Testing an El84 Vacuum Tube with 9v Batteries

I  have always been interested in vacuum tube amplifiers, however the high voltages from the mains has always kept me from experimenting with them. About a year ago I played with an low power tube the 1t4 in a regenerative receiver.  The B+ was set to 45V powered from an array of 9v batteries.  I decided to revisit an old guitar amplifier project I started in high school using an EL84 power pentode.

The first part was to figure out a way to mount the tube so I could experiment with it. I found an small metal box that worked well for mounting the tube socket. I then took some wires from a broken ATX power supply and soldered them to the tube socket.

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After I had everything assembled I brought it to the test bench and wired up the test circuit below. I measured the current at the anode versus the voltage at the gate. I kept adding 9v batters to the B+ supply and noted the results.

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Below are the results I noted along with a graph. I also included the triode configuration graph from the original datasheet.

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So far the data looks consistent with the original datasheet for the tube in triode mode. The tube can sink a lot more current when VG2 is around 350V running in the full pentode configuration. Even with all 12 9V batteries I might be able to drive about 1watt of audio to a speaker. I was also surprised that the heater alone took around .7 amps at 6.3V. This is definitely not the most efficient way to drive a speaker I hope to get an proper impedance matching transformer so I can hear how it performers soon.

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One Response to Testing an El84 Vacuum Tube with 9v Batteries

  1. Occulist says:

    You are correct, tubes are not very efficient. That was why so few are in use today. But some of the attraction for people to construct tube amps today is the sound quality. The differences are subtle in many ways, however there are significant toanl differences between solid state and vacant state when it comes to guitar amps and synthesizers. Overdriving a solid state preamp stage produces a tone quality that is harsh; overdrive a tube preamp and you get an effect people like to duplicate.

    There is something to be said for an amplifier or a radio for that matter that glows in the dark by it’s own internal light and you will rarely find a radio as sensitive as a tube radio with a decent antenna.

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