About Valve testing

[Mike Watterson]

First I'll outline kinds of valves
Second will be kinds of testers
Third will be some simple PSUs and leakage testers for ad hoc testing.

[Mike Watterson]

First Understand Valves

Valves can use a direct filamentary cathode or an indrectly heated cathode. The cathode to heater leakege needs tested and the maximum is in the data sheet. It can be from 50V to 500V. Higher insulation is needed for series valves and rectifiers using the main heater supply and this makes the valve slower to turn on. This why some 6.3V rectifiers have their own winding. Direct filament rectifiers always need their own isolated supply.

Diode
A diode has only an anode and cathode. It only needs a small positive voltage and the current rise with voltage is exponential until the filament or cathode is damaged or reaches saturation. A rectifier is a higher power diode. An FM detector needs isolated diodes.

Obsolete three electrode valve, but not Triodes
A true bi-grid or dual plate valve has a plate or grid either side of the cathode. One plate is control with the other plate as output. Some dual anode rectifiers can be used like this. The idea is obsolete.

Triode
Next is the triode. The grid is 0V for maximum current and -5 to -20V to turn off the valve. The grid is usually closer to the cathode than anode (or plate). At 11V to 14V supply (old car radios) the grid voltage may be as high as +2V. The grid connected to 0V via a large value resistor will go negative due to space charge. This can be substantial like -9V on an RF oscillator, so some battery valve sets use a series R C filter to bais the output stage. Dual control grid triodes exist but are rare.

Tetrode
The triode has too much input to output capacitance at RF, so a second grid decoupled with a capacitor to 0V was added. It was also found that the positive voltage on this could vary the gain. This is the true Tetrode. It has a negative resistance region which causes self oscillation caused by secondary emission from the anode to the g2, or Screen grid. The original grid is g1 or control grid. A Beam Tetrode is unrelated.

Pentode
The Pentode has a third, or suppressor, grid to stop the anode secondary emission. Normally this is at 0V and may be internally connected to cathode. On any RF valve it needs to have a separate pin as the cathode can be the input and the g1 set to 0V. This is a common grid amp, which has only voltage gain and a low input impedance, but as the grid is at 0V, the input-output capacitance is very low, thus better RF performance.

The Beam Tetrode is using electrostatic plates to replace g3. It's really a kind of Pentode.

Pentodes (and Beam Tetrodes) can be used as triodes by shorting g2 to the anode. The g3 might also go to anode or to ground depending on the design of tube. You can't reliably simulate a tetrode used as a negative resistance oscillator.

Triodes have an exponential type curve for anode voltage till saturation or destruction. The Tetrodes, Pentodes and Beam Tetrodes initally are exponential and then a near constant current set by the g1 and/or g2 voltage.

Magic Eye
The magic eye is an electron gun where the glow is the anode target on older types. Deflection pins cause the shadow to increase or decrease. Usually there is a triode included that drives the deflection pins which must have a resistive load. The connection is oven internal. Most have the beam grid tied to cathode, but a few have it to a pin to control brightness, Few have the triode anode and deflection separate. It's usually driven by AGC, so a strong signal turns off the triode and deflection pins are equal to gun anode volts giving no shadow. No signal is 0V, so triode is fully "on" and thus the deflection pin voltage is much lower and there is the large shadow. The DM70 and DM71 work on a different principle. The DM70 is a triode with a keyhole shape in a grid plate angled from the filament cathode. So as grid varies more length passes to the anode which has the phosphor. Thus the shadow / fully lit is opposite to a regular magic eye, The Magic eye also gives an amplified inverted AGC. A valve tester needs an added resistor to test a magic eye.

Hexode
The Hexode (hex meaning six) has a 4th grid used for mixing, though g2 will work on a Pentode (but needs about 20V drive) and g3 can be used too. They can be used for voltage controlled gain.

More than 4 grids
The heptode and octode have 5 or 6 grids. In reality the heptode and octode are like two valves in series. The 2nd grid may be just rods and is essentially a triode's Anode. So by suitable wiring and no component changes, you can use a heptode or octode instead of a triode oscillator and pentode or hexode mixer. The advantage of the heptode for a superhet originally was cheaper than a separate mixer and oscillator and less filament current in a battery set. The disadvantage is the oscillator leaks out of the RF in grid (g4 or g5) and a strong RF causes Frequency Modulation of the oscillaror. Both Pentode-triodes and hexode-triodes exist in a single valve for better performance and more expensive sets use separate mixers and oscillators. A different technique is used for VHF/UHF. There are also nonodes, which have 7 grids. They were short-lived fashion for FM detectors and colour decoders because transistors and ICs replaced them.

Bias
At higher power a negative bias is required for g1 either from a negative supply or puting a resistor in series with the cathode.

