Audio synthesis via vacuum tubes/Using sheet-beam tubes (view source)
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'''Using sheet-beam tubes''' has no parallel in the transistorized age. Between the mid 1930s until integrated circuits became commonplace in the 1960s, special electron tubes were designed to take advantage of specific effects of electron-beam ballistics. Essentially, these tubes were "integrated circuits" before the term even existed. And they did fairly complex things using very simple designs.<ref name="usbdt">[https://web.archive.org/web/20171020033858/http://www.cgs.synth.net/tube/beam.html Using special beam-deflection tubes]
== Background ==
[[File:cgs_photo_6BN6.jpg|162px|thumb|right|6BN6 sheet-beam tube]]In 1954, the expanding need for single-sideband communications equipment caused RCA to introduce a special tube for use as a balanced modulator. The 7360, the first of the "sheet-beam" tubes, is often seen in Collins and other two-way radio equipment of the period. Because there are obsessive Collins collectors, 7360s have become scarce and costly. However, the tube manufacturers were always looking for new markets that could be satisfied with tubes made on their existing production equipment, especially the huge television business. So lower-cost versions (with a different pinout, to prevent cross referencing) of the 7360 were made, primarily under the numbers 6AR8 (believed to be a Tung-Sol product), 6JH8 (Sylvania) and 6ME8 (apparently GE). These tubes were almost identical to each other, even though they were made by different factories--typical of the tube business--standardization was forced upon them by TV set manufacturers, who wanted to use these beam tubes as video demodulators. Another family of sheet-beam tubes was the 6HW8, believed to be European in origin and quite different in pinout from the others. And there's the obscure 7763, having yet another pinout.<ref name="usbdt"/>
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The sheet-beam tubes were modified pentodes. Between the suppressor grid and a pair of matching anodes, there were placed two beam-deflection plates. By putting a push-pull modulation AC signal across these two deflection plates, the electron beam could be "forced" to switch from one anode to the other; giving a very good ring modulator effect, with reasonable carrier suppression (provided the input and modulation signals were fairly high in peak voltage). So far, this sounds similar to the differential-amp balanced modulator seen in IC form.<ref name="usbdt"/>
However, the tube has something the IC doesn't.......... a screen grid. By varying the screen voltage between the plate supply voltage and the actual plate idle voltage, drift effects take over, and the peaks of the waveform start to fold over on themselves. Moreover, this folding is not sharp, as in the famed [[Ken Stone/1973 classic Serge/Serge tripple wave-shaper|Serge waveshaper]] circuit of the 1970s. It also happens on top of the modulation, causing some very strange and distinctive effects.<ref name="usbdt"/>
== Circuits ==
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== External links ==
* [http://metasonix.com/ Metasonix]
* [https://www.modwiggler.com/forum/viewforum.php?f=13 Metasonix] subforum at Mod Wiggler Forum
* [http://www.nj7p.org/Tubes/PDFs/Frank/093-GE/6BN6.pdf 6BN6 data sheet]
* Mouser [https://www.mouser.co.uk/ProductDetail/RECOM-Power/R12-100B?qs=%2Fha2pyFaduikktQIuFSUoAlyCfBOgsEWTQI0oSVVCeJHhELUNNCVHQ%3D%3D RECOM Power R12-100B 5W DC/DC converter] - Takes 12 VDC and outputs 135V at up to 50mA, or [https://www.mouser.co.uk/ProductDetail/RECOM-Power/R15-100B?qs=%2Fha2pyFadujT2%2F2uiGzLeAHh0JKwi3RAPisAPJgodY8%3D R15-100B] 15V input version."<ref>[https://www.modwiggler.com/forum/viewtopic.php?f=17&t=219618 Re: TS-23 Tube VCO help] by metasonix, Mod Wiggler Forum, 22 May 2020</ref>
[[Category:Vacuum tube synthesizers]]
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