Diode-Matrix and Transistor Organ-Control Systems

A diode matrix is a panel of semiconductor assemblies that performs the key switching and stop control a contact relay used to perform. Peterson describes its Diode Matrix Switching System as a custom-built electronic panel carrying out the key switching and stop control functions of a pipe organ, made up of modular assemblies of transistors, diodes, integrated circuits and resistors mounted on printed wiring boards in octave groups. In Peterson’s account the system replaces thousands of mechanical switch contacts with those semiconductor devices and is supplied completely wired, tested and documented.

Functionally, then, a diode matrix is a relay. It sits where the contact relay sat, between the console and the chest magnets, and it decides which magnets receive current; the deciding is done by semiconductor junctions instead of by contacts closing. The octave-group layout is a useful recognition cue, a rack of similar boards each handling a span of notes, but it is not unique to Peterson, so boards arranged by octave identify the generation more reliably than they identify a maker.

Removing the moving contact removes the wear that goes with it. Peterson claims for its Diode Matrix system silent operation with no moving parts, no contact sparking and therefore no pitting or burning, lower power consumption that avoids the need for large rectifiers, and expandability that permits future additions or changes without a cost penalty. In its specification wording, thousands of switch contacts are replaced by semiconductor devices that “can never pit, corrode or require adjustment.” Those are the manufacturer’s statements about its own product, and the durability language is Peterson’s rather than an independent finding.

The physical logic behind the central claim is straightforward. A junction that switches current without two surfaces parting cannot arc across a gap, so the pitting and burning that a contact accumulates over years of make-and-break do not occur. The trade is not free, though. Semiconductor switching has failure modes of its own, among them semiconductor and solder-joint failure and supply transients, and the absence of pitting is not the absence of faults.

In Burton’s key circuit a power transistor does the switching a key contact used to do. The transistor’s collector is connected to the negative supply. The key switch sits between the negative supply and the transistor’s base, and a resistor biases the base positive so that the transistor is held at cutoff, passing no current, until the key closes. When the key closes the transistor conducts, and current flows from it to the chest magnets. The key contact’s job is reduced to operating the transistor’s base; the transistor, not the contact, carries the current that reaches the magnet branches.

Each chest magnet hangs off the emitter as one parallel branch, and every branch contains a diode in series, with its cathode toward the emitter and its anode toward the magnet coil. That diode is the second half of the design, and the reason for it is the subject of the next section.

The series diodes are there to stop current taking paths the designer did not intend. The patent states that they are provided to prevent “sneak or phantom circuits through which current could flow in reverse to actuate some of the chest magnets and possibly cause damage to the transistors.” With many magnets sharing common supply lines, current that found a reverse path through one branch could energize a magnet nobody had played and load a transistor in a way it was not built for. A diode in each branch passes current in one direction only, so a branch can receive current but cannot become a return path for another.

The patent also notes a consequence of putting one diode in every branch: if a diode fails, the effect is confined to one note in one chest. That is a property of the architecture, a fault stays local to the branch it is in, and it is worth knowing as a matter of how the system is built rather than as a diagnostic method.

Stop control in Burton’s design is a second switching stage in series with the key stage. A second power transistor energizes a relay whose contact completes the circuit between the magnet common and the positive supply. A chest magnet therefore operates only when two conditions hold at once: the key transistor is conducting and the stop relay has closed. The stop side makes a magnet common live, the key side makes an individual note path conduct, and the magnet at the intersection is the one that pulls.

That is the stop switching function of a relay, carried by a transistor and a relay contact rather than by a stack of stop switches. In this particular design the stop stage still closes a relay contact; it is the note-by-note key switching that goes over to transistors. The patent states that its circuits apply to both direct-electric and electro-pneumatic pipe organs.

Diode keying changes the wiring rule because one key contact can drive many diodes in parallel without interaction between the circuits they feed. One practitioner’s published account of diode keying wiring calls this a fundamental change: with a diode isolating each destination, a single wiring harness of typically 61 wires per keyboard suffices regardless of how large or complicated the organ is.

The consequence is that the keyboard’s harness stops growing with the specification. Whatever else the organ is asked to do from that keyboard is handled downstream of those 61 wires, at the diodes, where each added function is another diode fed from the same contact rather than another contact under the key. This is the reason a diode system’s console wiring looks different from a contact system’s, and it is the practical gain that defines the generation.

Eliminating the contact did not eliminate the wire. Writing in Theatre Organ, the American Theatre Organ Society journal, in 1986, De Vault records that the first generation of solid-state relays used diode switches which “eliminated all moving contacts and increased reliability” but “did little to reduce the number of wires in cables and the number of solder joints.” The keyboard harness rule above fixed the console side; the cabling beyond it, and the solder joints at each end of every conductor, stayed.

