![]() The TTL output doesn’t really care if its sinking a hundredth of 20 mA or a four hundredth of 20 mA so regardless of exact transistor beta value it’ll be “good enough” its going to sink a lot less than old fashioned TTL max of 10 mA or 16 or whatever it was back when there was only TTL and not a dozen extended logic families… So there were entire databooks on the topic of which transistors were “mighty similar” to each other enough to make it worth considering a substitution. Both of them have a Vce WAY way WAY over 5 volts, both have an Ic-max that laughs at LED levels of current. On the other hand there were designers who were, like, flicking on a LED, there’s not a major difference between a 2n3904 and a 2n5089. So you could pay a mfgr a lot of money for “matched pair” packages of two transistors for like 10x the cost of one, or you could buy the rat shack qty 25 “best of luck to you” generic NPN transistor bulkpack and spend some labor making your own matched pairs. ![]() The manufacturers wanted to ship anything that wasn’t technically an open circuit or short circuit so datasheet specifications were … a little wider than you’d see on a 2021 transistor datasheet, things are a little more constrained now and more likely to match out of the box, but not so much in 1970. On one hand, some designers would make push-pull output stages with minimal feedback and “wildly imbalanced” Vbe and hFE values on the two transistors would result in audible distortion. Posted in Arduino Hacks, Tool Hacks Tagged tube, tube tester, vacuum tube Post navigation If you don’t remember tube testers in drug stores, you might find that TV repair, at one time, was a big business. One of the problems with a truly general-purpose tube tester is connecting to the different pinouts. He has plans to maybe make a more general-purpose tube tester. However, as points out, the tester is very specific to his application. Grabbing the data into a spreadsheet allowed some curve tracing which looked useful for matching. The converter goes through a voltage divider so the maximum 24 volts won’t overload the Arduino. The tube is wired up in a particular configuration and the Arduino makes a few measurements while changing the operating bias conditions. The bulk of the circuit is creating the voltages required, including a 555 charge pump to generate around -10V. He does have a traditional tube tester, but it uses 100s of volts which is a different operating regime. The tester only uses 24V, but for the projects he’s building, that’s close to the operation in the real circuits. You can see the results in the video, below. As has been working on some tube-based projects, he decided he needed a tester so he built one. We aren’t sure that’s a testament to capitalistic ingenuity or an inditement of tube reliability - maybe both. There was a time when people like us might own a tube tester and even if you didn’t, you probably knew which drug store had a tube testing machine you could use for free.
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