Beam Switching Tubes

For technical data, please refer to the Burroughs MBST Specifications reference.

Beam switching tubes are thermionic decimal counting devices used in high speed applications through the 1950s and 1960s. They feature counting speeds up to 10 MHz (versus 1 MHz for the fastest dekatrons) and high electrical efficiency. Many types are direct-drive, producing output signals which can be coupled directly to a Nixie tube with minimal intermediate components.

The magnetic beam switching tube, alternately referred to as a 'magnetron beam switching tube', 'crossed field counting tube' or 'trochotron', operates on the principle of crossed electrical and magnetic fields: an electron beam is emitted from a centrally heated cathode, spirals outward under the influence of a permanent magnet, and sequentially strikes a series of spade electrodes. The potential between the cathode and a selected spade causes the beam to lock onto that spade, which passes current to its adjacent target electrode. Adjacent to each spade/target pair is a switching grid electrode which controls the positional advancement of the electron beam. Early MBSTs have a large cylindrical permanent magnet which encircles the glass envelope, while later types have 10 discrete internal magnets which also function as the target electrodes.

In addition to the conventional trochotrons featured here, National Union designed a number of 'radial beam' or 'commutator' tubes, a series of 18- and 25-position devices somewhat similar in construction to a trochotron, but with an external polyphase wound stator instead of a permanent magnet. Two electron beams are generated within the tube, spaced 180 degrees from each other. Radial beam tubes have a much simpler internal construction than a conventional trochotron, as beam focusing and rotation functions are controlled by the external stator. Radial beam tubes were not widely used, and cannot achieve the MHz-range switching frequencies of trochotrons.

Another somewhat similar device is the electrostatic beam switching tube, of which only one model was produced: the E1T. The E1T uses no magnets, and has a side-viewing phosphor screen which indicates count position.

Alfven-Romanus (Linear) Architecture



Ericsson engineers Hannes Alfven and Harold Romanus patented the first linear trochotron in 1946. These early trochotrons did not function as a ring counter, but were instead of linear construction, with an axially-positioned cathode. Although Ericsson developed at least one functional prototype, It was once believed that no linear trochotrons were ever commercially produced. This changed with the recent discovery of the Melz LP-4, an unusual Soviet design in a bizarre acorn-style envelope.

Backmark (Two Electrode) Architecture



The first cylindrical trochotron was invented in 1949 by Nils Backmark of L.M. Ericsson. Backmark's simple cylindrical trochotron was derived directly from the Alfven-Romanus linear trochotron, but introduced a centrally-positioned cathode which allowed fully circular ring counter operations.

The Backmark trochotron was intended for use as a selector in telephone switching systems, and had electrical characteristics which made it unsuited for computer use, namely the absence of a switching grid and a high amount of crosstalk. Both Ericsson and Burroughs experimented with two-electrode designs through this period; some designs featured ten discrete anode targets as specified in the original Backmark patent, while other designs had a single sleeve-type anode surrounding the spade cage. Ericsson also developed the RYG10, a MBST with a sleeve-type anode and phosphor-coated targets, which would allow the count position to be viewed in the same manner as a Dekatron.

Ericsson later produced a line of more conventional beam switching tubes under the Trochotron brand name, but only after incorporating a switching grid, developed at Burroughs several years after Backmark's original invention.

Fan-Kuchinsky (Three Electrode) Architecture



A somewhat improved magnetic beam switching tube was invented in 1952 by Saul Kuchinsky of the Burroughs Corporation. These early Kuchinsky tubes were still fairly undesirable in terms of electrical characteristics, but with the collaboration of Burroughs engineer Sin-Pih Fan, the Kuchinsky tube was approached with the following improvements: (1) reduce crosstalk noise to a minimum; (2) increase output current; (3) isolate beam locking, beam switching and other independent tube functions from each other; (4) provide a tube structure in which a change of potential at the output electrodes does not substantially tend to change the output current afforded by the beam over a wide range of output potential variations; (5) provide a tube structure which is capable of handling a signal having a high percentage of modulation without affecting stability of operation; (6) provide a tube structure in which the beam switching may be rapid and in which the unmodulated output signal approaches a square wave in form; (7) provide a tube structure which requires a minimum of external circuitry for operation. The product of these improvements is the Fan-Kuchinsky magnetic beam switching tube, suitable for use in computers and other precision applications.