De Vault attributes the wiring reduction to the later multiplexed integrated-circuit systems, which cut the cable at the cost of “very complex circuits” requiring large quantities of ICs, transistors and diodes. That is a 1986 snapshot, and the specific technologies it describes have been superseded, but the sequence it records is the one that matters when reading hardware: diode and transistor switching removed contacts, and multiplexing, a separate and later step with its own subject, removed conductors. A diode matrix in a console does not imply a multiplexed transmission to the chamber.

For its Diode Matrix Switching System, Peterson documents an operating range of 12 to 18 volts DC. Its standard conventions are a positive key common, a stop common that is positive as standard or negative to order, and a chest return common that is negative as standard. Those figures and polarities are Peterson’s specification for one product line. They are what the documentation leads a technician to expect on a Peterson diode-matrix installation, and they should not be read as a standard for diode or transistor switching in general.

What the specification does illustrate is that in this generation the polarity of each common is a designed parameter, chosen and written down, rather than an incidental fact of how the wires happened to be run.

Polarity matters on a semiconductor switching system because a diode passes current in one direction and blocks it in the other, so the circuit only works, and only stays protected, when current flows the way the designer intended. A contact is indifferent to which way current passes through it. A diode is not, and that is the point of putting one in each magnet branch: Burton’s patent places the series diodes there precisely to keep reverse current from reaching magnets and transistors through sneak paths. Reverse the intended polarity and the devices that were placed to block current now sit in the path of it.

The point is older than the transistor era. Alan Jackson, writing on servicing Casavant instruments for the Journal of American Organbuilding, states that Casavant used selenium diode relays as early as 1956 and that polarity must therefore be observed when working on those instruments. His article is undated and concerns one builder’s products, but the principle it states belongs to the device rather than the builder: once a diode is in the switching path, the direction of current is part of the design. An older diode-based relay is therefore not a place to assume that supply leads are interchangeable, and its polarity conventions are part of what has to be documented before any work on it is scoped.

The presence of diode or transistor switching establishes that the organ’s control work belongs to the semiconductor generation, and it identifies which kind of switching layer the instrument has. A transistorized relay is still the relay. An Organ Historical Society database record of a 1981 Wicks control-system change quotes a contemporary newspaper account that “the relays have become completely transistorized, which means there will be no moving parts to the main relay box,” a box a named Wicks representative called the instrument’s “electronic heart.” That box was doing the switching job the earlier relay had done, without contacts. The project came with a completely new console, and it is one instrument, quoted at second hand through a 2015 database note, not a statement of general Wicks practice in 1981.

Dating the generation itself rests on attributed accounts rather than a single settled date. Reuter states that after 1945 it pioneered diode relay technology, which it describes as the precursor of modern solid-state switching; that is a company self-description with no date, patent number or instrument named, and it establishes what Reuter says about itself rather than priority. Alan Jackson’s Casavant account places selenium diode relays as early as 1956. Burton’s patent was filed in 1965 and granted in 1968.

The start date of one long-running supplier of diode couplers is itself unresolved in the public record. As of September 1, 2026, the Solid State Organ Systems website states that the company has supplied pipe-organ control systems since 1968, and its Diode Coupler page states that it has built thousands of diode couplers since 1968. As of the same date, The Diapason’s directory entry for Solid State Organ Systems gives a 1969 founding, and an encyclopedia article on Solid State Logic states that company was founded in 1969 by Colin Sanders to make solid-state control systems for pipe organs, with the name coined to explain transistor and FET switching to organ builders. The supplier’s own material says 1968; the directory and encyclopedia accounts say 1969. Both are given here with their sources, and neither is treated as settled.

Semiconductor switching does not establish who built the organ or when, because in the documented projects it arrived as later console work on an instrument that already existed. The Wicks project above was a control-system change on an existing instrument, with the transistorized relay and a new console added to an organ that was already there. Patrick J. Murphy & Associates, describing one of its rebuilding projects as of September 1, 2026, states that the console shell and keyboards were retained while most console mechanisms were replaced, with Peterson solid-state relays and combination actions installed. The boards in that console date the console work, not the pipework or the original builder.

Two documented projects establish that this pattern occurs, not how often it occurs. They are enough to fix the rule, though: a diode matrix or transistor relay dates the control system, and a control system is not the instrument. How original and retrofit control systems are told apart, and which builders or suppliers used which hardware, are separate subjects and are not answered here. What the boards do tell an owner is which generation of switching has to be documented before any work on the console is planned.