Some of you may remember a recent project that featured on these pages, a 555 timer reproduced using vacuum tubes. Its creator [Usagi Electric] was left at loose ends while waiting for a fresh PCB revision of the 555 to be delivered, so set about creating a new vacuum tube model of a popular chip, this time the ubiquitous 741 op-amp. (Video, embedded below.)
The circuit is fairly straightforward, using six small pentodes. The first two are a long-tailed pair as might be expected, followed by two gain stages, then a final gain stage feeding a cathode follower with feedback. It’s neatly built on a PCB with IC-style “pins” made from more PCB material, then put in a huge replication of an IC socket on a wooden baseboard.
The result is an op-amp, but not necessarily a good one. He looks at the AC performance instead of the DC even though it’s a fully DC-coupled circuit, and finds that while it performs as expected in a classic op-amp circuit it still differs from the ideal at higher gain. The frequency response is poor too, something he rectifies by replacing the feedback capacitor with a smaller value. Sadly he doesn’t look at its common mode performance, though we’d expect that without close matching of the tubes it might leave something to be desired.
It’s obvious that this project would never be selected as an op-amp given the quality of even the cheapest silicon op-amp in comparison. But its value is in a novelty, a talking point, and maybe a chance to learn about op-amps. For that, we like it.
We covered the vacuum tube ..
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