TubeTime

We’ve done it again! My friends at Evil Mad Scientist Laboratories have a new kit for sale. Following on the success of our Three Fives discrete 555 timer kit, we’ve had a lot of requests for a discrete 741 op-amp.

The XL741 is based on the datasheet schematic of the original uA741 op-amp IC from 1968. You can wire it up in a classic op-amp circuit and probe nodes inside the IC so you can see how the chip works. Play with differential pairs, modify the compensation, and change bias currents to your heart’s content!

At the electronics flea market on Saturday, I found a 3ABP7 dual gun CRT. This one was built by DuMont, most likely intended for the 3″ version of their 5″ Type 279 Dual Beam Oscilloscope.

So of course I had to fire it up. There are two sets of deflection coils, so I drove them with one deflection board and cross-wired the deflection coils to flip the image around on the second gun.

The guns themselves have a common cathode connection and separate grids, which forces me to drive them both in parallel since my deflection board video amplifier keeps the grid at a constant potential and drives the video onto the cathode.

Here’s a closer look at the guns. The filaments are connected in parallel so this tube uses twice the normal current.