selecting the transistors. Raw stock is available on the Stock List and audited pairs will follow soon. The right-hand pic shows the board with connectors installed for the pots, transistors installed for the Oh My Darling and jumpering done for that version. The layout drawings that follow show the jumpering possibilities that I worked out. When setting up the Sziklai, note that the transistor orientations are reversed from the Oh My Darling.

I chose a 3mm LED for this build. To install it, I first covered its leads with spaghetti. A couple of scraps of installation fit fine. I left about 1/8" bare on the negative lead, a little more on the positive. Then I bent the leads at right angles to each other before soldering to the pads on the bottom of the board. I eyeballed the location for the LED hole by holding the board in place in the enclosure. With that hole drilled, the LED and the board fell right into place.

I chose shrouded jacks for this build, as used in most "name" pedals. These have a chamfer on the sleeve pin edge, which is helpful for identifying the contacts. Pay close attention to which contact of each jack is soldered to which contact of its associated plug.

The pot board is prepared by soldering the pots and headers in place and boring out the center hole of location VR3. This will pass a standoff for stability when the pot board is installed. You may substitute 250K audio for R2, though I found that the "spread" of control is better with reverse audio.


 

 

 

 

 

 

 

 

 

 

The stomp switch and DC power connector are prepped in the same way as the input and output jacks by wiring connectors to them. Any panel-mount DC power jack is OK, but the pre-drilled hole has to be enlarged for the all-plastic types. If you use my SKU 0612D as shown here, the shoulders of the washers need to be filed down slightly because the panel in front is a little thinner than on some other enclosures. The notes in the layout drawing should help with getting the connections right.

 

Put It Together To Test

I worked from front-to-back and bottom-to-top as shown in the following pics. Before inserting the pot board, set a standoff in place without exposing its adhesive as shown in the second pic. Plug in the off-board parts,  install the board with minimum screws for now and screw nuts on the pots finger-tight.

At this point, I connected my gear at got...nothing. So I did what I tell everyone else to do: lifted the works out, started testing continuity point-to-point and marked off connections with a highlighter. I found a connection that I had forgotten to solder and a couple of shaky joints on the pot board. Fixed those, set up again and got action. The drawings above are exactly what I built, so you should be good if you follow precisely. If you have built a Rangemaster, the transistor voltages at Collector, Base and Emitter are the same.

Finishing

The pluggable construction makes the Oh My Darling very easy to strip down. I prepped the enclosure in the usual way: Sand to 600 grit, acetone wash, spray paint, add decals and clear coat with Krylon lacquer. Incidentally, if you are not comfortable with drilling and coloring, pedalpartsplus.com will do these steps for you. Their prices are reasonable and the quality is quite good.

 I rested the case on a soft cloth so that I wouldn't scratch the finish while poking around with a meter. I hope you enjoy being able to build in this modular fashion. Comments and suggestions are welcome at smallbearelec@ix.netcom.com.

Parts List/Bill Of Materials For The Oh My Darling

Reference or Quantity

Description

Small Bear SKU

 

Fixed resistors are watt, 5% tolerance, carbon film or 1% metal film.

 

R1, R5

1 meg

R2

250K Reverse Audio Taper Potentiometer

 1012A

R3

470K

R4

10K Audio Taper Potentiometer

 1012A

R6

10K (for a 3mm high-brightness clear LED)

 2304
Rb,Rc,Rd,Re,Rf Selected bias resistors (see text)

C1

100 mf. 16 volt radial electrolytic

 1404

C2

.0022 mf. poly-film or mylar

 1150

C3

.015 mfd. poly-film or mylar

1101B

C4

47 mf. 16 volt radial electrolytic

1404

C2

.22 mf. poly-film or mylar

1103B

D1

1N5818 Schottky diode

2215A

Q1,Q2

Selected Germanium or Silicon (see text)

 

Pot Board and Connectors

 

1

"5-Pots" Board

0353

1

3/8" Mini-PCB standoff

0415N

2

Molex 3-pin vertical headers

0614S

2

Molex 3-pin plugs

0614U-2

6

Terminated leads

0614O

 

Main PCB and Connectors

1

Pad-per-hole perfboard or PCB

0355
2 Molex 2-pin horizontal headers 0614R-1

1

Molex 3-pin horizontal header

0614S-1
1 DC power connector 0614M
1 Input connector 0614N
1 Output connector 0614N1

1

Molex 6-pin horizontal header

0614T-1

1

Molex 6-pin plug

0614U-3
1 Strip of single-in-line socket pins 0706

6

Terminated leads (for the stomp switch)

0614O
Bare tinned wire 0509

Parts For The "Shell"

1 Bare Box #1 Enclosure 0350

J1

1/4" Shrouded Stereo Jack, Switchcraft #112B or similar

0604

J2

1/4" Shrouded Mono Jack, Switchcraft #111 or similar

0603

J3

DC Power Jack

0612D

S1

3PDT stomp switch

 0200

 2

Knobs for " shaft