This is and old black box I had in the strange things bin.
My interest for it resurrected when I was playing with a pulse generator and
then I tried to hook up a led to that... but this is an other story.
The black box contains an RCA 931 tube, it's a photomultiplier device,
seem it was relatively common back then and it's easy to find some
selling on the bay.
The “eye”, from here the light enters the sensing side of the tube:
Le'ts open it:
And here is the old RCA931A in all it's glory:
This is the pcb, the opamp suggests that the tube was used for high sensivity
applications, the resistors generate the voltage ladder that supplies the multiplier
stages:
The box contains also a small lamp, self testing perhaps:
The photomultiplier tube is a very interesting device,
because it combines high sensivity, high speed and
easy coupling to 50Ohm scope inputs, the only
requirement is to have at hand an high voltage supply
of around 1kV.
This old tube (the tube itself was build in 1978, but
the datasheet specs are of 1959) is capable
of a risetime (well, better call it a falltime)
of 2nS and a transit time of 8nS.
Let's test it with a pulse generator driving
a small led, the PMT was biased with a voltage
of 900V:
Another fascinating thing about tubes is that
they make visible “single events”, drive the
tube with high voltage and the output shows
random single events where an electron gets
multiplied and generate an electron shower on
the output. In this case the event is very fast
and this can be a good reference point to calibrate
the circuit for maximum speed.
I modified the biasing of the electrodes,
I adapted the circuit in app note TP-133 from Burle:
added some resistors and capaticors to last 4 dynodes
stages and shortened as much as possible some of
the components, now the pulse is much cleaner:
the light source
is an orange led connected to the output of an
avalanche pulser:
Not bad from an old vacuum device
Fabio.
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