Found this very simple EHT meter in the third edition of the Quarrington Radio and Television Circuits and Data book.
It's an easy enough instrument to construct, the only item of concern is the 200Mohm resistor in series with base of the OC71 transistor. Max reading is 20KV. No doubt it can be modified for >25KV measurements.
Till Eulenspiegel.
I didn't know Quarrington got to an edition late enough to have transistors in it!
Mine is the first edition, and 'reads' very pre-war, although it has the circuit of the Ekco 'chamber pot' A22 in it...
20kv on the base of an OC71......hmmmmm 
To understand the black art of electronics is to understand witchcraft. Andrew.
20kV minus about 150mV, Andrew 
Well if you consider the BRC 2000 and those EHT bleeder resistors one being Z501 in the tripler unit and Z500 the focus control supplied the bases of transistors VT1 and VT2. The feedback current is only in the order of microamperes.
Till Eulenspiegel.
Well if you consider the BRC 2000 and those EHT bleeder resistors one being Z501 in the tripler unit and Z500 the focus control supplied the bases of transistors VT1 and VT2. The feedback current is only in the order of microamperes.
Indeed it is, but whilst squinting at the provided circuit you can see the potential applied to those transistors is at least 'potted down' by a relatively small value resistor on the focus panel. In the Quarrington circuit - which is of astonishingly poor design, incidentally - only the resistor stands between 20,000 volts and the OC71's base.
I'm wondering how many of these were built, and how many found three wires in the air after a bit of bad luck...
I'm wondering how many of these were built, and how many found three wires in the air after a bit of bad luck...
Indeed, did they actually realise a working EHT tester? I'd guess this instrument would be OK for measuring negative EHT supplies, the type found oscilloscopes. However, change the transistor to a NPN type, reverse the polarity of the batteries and we can now have an instrument we can safely use on TV EHT systems.
Till Eulenspiegel.
Several decades ago I made a HV meter out of 100 10MΩ resistors in a piece of electricity pipe and a yarn spool.
It has served me well for making masurements on old TVs.
The "output" goes into a multimeter.
Calibration is easly done by measuring a relative low voltage and adjusting the voltage divider resistors accordingly.
Jac
Still, it is a strange circuit, Till.
I was wondering if someone actually built it.
I've never seen a circuit like it, most just have the voltage divider and a digital meter or just a large input resistor and a moving coil meter.
I looked in a (Dutch) parts catalog of the early '50s and in one of the early '60s, but I couldn't find a resistor over 22MΩ.
The 200 MΩ resistor must have very difficult to obtain.
Jac
Good evening Jac,
I often wonder if these circuits were ever realised as working projects. First question: is a transistor really necessary? Most EHT testers and probes get by without such a device. Second question: should we attempt ever to make an EHT tester utilising this circuit?
I'd reckon a 200Megohm resistor is a very rare device.
From the same book a circuit of a valve voltmeter which employs the DA90/1A3 diode. I'm absolutely sure the clamp diode is inserted into the input the wrong way around. As it appears in the circuit shown in the attachment the diode will conduct on the positive tips of the waveform thus DC restoring the waveform negative below ground potential and under that condition the triode valve will be driven away from conduction. Turn the diode around and the waveform will be positive above ground.
Of course I could be wrong.
Till Eulenspiegel.
Here is a snippet from the AVO Electronic Testmeter manual. The diode is clearly the same way round.
Just to satisfy my curiosity I connected up a series capacitor, diode and resistor in the manner shown in the input circuit of the valve volt meter. I supplied 6.3 AC from the heater supply of a record player.
The scope waveforms confirm that the 50Hz sinewave is driven negative and the positive tips are now at zero level.
For 50Hz I found that the series capacitor had to be 0.47mfd. With 1000pF the positive part of the waveform is simply clipped down to zero level.
Till Eulenspiegel.
There we have it. I've reversed the diode and now the waveform is now moved positive above the zero level.
Till Eulenspiegel.