The key structural advancement over the Backmark trochotron is the addition of a switching grid, which provides independent beam switching control. Like the Backmark design, the Fan-Kuchinsky tube has a large cylindrical magnet surrounding the glass envelope. The Fan-Kuchinsky tube would remain the definitive state of beam switching tube structure for several years. Kuchinsky and Fan also developed several other MBST-derived devices which apparently never made it to production, including a multi-decade counter, a single-tube BCD encoder and several types of beam switching flip-flop.

Kuchinsky-Wolfe (Four Electrode) Architecture



In 1960, Saul Kuchinsky designed a total of six different concepts for an internal-magnet device, with the goal of developing a smaller, lighter tube with improved means for providing a magnetic field. Burroughs engineer Roger W. Wolfe then added signal-improving shield grid elements which reduce crosstalk by absorbing stray electrons that would otherwise splatter on an adjacent target. These two improvements encompass the Kuchinsky-Wolfe magnetic beam switching tube, commercially known as the Beam-X Switch, the ultimate improvement of Backmark's original cylindrical trochotron.

Beam-X tubes were highly suitable for use as a Nixie tube driver, at a time when the demand for computer tubes was imploding. Burroughs was fond of noting in various publications that a Beam-X tube could replace 43 transistors and some number of passive components, and described the BX-1000 as the ideal Nixie driver. Nonetheless, by 1965 the rapidly encroaching transistor had nearly entirely eliminated the beam switching tube market, and by 1970 Burroughs had completely abandoned the beam switching tube in favor of Trixie transistor drivers and BIPCO integrated driver IC sockets.

Van Overbeek E1T



In 1946, Philips engineer Adrianus van Overbeek designed a unique, albeit highly impractical decimal counting tube, the E1T. Many such impractical counting tube designs surfaced in the 1940s and 1950s, but the E1T was the only one to make it to production. Standing alone among the many successful dekatrons and magnetic beam switching tubes, the E1T is an electrostatic beam switching tube, equipped with a directional electron gun, focusing, acceleration and deflection grids, and a side-facing phosphor screen which indicates count position in a highly unintuitive fashion. Although the E1T has theoretical frequency limit of 1MHz, practical counting speeds are much slower. All examples of E1T, regardless of brand name, were produced by the Philips sample department; the E1T was a difficult tube to manufacture and never went to large scale production.

Burroughs HB-101 / MO-10 / 6700
 Burroughs HB-101 / MO-10 / 6700

Devices included in this entry:

Haydu 6700 unshielded MBST (pictured in thumbnail)
Burroughs MO-10 unshielded MBST
Burroughs 6700 unshielded MBST (red label variation)
Burroughs JAN-CBSC-6700 unshielded MBST


The 6700 is the first mass-produced beam switching tube, developed by Burroughs as a 2 MHz counter for use in computers. All subsequent beam switching tubes developed by Burroughs are direct descendants of the 6700.

While many beam switching tubes are intended for computer use, the 6700 is currently the only decimal counting tube of any type which is confirmed to have been used in the central processor of a commercially produced general purpose computer.

Haydu Bros. began mass-producing Burroughs tube designs shortly after they were acquired by Burroughs in 1954. The earliest Nixie and beam switching tubes have dual branding, with Haydu in the foreground and 'Subsidiary of Burroughs' condemned to fine print.

This inexplicable inversion of due credit still causes confusion to this day, as many sources either partially or completely credit Haydu with the development of these tubes, yet there is no evidence that Haydu was involved in their development at all. The earliest known advertisement for the Haydu 6700 states that the 6700 was perfected by the Burroughs Research Center, and mass-produced by the Haydu tube division. The extensive patent history of Nixie and beam switching tubes also supports this assertion, with all related inventors listed as assignors of Burroughs Corporation.

By 1957, the misleading Haydu branding was abandoned. As a result, the 6700 and low voltage variant 6701 are the only Burroughs beam switching tubes known to be branded as a Haydu product.

Burroughs 6700 Datasheet (PDF)
Criteria for Selection of the Magnetron Beam Switching Tube as a Circuit Component (PDF)
Beam Switching Tube Circuits for MBS Tubes Types 6700 - 6701 (PDF)

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Burroughs 6701
 Burroughs 6701

Devices included in this entry:

Burroughs 6701 unshielded MBST (red label variation; pictured in thumbnail)
Burroughs 6701 unshielded MBST (black print variation)
Burroughs 6701 unshielded MBST (silver print variation)


The Burroughs 6701 is a low voltage variant of the original Burroughs 6700 with a maximum counting speed of 1MHz The 6701 also adds a 'grid zero' reset function.

