If you connect the filament pins (2 & 7) together a 1B3GT, or similar vacuum diode makes an excellent HV capacitor capable or at least 15KV. Filament power is not applied. You can then use it as the upper cap in a high voltage capacitive AC voltage divider. The plate to heater capacitance is typically 3-4 pf. With a 100 pf capacitor in series you will have an approximately 34:1 divider. Note that the lower (larger) capacitor does NOT have to have an especially high voltage rating; a 100 pf 500 volt cap will work fine. If you use a 1000 pf lower cap for a 340:1 divider a 50 volt cap has sufficient ratings. The usual warnings apply; Don’t touch high voltage devices or associated circuitry when under test.
An interesting box rarely seen today. Internally, it’s just an attenuator and a tunable bridged T network. Operation is simple: Select 400 Hz or 1 KC. Apply signal to the input terminals at corresponding frequency. View residual on a ‘scope. Fine tune oscillator/source frequency for minimum deflection. Adjust the balance control for minimum deflection. Repeat if needed. Percent distortion is peak-to-peak output voltage divided by input voltage times 100. Unlike HP’s active distortion analyzers this does not suffer from ground loops. The 320A can also be use as an attenuator. I am not sure when this was first manufactured but it is mentioned in the August 1951 issue of The Hewlett Packard Journal.
From the sublime (866) to the insane: Tung Sol 7791A hydrogen diode. Current rating: is 8 peak/2 ave. amperes and 25000 peak inverse voltage. Filament eats 15 amperes @ 5 volts. At 20 amperes forward current (Way outside its ratings) the 7791A drops only about 20 volts. It’s not as big as you would think: approx. 15 cm high, base flange 11.5 cm diameter.
The 3B28 has the same pin out and filament voltage (2.5 volt @ 5 amps) as the 866 but is filled with Xenon gas rather than mercury. Peak inverse anode voltage is 10000 volts at peak plate currents of 1 ampere or less and 5000 volt at peak plate currents of 2 amperes.
The 3B28 can operate over a wider ambient temperature range (-75 deg C to 90 deg C) than the 866 and can be operated in any position; the 866 must be operated vertical, base down. Contrary to popular belief 3B28’s do require warm up before high voltage is applied. The Altec 260A amplifier uses a pair of 3B28’s in its full wave power supply to provide B+ to a pair of 813B tubes operating push-pull.
Here’s an ancient RCA 866 (non-A, S envelope) being given a quick test on Triplett 3444A tube tester. At the top of the envelope you can see the characteristic blue glow of mercury vapor. Not a good idea to stare for a long time at the tube as the light emitted is rich in UV. Photo, of course, is safe.
- One bigger tin can
- One smaller tin – small enough to fit inside the big one without touching the side
- two short pieces of wire
- a sandpaper
- a soldering iron
Wash the cans and make sure they are thoroughly dry. Use sandpaper to rough up and clean small patches on the outside ends of both cans. Tin a small spot on the roughed up section with the soldering iron. Solder a short wire to both ends. Use some kind of insulator so that the cans don’t short together – I used two cardboard strips. These have the added benefit of making the capacitor a variable one by pulling the small can out of the larger one (C decreases). My capacitance meter tells me this one is about 36 pf. Hi Pot test say it’s good for about 2 kilovolts. I went higher, it arced over and I smoked part of the cardboard. Small pieces of fiberglass or nylon would make better spacers.
An old vacuum rectifier. Filament: 5 volts @ 4 amperes, Peak inverse plate voltage: 20,000 volts. This version has uranium glass seals around the pins. (Greenish glass ) Uranium glass is also known as “Vaseline glass”. It is made by adding uranium, usually in the form of uranium oxide diuranate to glass. Uranium glass will glow under ultra violet light. There were several types of table wear made with uranium glass in the early to mid 20th century, the most famous being Fiesta Wear.
The EIMAC 15R was used in the WW II era ASB-5 radar display unit.
First person to visit today gets a EIMAC 15R for free!
Another beauty. Model 432 No. 13661. Weston made several versions of this meter with different full scale and voltage/amperage ratings. On the left are terminals for 75 volts and 150 volts. On the right are terminals for up to 30 amperes RMS. Full scale is either 1.5 KW (75 volts) or 3 KW (150 volts). Size 12.7 cm high, 12.7 wide and 8 cm deep.