Editing Whirlwind I (section)

=== Vacuum tubes ===
The design used approximately 5,000 [[vacuum tube]]s.<!-- Ulmann has 4,221 in 1950 in table p. 62 -->

The large number of tubes used in Whirlwind resulted in a problematic failure rate since a single tube failure could cause a system failure.  The standard [[pentode]] at the time was the 6AG7, but testing in 1948 determined that its expected lifetime in service was too short for this application.  Consequently, the 7AD7 was chosen instead, but this also had too high a failure rate in service.  An investigation into the cause of the failures found that [[silicon]] in the [[tungsten alloy]] of the [[heater filament]] was causing [[cathode poisoning]]; deposits of [[barium orthosilicate]] forming on the [[cathode]] reduce or prevent its function of emitting [[electron]]s.  The [[7AK7]] tube with a high-purity tungsten filament was then specially developed for Whirlwind by [[Sylvania Electric Products|Sylvania]].<ref name=Ulmann>Bernd Ulmann, ''AN/FSQ-7: The Computer that Shaped the Cold War'', Walter de Gruyter GmbH, 2014 {{ISBN|3486856707}}.</ref>{{rp|59–60}}

Cathode poisoning is at its worst when the tube is being run in [[Cut-off (electronics)|cut-off]] with the heater on.  Commercial tubes were intended for radio (and later, television) applications where they are rarely run in this state.  Analog applications like these keep the tube in the linear region, whereas digital applications switch the tube between cut-off and full conduction, passing only briefly through the linear region.  Further, commercial manufacturers expected their tubes to only be in use for a few hours per day.<ref name=Ulmann/>{{rp|59}}  To ameliorate this issue, the heaters were turned off on valves not expected to switch for long periods.  The heater voltage was turned on and off with a slow [[ramp waveform]] to avoid [[thermal shock]] to the heater filaments.<ref name=RichTaylor>E.S. Rich, N.H. Taylor, "Component failure analysis in computers", ''Proceedings of Symposium on Improved Quality Electronic Components'', vol. 1, pp. 222–233, Radio-Television Manufacturers Association, 1950.</ref>{{rp|226}}

Even these measures were not enough to achieve the required reliability.  Incipient faults were proactively sought by testing the valves during maintenance periods.  They were subject to [[hardware stress test|stress tests]] called ''marginal testing'' because they applied voltages and signals to the valves right up to their design margins.  These tests were designed to bring on early failure of valves that would otherwise have failed while in service.  They were carried out automatically by a test program.<ref name=Ulmann/>{{rp|60–61}}  The maintenance statistics for 1950 show the success of these measures.  Of the 1,622 7AD7 tubes in use, 243 failed, of which 168 were found by marginal testing.  Of the 1,412 7AK7 tubes in use, 18 failed, of which only 2 failed during marginal checking.  As a result, Whirlwind was far more reliable than any commercially available machine.<ref name=Ulmann/>{{rp|61–62}}

Many other features of the Whirlwind tube testing regime were not standard tests and required specially built equipment.  One condition that required special testing was momentary shorting on a few tubes caused by small objects like lint inside the tube.  Occasional spurious short pulses are a minor problem, or even entirely unnoticeable, in analog circuits, but are likely to be disastrous in a digital circuit.  These did not show up on standard tests but could be discovered manually by tapping the glass envelope.  A thyratron-triggered circuit was built to automate this test.<ref name=RichTaylor/>{{rp|225}}