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Some experiments with vacuum tubes

I've wanted to build a radio (either RX or TX) with a vacuum tube or two. This page will document my attempts at getting familiar with this old technology.

The basics

Looking in my collection of tubes, I had a bunch of common types to pick from for experiments, most being pentodes. I hoped that ignoring the grids (leaving them floating) while putting some volts between Anode and Cathode would cause a current to flow when the filament was lit. I tried this with the couple of EF91s I had in the collection with no results; it's possible the tubes are dead.

The other tube type I had a few new old stock (never used, still in original packaging) were EF86 low noise audio amplifier pentodes. Again leaving the grids floating didn't let any current flow though the tube, however unlike the EF91s I tried, grounding the grids did cause a current to flow.

Simple grounded grids pentode-diode

The resistor and neon combination limit anode current to around 50 µA. If the HT is switched on before the heater voltage, the current slowly climbs as the cathode is heated and begins emitting electrons. The neon starts to glow before the meter needle even starts to move. The voltage measurements at various points in the circuit look reasonable, though I was surprised with the large voltage drop across the tube, I'd expected it to be lower resistance than the anode resistor. This may be an effect of the grounded grids.

Circuit node Voltage (V)
Vpsu 208
Vresistor 21.3
Vneon 56.2
Vtube 123.3

This circuit was lashed together on the bench with crocodile clips connecting to the tube pins. I'll need some proper bases before I can try to make the valves do my bidding - crocodile clips are a bit cumbersome to say the least.

A simple pentode audio amplifier

This circuit is taken directly from the Mullard application notes for the EF86. As soon as I get a proper valve base I'll build and test the circuit.

EF86 Audio Amp

The application note offers two parts lists, depending if you want high gain or lower noise.

Part Number High gain Low Noise
R1 390k 1.0M
R2 100k 220k
R3 1.0k 2.2k
R4 330k 680k
C1 220nF 220nF
C2 1uF 1uF
C3 100uF 100uF
C4 1uF 1uF