Rod Pentodes
Rod Pentodes are not actually Pentodes at all. They are dual beam electron guns with electrostatic focusing. The g1 is a pair of plates either side of the filament (they can only work with a filament, an indirect cathode is too large), that work a little like the original three element biplate tube. The g2 is rods like some heptodes or octodes and the g3 are beam plates (rods) like on a Beam tetrode. There are only rods and plates. So they can be tested and used like pentodes and can be "triodised", but g3 is always 0V, not tied to anode (as is the case with a Beam Tetrode used as a triode). Like Pentodes and Beam Tetrodes they can't be used as a Tetrode negative resistance oscillator. The dual g1 Rod Pentodes are the opposite to a differential amp, it's the same as any rod pentode except normally the plates either side of the cathode are tied together internally. Any differential voltage, in either polarity, reduces gain. Common mode voltage is amplified. Thus if there is a transformer with a centre tap between the pair of g1, the centre tap signal is amplified normally and the voltage on the transformer is multiplied,

Simple Beam Switches
Beam switching tube: Example is the 7660. This is an electron gun with two anodes. Deflection plates switch the beam between the anodes. So the signal feeds the grid and an oscillator feeds the deflection (usually using an RF transformer). It's a very high performance mixer. Two of them fed in anti-phase is a full double balanced mixer.

Other devices
CRTs: These can be tested like a triodes, tetrode or pentode. The voltages may be high and there is a risk of burning a spot on the face when there is no deflection. Best tested in a set.

There are many other kinds of valves. A Magentron can be tested at lower voltage as a diode. A klystron can be tested like a triode or tetrod or pentode. A travelling wave tube is like a CRT, but with an earthed headsink spike as anode and negative kV on cathode. All RF power devices should be tested in equipment other than filament and leakage and may have X-ray, RF and high voltage hazards.

Gas discharge devices such as dekatrons, nixies, thyratrons, Xenon lamps, mercury vapour rectiers etc should be tested in circuit, not a valve tester.

Some kinds of transistors can be tested in a valve tester, and with modification, all kinds.

Next is types of tester.

[Mike Watterson]

Kinds of Valve tester

Simple Go/Fail
Hickock and very early AVOs
Very limited and may "fail" usuable valves. Only much use for parts and sockets.
I've used a couple of these. Horrible.

Advanced Go/Fail
Mullard High speed tester.
Very rapid but can't test anything not programmed, You can't do what if. Designed for production or testing stock or TVs. Not a great idea for home workshop.

Characteristic tester
The AVO models from mid 1950s. Later more advanced models added AC gain and electrode leakage tests. They are calibrated with direct manual separate controls for heater/filament, g1 volts, g2 volts and Anode. Usually have links that can be replaced by a resistor for magic eyes or pentode-triode or hexode-triode mixers. Can test any valve.
You can manually curve trace by writing it all down (or spreadsheet). Note a DVM will give a different reading to the built in knob legends except for heater / filamment as they may use a half wave unsmoothed supply.
https://www.radiomuseum.org/r/avo_valve_...met_2.html
Read the explination.
Probably the ultimate original tester. I had extended loan of a VCM163. I had to replace both meters and make new scales.

Also a lesser AVO where I replaced the nearly dead meters with easy to get cheap one and an op-amp driver.

A problem with old testers is failure of the rotary switches that select socket to voltage. On a self-build use two sets of 2mm wander plugs and sockets.

Meter failure or reduced sensitivity is common on old AVO testgear except the actual AVO meters.

The Sussex tester is somewhere between a Hickock and VCM163. It's really late 1960s home brew.

Curve Tracer
A microprocessor based tester is now the cheapest solution.
https://www.dos4ever.com/uTracer3/uTracer3_pag0.html

There are now several versions. You can peer an unmarked valve and figure out possible connections, and measure for filament/heater and connect to a bench PSU (start at 0.5V) to see likely voltage. Filaments/ heaters vary from 0.7V (two hearing aid valves in series off 1.4V and a 22V anode) to 117V for some direct on USA mains rectifiers.

I made a mistake wiring mine like an AVO. I should have used a plug-in with the valve socket. The inductance is too high on some of the sockets and this reduces the heater voltage.
https://www.youtube.com/watch?v=JhNvdOW2zX4
On here http://www.radicalvalves.com/index/

Test in the equipment
This is simplest if the equipment voltages make sense. Tektronix argued this was the best way for repair shops.
Check value of cathode resistor, g2 resistor with set off.
Check all the socket voltages with valve unplugged (use separate socket and wires for series heater/filament). The g1 must be zero or less unless a car radio.
See volts on cathode resistor and calculate anode current.


Ad Hoc tester
A collection of PSUs, meters etc to test occasionally.

Next some PSU and leakage test ideas for Ad hoc testing.

[Mike Watterson]

Ad hoc tests for Diodes & Rectifiers
The only high voltage thing needed is a leakage tester.

Gear
1) PSU for heater/filament. AC or DC
2) Anode power supply. 0 to 30V DC. Less than 50mA. Possibly a wirewound pot on a 12V PSU
3) Leakage tester. Can be made easily from the flash in a single use film camera! Should have 2M Ohm series resistance.
4) DMM to measure Anode voltage.
5) DMM to measure Anode current.

Procedure:
1) Check filament. Resistance is about 1/2 when cold. Power up slowly. Stop if it seems too bright and recheck valve type and spec.

