# Vacuum tube DC amplifier circuit

I am currently working on a project to amplify a very small signal (about 10 millivolts) using a vacuum tube. I understand vacuum tubes were used as amplifiers and transistors back in the day, but I can't figure out how they were used to power anything with the tiny amount of current they can deliver through the anode/cathode.

I'm looking to power a small relay with the amplified pulse.

I'm obviously missing something.

Can someone explain to me just how these were wired to perform the same function as transistors?

• Why are you wanting to use vacuum tubes (given the known problems of heat, inaccuracy at DC, non-linearity, repeatability, cost, inefficiency, large carbon footprint and complexity) Feb 1, 2021 at 17:39
• I think you might not be missing anything. Vacuum tubes weren't really meant for high currents like power transistors nowadays. But a small relay would be doable I would think. en.wikipedia.org/wiki/Amplidyne But take what I say with a grain of salt. I know almost nothing about using vacuum tubes. Feb 1, 2021 at 17:40

First of all, vacuum tubes were not used as "transistors". They were, and still are, used as vacuum tubes. Transistors and vacuum tubes are different things but either can be used as an amplifier.

Where transistors are primarily current amplifiers, vacuum tubes are voltage amplifiers. The closest solid-state device to a vacuum tube (specifically a triode) is the JFET.

It is possible to get large amounts of amplification with vacuum tubes and examples are RF transmitters and RF amplifiers. Even today there are some applications, such as very high powered radio transmitters that still use vacuum tubes.

To understand how you get this to happen, remember that P = VI. To get more power you can increase either the current (I) or the voltage (V). Transistors increase the current. Vacuum tube amplifiers increase the voltage and can operate at very high voltages. In some cases this is what you want but with others you want a relatively low voltage output. For example audio amplification usually requires a low voltage low impedance (i.e. 8 Ohm) output. To match the HV high-impedance output of a tube amplifier you will almost universally see output matching transformers on the output stages of a tube amplifier. This steps-down the high voltage/low current signal to a low voltage/high current signal that you want for a speaker.

• Wow, the questioner really wants to do this the hard way! Building a DC amp, he's going to encounter offset and drift big time. Not much to add to @jwh20 excellent explanation, except to suggest using a "communications relay" with your vacuum tube circuit. These have coils with many turns, and can operate with a few milliamps of coil current. This is just one example for \$5: te.com/usa-en/product-1-1462000-8.html Feb 1, 2021 at 18:44
• Perhaps he's just wanting to learn about vacuum tubes. While generally obsolete and often totally ignored in engineering schools these days, they are still interesting and in some applications, indispensable. Feb 1, 2021 at 18:46
• I have a soft spot for those tubes too. Engineer for AM stations in my high school years, loved to watch the dull red glow of the plates keeping time to the music. Feb 1, 2021 at 18:49
• High-end audio, musical instrument amplifiers (especially guitar amps), and RF transmitter power amplifiers are the last frontier for these gems! I've never worked with them professionally but vacuum tube technology was still a part of entry-level EE curriculum when I was attending. Feb 1, 2021 at 19:26

I understand vacuum tubes were used as amplifiers and transistors back in the day, but I can't figure out how they were used to power anything with the tiny amount of current they can deliver through the anode/cathode.

Vacuum tube circuits typically ran at much higher voltages than today's transistor circuits. Typically at rectified and smoothed mains voltage, which in the US might be up to 170V depending upon the load and smoothing capacitor. Then, if the output needed to be of lower voltage and higher current (for example to drive an 8 ohm speaker), there would be a step-down (audio-frequency) transformer after the amplifier.