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Photoelectric valves?


2004-12-25 8:36 pm
I had a cool idea today - instead of relying on the termoelectric effect to get electrons into a vacuum, why not use the photoelectric effect? You can get UV LEDs these days, you could focus one onto a peice of metal with a low work function, and hopefully the UV light would knock lots off the electrons off of its surface - this would be your cathode! You could even try amplification by varying the brightness of the LED instead of the voltage on the grid if the LED was fast enough. This seems like a really interesting idea. I'm sure a grid would probably work better though. Think about it - no worn out heaters, just a simple LED you could replace! (Would need clever design of the tube so the LED wasn't in the vacuum). Any thoughts?
hi!, i think they already exist!
There is a dutch article on internet....i'll try to translate it into english.....

Operation of a photomultiplier tube:

A photomultiplier tube or PMT is a detector which uses the photo-electric effect to convert light into an electrical current.

The main parts of a PMT are the photo-cathode, the dynodes and the anode. They have been brought into a vacuum glass tube

The Dynode-stage:multiplying electrons
A photon goes into the tube trough the glass. it loosens an electron from the photo-cathode

The electron is led into the dynode-stage by a 'focus-electrode'
Because of the electro-magnetic field the electron is accelerated before it strikes the first dynode. Here the impact will loosen more electrons.

These electrons in turn will by attracted by the next dynode and be accelerated..
At every dynode more electrons will be loosened until all electrons are caught at the anode

Because of the number of electrons now available are enought to measure a current....out of the 1 foton it began with...

Well it's quit a long article, i'll summarize the rest.

The dynodes are on high + voltage, the photo-kathode is held at a very negative voltage.. to get the electrons to go to the next dynode every following dynode will be on a higher voltage then the one preceding it...(else they would stick to one dynode)
These voltages are created using a voltage divider

Because the density of the electrons getting closer on the way to the anode, the em-field there will be disrupted by the space-charge, by using bigger values of resistance at the anode the em-field will be restored which will benefit respons and linearity...

The basic idea seems workable, but some practical problems are an issue I think. The grid will cause a lot of shadowing of the cathode, so will reduce efficiency. For an LED outside the tube, most glass materials block UV. Driving the LEDs with a signal seems a little pointless, takes more power than the grid. The fabrication of photoelectric surfaces probably isn't much simpler than the new room temperature thermoelectric emitters.
See this:

Photoelectric emission is very inefficient. With so little emission, it's always saturated (plate voltages are often relatively high to scavenge as many emitted electrons as possible). You could add a grid and make a phototriode, but I don't know why you'd want to.

The frequency of the light and the work function of the cathode determines the initial energy the electrons have. This is like an extra volt or three of bias for the price of zero, as opposed to the thermionic 1V or so virtual cathode effect. (Both of these allow a very small zero-voltage (short circuit) cathode current effect.) Given the pull of 50+ volts from the plate, UV isn't necessary to get the space charge moving.

Photo cell

Ive come accross these a while back, whilst running Cinemas. They are used in the Pre Stereo days for sound pick-up

Normally operate at around 90V, and currents in the micro-ampere range, Used to go microphonic and pick up projector mechinism noises and also some would make a rushing noise like water, especially if over-voltaged.

Not a very good HF response either--but that may just have been the sound-head optics