Re: Tektronix 570 Vacuum Tube Curve Tracer on eBay


 

Hi Greg,

That is less likely than you think. Almost since the beginning Tek has designed its circuitry using what is known as a "Long-Tailed Pair". One of the features of this circuit design is its high immunity to the parameter changes a tube undergoes as it ages. The circuit is stable until the tube is worn out. So you have no guarantee that a tube you pull out of a Tek scope will work. In addition there are so many people selling used tubes that there isn't much profit in it unless, like the eBay seller that was mentioned, he uses a 570 to match a pair of tubes and certifies that they meet the basic manufacturers specs for transconductance (gm), voltage gain (mu), and plate resistance (rp). That is a time consuming process requiring a very expensive tube tester. So this isn't a lucrative pastime.

Dennis Tillman W7PF

Here is an excerpt from Wikipedia describing the history and benefits of the "Long Tailed Pair":
The long-tailed pair was developed from earlier knowledge of push-pull circuit techniques and measurement bridges. An early circuit which closely resembles a long-tailed pair was published by British neurologist Bryan Matthews in 1934, and it seems likely that this was intended to be a true long-tailed pair but was published with a drawing error. The earliest definite long-tailed pair circuit appears in a patent submitted by Alan Blumlein in 1936. By the end of the 1930s the topology was well established and had been described by various authors including Frank Offner (1937), Otto Schmitt (1937) and Jan Friedrich Toennies (1938) and it was particularly used for detection and measurement of physiological impulses.

The long-tailed pair was very successfully used in early British computing, most notably the Pilot ACE model and descendants, Maurice Wilkes’ EDSAC, and probably others designed by people who worked with Blumlein or his peers. The long-tailed pair has many favorable attributes if used as a switch: largely immune to tube (transistor) variations (of great importance when machines contained 1,000 tubes or more), high gain, gain stability, high input impedance, medium/low output impedance, good clipper (with a not-too-long tail), non-inverting (EDSAC contained no inverters!) and large output voltage swings. One disadvantage is that the output voltage swing (typically ±10–20 V) was imposed upon a high DC voltage (200 V or so), requiring care in signal coupling, usually some form of wide-band DC coupling. Many computers of this time tried to avoid this problem by using only AC-coupled pulse logic, which made them very large and overly complex (ENIAC: 18,000 tubes for a 20 digit calculator) or unreliable. DC-coupled circuitry became the norm after the first generation of vacuum tube computers.

-----Original Message-----
From: Greg Muir
Sent: Sunday, January 05, 2020 9:44 AM
Subject: Re: [TekScopes] Tektronix 570 Vacuum Tube Curve Tracer on eBay

I'm sure that he probably has numerous Tek scopes and plugins in his back room missing all of their tubes. They will most likely be the next thing to show up on ePay.

Greg




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Dennis Tillman W7PF
TekScopes Moderator

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