Copyright 1999-2002 R.G. Keen. All Rights Reserved.
Before digital delays, before analog bucket brigade delays, there was an effects technology that subbed in for tape delays that was portable and relatively easy to use for floor mounted items. This was the rotating oil can delay, and here's how it works.
Everyone is familiar with magnetic storage - you move a substrate containing magnetizable particles past a recording head that has an alternating magnetic field in it. The magnetic field polarizes the magnetic direction of the particles on the substrate and an "image" of the magnetic field alternations is thereby stored in the particles. Reading is the reverse - you run the substrate past a pickup head with many turns of hair-thin wire and the magnetic field in the particles causes a voltage to be induced in the coil of wire, reading the info that was recorded.
There are usually "dual" operations for all magnetic and electronic operations, interchanging electric field for magnetic field and capacitors for inductors. This is no exception. If you put charge into a capacitor, it holds the resulting voltage, a crude form of storage. If you have many incredibly tiny capacitors, you can start making a fairly good representation of a varying voltage. This is in fact the way bucket brigade delay chips work.
There is another way to do electrical field storage. Insulating materials can be given an electric charge, as we all know. Just wear rubber soled shoes and walk across pile carpeting on a dry winter day, then touch a doorknob. The motion of the shoes across the carpet stored a charge on the shoes (and then you) that was expressed visibly and audibly when you touched the doorknob.
In a similar way, if we have a fine brush of conductive wires, and arrange an insulating belt to be moved past, just touching the brush, we can put a large AC voltage on the brush and some of the electric field will be captured on the surface of the belt. Since the belt is an insulator, the charge can't go anywhere, so the electrical charge forms a replica of the voltage on the brush. Each tiny area of insulating surface is in fact acting like a micro-miniature capacitor, storing the value of the voltage from the brush at the instant the brush moved away from it, just like the magnetic particles in a tape machine store a replica of the magnetic field from the record head.
The tiny voltage-carrying capacitors are carried off as the insulating belt moves. The voltage would eventually leak off into the air if we let it. We can instead choose to keep it in a dry environment for a while, and "read" it later with a very high impedance amplifier. It turns out that vacuum tubes are ideal for both the writing (at high voltage) and reading (very high impedance) of such capacitive storage, and indeed the first oil can delays were tube based. Later as semiconductor technology got better, transistor and FET read and write amplifiers were made for the oil can delays.
So why the oil? What's that do? Remember that business about leaking into the air? The oil provides a sealable insulating layer over the insulating belt so the charge is trapped inside and has a hard time leaking into the air. The brushes reach right through it to put in/take out charge, and the voltage is protected from leaking away.
The oil is the center of a controversy - the original oil is reputed to be a hazardous material, carinogenic, etc. Is it? Maybe. The best insulating oils available at the time the oil can delays were designed were transformer insulating/cooling oils. These were definitely polychlorinated biphenyl based - the same "PCB's" that are now banned from all use as containing deadly dioxins. The only question is whether the oil can delay makers used that stuff or something else entirely.
If you're restoring an oil can delay that is now dry, what oil do you use? I've heard of using mineral oil from a pharmacy, Singer brand sewing machine oil, even 20 weight motor oil, all said to work to some degree. However, the reports have been decidedly mixed.
I recently stumbled onto the Tel-Ray page (http://www.geocities.com/tel_ray/home.html) where some of my intuitions on oil can delays were confirmed, and where I found a reference to the original patents on the technology. You can look them up at the US patent office web site if you're interested. Look for US Patents 2892898 and 3072543. From what I read, we might be able to make new, functioning oil cans - they don't look like rocket science ( as old-hat as rocket science seems now, even ).
The principals in the Tel-Ray page have now confirmed some of my guesses, and have graciously extended the info a great deal. From the patents, it is clear that the original oil can delays were just as I guessed, capacitive storage devices. However, the second and third patents delve further into the lubricating medium. It seems that by carefully dinking with the lubricating/insulating oil and doping it with various things to get conducting particles spread out in the oil, you can make for a higher signal level stored in the rotating capacitor, and hence better signal recovery, lower noise, and all-round better performance.
Zak Izbinsky, Richard Bills and Jamie Ray dug out the detailed info, as
displayed at the Tel-Ray site. The "real stuff" replacement oil for the oilcan effects is Union Carbide "Ucon" LB-65 oil.
It is not carcinogenic, and not PCB oil.
The second method
I've also come across a second method for "oil can" delays. I put the quotes in because it doesn't use any oil. I got a Vox Echo-Reverb model V807 recently. It had a typical oil can delay... until I looked further. There's no oil, but not only that, it looks like there never was any. Multiple spring contacts are held against a rotating disc in a machined aluminum housing with a counterbalancing spring mechanism to keep them all just touching. It looks like this one is intended to get around the Tel-Ray patents. I'll know more when I get the bugs fixed and get it running.