Burroughs 6701 Datasheet (PDF)
Criteria for Selection of the Magnetron Beam Switching Tube as a Circuit Component (PDF)
Beam Switching Tube Circuits for MBS Tubes Types 6700 - 6701 (PDF)

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Burroughs BD-401 / 6702
 Burroughs BD-401 / 6702

The Burroughs BD-401 is specially modified 6700-type beam switching tube, designed to operate as a wide-band noise generator for use in AM/FM radio jammers. The BD-401 has only two functional sets of spade / target structures, with the remaining structures being replaced by anode elements. The anode supply voltage is set for cutoff operation, and the biasing of spades and targets is set to prevent stable states.

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Burroughs BD-301 / 6703
 Burroughs BD-301 / 6703

The BD-301 is an early shielded model with a large envelope and magnet, similar to a shielded 6700. It should be noted that at least one BD-301 print style includes a 6700 dual part number. This is odd, because the electrical characteristics of the 6700 and BD-301 are similar, but not identical.

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Burroughs MO-10R / 6704
 Burroughs MO-10R / 6704

The MO-10R is a high speed variant of the 6700, with internal 100K spade resistors and an atypical 20-pin base. The MO-10R is believed to be the fastest counting tube ever produced, with a maximum counting speed of 10 MHz. There is also a shielded variant of the MO-10R, the BD-309, which resembles the BD-301. The internal spade resistors are mounted at the top of the tube, between the upper mica sheet and the getter.

Criteria for Selection of the Magnetron Beam Switching Tube as a Circuit Component (PDF)

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Burroughs BD-203 & BD-316
 Burroughs BD-203 & BD-316

Devices included in this entry:

Burroughs BD-203 unshielded MBST (pictured in thumbnail)
Burroughs BD-316 shielded MBST


The Burroughs BD-203 and BD-316 are the only known 'compact' models of beam switching tube, with a slender envelope and external magnet. The series consists of the unshielded (and extremely rare) BD-203, and the shielded BD-316. The BD-203 and BD-316 are capable of counting at 1MHz.

The short-lived BD-203 is noteworthy as being the direct predecessor to the internal-magnet BX-1000, with which it shares similar electrical and physical specifications.

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Burroughs BX-1000 Series
 Burroughs BX-1000 Series

Devices included in this entry:

Burroughs BX-1000 unshielded Beam-X Switch (small shield grid variant; pictured in thumbnail)
Burroughs BX-1000 unshielded Beam-X Switch (large shield grid variant)
Electronic Enterprises BX-1000 unshielded Beam-X Switch (large shield grid variant
Burroughs BX-1003 unshielded Beam-X Switch
Burroughs BX-2000 shielded Beam-X switch


The BX-1000 is the first and best-known of the Beam-X Switch tubes. All susequent BX variants retain a nearly identical construction, though material and internal dimensional changes in later BX types result in various application-specific characteristics. The BX-1000 is capable of stable operation up to 3 MHz, and the tube's theoretical limit is around 4.5 MHz, assuming an ideal drive circuit with zero external spade capacitance. Burroughs never published a speed rating higher than 2 MHz, but internal developmental documentation confirms the higher attainable speeds. The earliest BX-1000s have smaller shield grid dimensions than later examples.

The BX-1000 is perhaps the most noteworthy for being far more substantially documented than any other type of beam switching tube. This is in stark contrast to subsequent Beam-X types, many of which are unregistered and seemingly undocumented part numbers.

Subsequent variations of the BX-1000 include the shielded BX-2000 and uncharacterized BX-1003.

Burroughs 6710 (BX-1000) Datasheet (PDF)
Burroughs ETM-149: Characteristics of Beam-X Switches in the Megacycle Range and Summary of Basic Operation (PDF)
Nixie Indicator Tubes: Characteristics & Circuit Design Data (PDF)

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Burroughs BX-1204
 Burroughs BX-1204

Little is known about the BX-1204, but it is believed to be a specialized noise generator, which may function in a similar manner to the BD-401 pictured above. The entire known part series (BX-1202, BX-1203 / 6713, BX-1204) is represented by only a few surviving examples.

