DOD / MXR: 250, Distortion + component values table

This is probably where it all started in terms of opamp-driven distortion -  in one side of 741 opamp, out the other with some hard clippers.  

DOD 250 / MXR DISTORTION + SCHEMATIC

Direct link to Google Doc

741 SPECS

Numbers vary slightly by manufacturer and grade, but these headline specs are what give the 741 its “personality” in effects land:

• Supply voltage: works on dual or single supplies (it was designed with ± supplies in mind, but can be run single-supply with proper biasing).

• Gain-bandwidth product: around ~1 MHz class.

• Slew rate: about ~0.5 V/µs typical.

• Input bias current: tens of nA typical (bipolar input).

• Not rail-to-rail: inputs/outputs can’t swing anywhere near the rails, especially on a 9V battery. 

What that means in practice: a 741 can sound/feel a bit rounder and more easily pushed in simple dirt circuits, partly because it’s relatively slow and because it doesn’t have huge clean output swing on a 9V single supply.

LM741 PINOUTS

Note the little tab on the side of the opamp is pin 8.

OFFSET NULL

Offset null is a pair of pins on the 741 that lets you trim out its small DC offset so the output sits where you want it (often 0V on a dual-rail supply). 

In pedals it’s usually ignored because stompboxes are typically single-supply with a virtual ground and AC-coupled stages, so capacitors block DC and the tiny offset isn’t audible.

While the pins aren't used, be sure to isolate them anyway.

GEC: Germanium Signal Transistors (1971 Semicondcutor Handbook)

From the General Electric semiconductor handbook, 1971

BE: Fuzz Buffer

Not that anyone is likely to, but please don't rush out to build this - it's a concept that should work, but I have not tested it yet.  Might need some fine-tuning.

It's a combination of the AMZ pickup simulator and a Pete Cornish buffer - so basically a fuzz buffer to drop in front of anything that does not like being behind other pedals.  Typically a fuzz face.

out from previous pedal  >  buffer  >  into a transformer (pickup coil)  >  regular guitar volume and tone control > into the fuzz pedal.  

I have a few transformer options to experiment with, so see how that works out.  Switching might be interesting, as I think a loop would be good here.  i.e.  switch the fuzz on/off via the buffer.

A PNP buffer can be done by inverting the power supply and using a PNP transistor (works on LTspice).  This might be handy for building into PNP fuzz, with needing an inverter.

This is similar in function to the Aion FX Proteus, which uses a 42TL019 transformer.  

BE: Fuzz Face with external bias control

Just jotting down some notes, cause I doubt anyone else will be using this layout.  It's a little impractical in parts, but I just had some spare board this size, and a pair of transistors that I liked on the breadboard that needed a home.  It's a progression on this somewhat more practical layout.   

If you're wondering what the BE part is about...  

BE FUZZ FACE V3 - EXTERNAL BIAS

Notes:  all links under the board.  Maybe reduce the 10k bias pot a little with a parallel resistor.  Cleans up well, solid fuzz.  Breadboard values: 1k / 6k8, maybe 5k6 at around 30 degrees.  10uf bypass cap, battery 8.5v

BELL & HOWELL: Model 601 amplifier

Picked up another Bell & Howell Gaumont today, with the speaker cab - a Model 601 from 1950 (based on the capacitor dates).  Nice to see the original speaker still in the cab - I gave a quick test, sounds fine.  

The 601 is the same amp as the 621 (full details here).  The 601 and 621 amplifiers share the same part number (202000); I believe the model relates to the projector more so than the amp in some cases.

It came as the complete package with the projector and what appears to be a really exciting Australian training film about bandages from 1975.

Not sure where I will go with a conversion just yet.  I did quite like the last one that I did using an early Gibson circuit - so that's probably top of the list at the moment.  

BELL & HOWELL PROJECTOR AMPLIFIER - MODEL 601

A few minor differences with this one - transformer ends are painted, and there are a couple of large wirewound resistors that I've not seen in these before - quite good quality by the look of them.  

OUTPUT TRANSFORMER MEASUREMENTS

~8.4K with 16 and 8 ohm loads

Primary DCR 470 Ω 

10.35 VAC across the primary and measured two secondary taps:

• Tap A: 0.454 VAC

  • Turns ratio (Vp/Vs): 22.80:1

  • Impedance ratio (N²): 519.7

  • 0.454 V with 16Ω → 8.32 kΩ p-p

• Tap B: 0.319 VAC

  • Turns ratio (Vp/Vs): 32.45:1

  • Impedance ratio (N²): 1052.7

  • 0.319 V with 8Ω → 8.42 kΩ p-p

So far, on all of these that I've had, the plastic on the fuse holder breaks when I try to check the fuse.  Not great, as this leaves an uninsulated 110v conductor sticking out the back.  Bit of a death trap, but it will be replaced with a new one.  

The really large tin can capacitor was not great - it was heating up at relatively low voltage, and did not want to reform.   So time to go.  

I like that, back in the day, someone clearly thought about protecting the speaker, adding this tea-cosy-style cover.   These are normally branded as B.A.F Type 60 (British Acoustic Film) - which look just like a Goodmans Axiom from the same time period. 

The valves mostly tested OK - one of the 6V6 is a bit average, but not from the output pair, so that's good.  There is a mix of English Mullards, Australian Miniwatts.

Bell & Howell Part Number 202000

BE: Fuzz Face, SZ-3 (Sziklai pair)

Just one of my little side projects - it's a pretty hot day here, so not one for playing with germanium transistors in a big black box with a tin roof (also known as my garage / workshop).  The leakage goes nuts, and they sound like balls...  

While playing around with a bunch of old low-gain silicon transistors with moderate success, I remembered the Sziklai pair.   A minute later, and it was silicon fuzz heaven, so I had to come up with a layout to build one, hence the post.

BALMORAL ELECTRIC, SZ-3 FUZZ FACE

There are a few random values used that obviously aren't for everyone.  They just happen to be what's on my breadboard.   But hey, the post isn't for everyone; it's mainly for me to come back to later.

Transistor types are not listed, in part cause I can't recall what is on the breadboard.  They are all very low gain silicon transistors - I think about 50hfe or thereabouts.  I'll update the post with details later.

The Szik collector measured about 4.3v on a 9v supply.

SZIKLAI PAIR

What's a Sziklai pair?  It's a bit like a Darlington pair, but it uses an NPN and PNP transistor to create a single high-gain transistor out of the two low-gain devices.   They tend to act more like a single transistor than a Darlington does, which makes it handy for a fuzz face.

Before you get too excited, this works best with silicon; it's not a magic bullet for not so great germanium transistors, as leakage and noise are also multiplied.  

FURTHER READING

Small Bear has an article on Darlingtons and the Szik