Don't Tell Ray
PT2399-Based Oil Can Delay
Overview
When it comes to delay, we are all aware of the tape, analog, digital, and even magnetic drum style delays like the Echorec. However, there is one design that has been almost lost to time, and that is the oil can delay. Invented by the founder of Tel-Ray, the oil can delay operates by using a spinning magnetic disk with carbonized rubber read and write heads. The disk spins in a can of special oil that helps to keep the rubber heads lubricated and to help prevent the leakage of the magnetically stored data.
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In the guitar pedal world, there has been a small resurgence of interest in the oil can delay due to its unique, murky sound. Old Blood Noise Endeavors and Catalinbread both have digital emulations of them, but as far as I could tell, no DIY designs existed. I decided to see what I could do with the ubiquitous PT2399. Over the course of a few months, I spent hours reading and watching everything I could find on them, detailing things like delay time ranges, RPM, methods of degradation, etc. so that I could take them into account as best I could. While I don't claim this circuit to be an exact emulation, it does a great job of getting some of that super old school vibe in a DIY-able circuit. And because it's supposed to mimic a Tel-Ray, I have decided to call it Don't Tell Ray.
How It Works
So what makes the Don't Tell Ray (DTR) different from other PT2399-based delays? Well, there are a couple of things and they have to do with some of the quirks of oil can delays. For example, many have a tube preamp which, in conjunction with the breakdown of the oil and rubber heads, can add some grit to the sound, so I decided to add a soft clipping set of diodes in the input buffer. Another thing about them is that, due to the spinning nature of the disk, modulation due to imbalances are sinusoidal, so the LFO is much more sinusoidal than the typical LFO scheme seen in other delays. So with that said, let's take a look at the circuit.
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As mentioned, oil can delays can have a gritty, dark texture to them, so I augmented the typical inverting input buffer with a pair of clipping diodes. This section doesn't get super dirty, as this isn't a distortion pedal, but it does give a little bit of hair if needed. However, because increasing the gain in the feedback loop raises the level of the output signal, we need to turn down the volume as the gain increases so that we don't have drastically different signal levels with different amounts of grit. To do this, I used a dual gang potentiometer with two additional resistors. One resistor sets the overall value of the potentiometer from lug 1 to 3 while the other resistor results in more of a quasi-log response to keep the volume effect smoother over the course of rotation. It's not perfect, but it works pretty well. After the volume control, we roll off a little bit of the high end, since oil can delays were typically rather dark. We don't want to lose too much, though.
Don't Tell Ray Input Stage
After the input stage, the PT2399 delay stage is fairly standard. The only item to note here is that the caps between pins 15/16 and 13/14 are larger than usual. This helps roll off just a little bit of the highs as well, in keeping with the general mellow tone of an oil can delay.
Don't Tell Ray Delay Stage
The remainder of the audio path consists of a tone control that allows for some real darkening of the tone, which can be useful if trying to replicate the really dark, murky tone of an oil can delay that has been around for years without the best of care. After that, it's a pretty standard mix control and output summing amplifier.
Don't Tell Ray Output Stage
One thing that really sets an oil can delay apart from others is the modulation produced. Because it is a disk spinning in a circle on a rod, the modulation tends to be sinusoidal and gets more pronounced if the disk becomes unbalanced. The balance control dictates the amount of this sinusoidal modulation. Because I sought to recreate the oil can delay, there are only two modulation speeds available. One corresponds to the guitar-oriented Adineko, which is about a 300 ms period, and the faster, organ-oriented Organ Tone delay, which has a wheel speed of about 150 ms. The LFO architecture is a truncated version of the exponentiated sine wave LFO from the ROG Tri-Vibe.
Don't Tell Ray LFO
The final portion of the circuit is the power section. This is a very standard PT2399 power section, with +9V, VREF, and +5V. Not a lot to say about that.
Don't Tell Ray Power Section
There we are, yet another PT2399 delay, but with a little bit of a twist. If you want to make one for yourself, you'll find everything you need right here. Happy swampi-ness!