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Burroughs BX-2004 & BX-2005
 Burroughs BX-2004 & BX-2005

Devices included in this entry:

Burroughs BX-2004 shielded Beam-X Switch (pictured in thumbnail)
Burroughs BX-2005 shielded Beam-X Switch


The BX-2003, BX-2004 and BX-2005 are unregistered, uncharacterized Beam-X variants, used in early Xerox high speed photocopiers.

Like the BX-2000, the BX-2004 and BX-2005 are equipped with a mu shield. Unshielded Beam-X types require a center-to-center spacing of two inches when installed adjacently, but shielded models permit much tighter adjacent installations.

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Burroughs BX-2012 & BX-2013
 Burroughs BX-2012 & BX-2013

Devices included in this entry:

Burroughs BX-2012 shielded Beam-X Switch (pictured in thumbnail)
Burroughs BX-2013 shielded Beam-X Switch


The BX-2012 is electrically equivalent to a BX-1000 or BX-2000, but has been 'specially tested for decoding BCD', which is Burroughs-ese for 'more fairy dust'. The BX-2012 is among the rarest Beam-X types, with very few known examples.

The BX-2013 is currently uncharacterized, but is believed to be a variant of the BX-2012.

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Burroughs BX-3000 / 6712 & BX-3001
 Burroughs BX-3000 / 6712 & BX-3001

Devices included in this entry:

Westinghouse BX-3000 unshielded Beam-X Switch (pictured in thumbnail)
Burroughs BX-3001 unshielded Beam-X Switch


The Burroughs BX-3000 is a high current Beam-X variant, known by only a handful of surviving examples. The BX-3000 may have been used as a microwave frequency noise source in radar jammers, as it was investigated for such use by the US Air Force in 1963. The Westinghouse BX-3000 shown here is particularly unique, because it is one of only a few known examples of a re-branded Burroughs beam switching tube.

The Burroughs BX-3001 is a completely undocumented type. The part number is not listed in available Burroughs documentation, and the device in our collection is one of the only known examples. If Burroughs numbering convention is being followed, the BX-3001 should be a variant of the BX-3000.

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Burroughs BX-4000
 Burroughs BX-4000

The BX-4000 is a low voltage variant of the BX-2000, and the only Beam-X tube with known specifications to be rated at 1 MHz instead of 2 MHz. However, the BX-4000 is capable of staying in the megahertz region at a much lower spade voltage and much higher spade resistance than the BX-1000. A BX-1000 powered to BX-4000 specifications would be riding the lower boundary curve of its stable operating region, and would not be capable of 1 MHz operation.

The BX-4000 was quickly superseded by the BX-4001, identical in electrical characteristics but capable of full 2 MHz operation. There are only a handful of BX-4000 / BX-4001 devices known to still exist.

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Ericsson RYG10
 Ericsson RYG10

Perhaps the most unique counting tube ever produced, the Ericsson RYG10 is an early two-electrode magnetic beam switching tube, with a sleeve-type anode similar to some Burroughs pre-production designs. However, the RYG10 incorporates a feature not seen in any other MBST: the RYG10 is capable of displaying its own count position, with a circular end-view readout comparable to a conventional Dekatron. Unlike a dekatron, which derives its readout from the ionization of its fill gas, the RYG10 is a thermionic device, and derives its readout from phosphor-coated targets located at each count position around the circumference of the anode sleeve. Each target is lit in succession as the electron beam sweeps the targets sequentially. The RYG10 has a maximum counting speed of 1 MHz.

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Melz LP-4
 Melz LP-4

The Melz LP-4 is a bizarre Soviet-produced linear trochotron in a large acorn-style envelope. Although the LP-4 is electrically similar to one of the linear designs described in the Alfven-Romanus patent, the LP4 is pseudo-circular, with its linear electrode array curved into an incomplete ring to facilitate installation within a compact cylindrical envelope.

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Philips Miniwatt E1T
 Philips Miniwatt E1T

The Philips E1T electrostatic beam switching tube works somewhat like a CRT, but instead of the beam being pushed around the screen by external stators, it is bumped along in discrete increments using electrostatic pulses. The pulse count is displayed at one of ten positions along a side-viewing phosphor screen with an externally applied digits label. Until the discovery of the rare Ericsson RYG10, the E1T was believed to be the only production beam switching tube with display capabilities.

The E1T did not see widespread use, as it is a temperamental device which offered limited advantage over more well-behaved alternatives. In recent years, the E1T has seen a surge of popularity among Nixie enthusiasts, owed to its on-board display characteristics and quirky appearance.

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