2) Check PIV when heater/filament powered with the leakage tester. Anode volts should be higher than PIV in data as that's a minimum.

3) Check heater/cathode with leakage tester, if it's rated as isolated.

4) Check Anode forward volts/current using LV PSU. Do not exceed current rating. Might be 25mA for a rectifier. If a signal diode does 1mA at 2V, is loads. We only care about the current, i.e. emission. A signal diode is unlikely to be a problem, ever, unless gone to air, bad leakage to heater or no heater/filament. A signal diode only needs about 25V PIV on AM and 40V on FM.
Power rectifiers can go low emission or fail on PIV.

The diode testing applies to diodes inside other valves, such as EBL, EABC, DAF, DAC, EAF etc. Just leave the other electrodes open circuit, though leakage test to diode anode & cathode agains other electrodes is worthwhile.


Semiconductor diodes can be tested with same gear. A 20M Ohm impedance DVM or VVM will display PIV.

I disconnect all metal/selenium rectifiers.


Next is Triodes.

[Mike Watterson]

Ad hoc tests for Triodes
The same gear as for diodes but with a high voltage PSU

Gear
1) PSU for heater/filament. AC or DC
2) Anode power supply. 10 to 300V DC. Less than 20mA.
3) Grid power supply. Either a 100K pot on 3 x 9V batteries in series or a 0 to 30V LV PSU and you earth the + out!
4) Leakage tester. Can be made easily from the flash in a single use film camera! Should have 2M Ohm series resistance.
5) DMM to measure Anode voltage.
6) DMM to measure Anode current.
7) DMM to measure the Grid voltage.

Procedure:
1) Check filament. Resistance is about 1/2 when cold. Power up slowly. Stop if it seems too bright and recheck valve type and spec.

2) Check grid to anode leakage when heater/filament powered with the leakage tester. Anode volts should be higher than rating in data as that's a minimum.

3) Check heater/cathode with leakage tester, if it's rated as isolated.

4) Set grid voltage to zero to check emission.
Check Anode forward volts/current using PSU. Do not exceed current rating. Might be 0.5mA to 10mA. We only care about the current, i.e. emission.
5) set grid to -1V and verify anode volts can be increased.
6) Set grid to -30V. Should be no anode current. Increase anode volts to max on data. (100, 200 etc). Verify anode current is still near zero.
7) Keeping anode volts in (6), reduce grid voltage slowly (less negative) and note grid voltage anode current starts to increase. This is the cut off voltage.

The triode testing applies to triodes inside other valves, such as EABC, DAC, ECC, triode-hexodes (ECH), triode-pentodes (ECF) etc. Just leave the other electrodes open circuit, though leakage test to grid against other electrodes is worthwhile.

You can do 4 to 7 with a 2N3819 / J310 or any depletion mode N JFET and a Low voltage PSU. Zero gate is quiescent current without bias. Enhancement mode FETs need a positive gate and bipolar transistors are easier to deal with using a variable resistor in series with 1.4V battery or a constant current source, though technically a 0V to 1V voltage source is better.

Make sure the heater/filament is within +/- 2.5% of rated 0.65/0.7, 1.15/1.2, 1.35/1.4, 1.8/2.1, 6.3, 12.6, etc voltage or that if a series valve that it's within +/- 2.5% of the 100, 150, 200 or 300mA series rating. Actual use is ideally at +/- 10% but you want it more accurate for tests.
Battery valves may be better tested at the lower number and octode/heptode mixer/osc should be also tested at 1.1 for 1.4V types and 1.8 for 2V types.·

[Mike Watterson]

Ad hoc tests for Tetrodes, Pentodes, Hexodes and Beam Tetrodes.
The same gear as for triodes but with a second HT PSU, of some sort. Usually only low current needed.
You can somewhat test Heptodes, Octodes, Nonodes etc.

Gear
1) PSU for heater/filament. AC or DC
2) Anode power supply. 10 to 300V DC. Less than 20mA.
3) Grid power supply. Either a 100K pot on 3 x 9V batteries in series or a 0 to 30V LV PSU and you earth the + out!
4) A screen grid supply. Can be fixed steps from 25V to 200V
5) Leakage tester. Can be made easily from the flash in a single use film camera! Should have 2M Ohm series resistance.
6) DMM to measure Anode voltage.
7) DMM to measure Anode current.
8) DMM to measure Screen Grid voltage
9) Optionally a DMM to measure Screen Grid current.
7) DMM to measure the control Grid voltage.

Procedure:
1) Check filament. Resistance is about 1/2 when cold. Power up slowly. Stop if it seems too bright and recheck valve type and spec.

2) Check each grid to anode leakage when heater/filament powered with the leakage tester. Anode volts should be higher than rating in data as that's a minimum. Also the screen grid to control grid, cathode and other grids if they exist.

3) Check heater/cathode with leakage tester, if it's rated as isolated.

4) Set grid voltage to zero to check emission. Set screen grid to maximum in datasheet and all other grids if they exist either to cathode or screen grid according to data sheet or example circuit. The final suppressor grid, if not already connected to cathode should connect to cathode.
Check Anode forward volts/current using PSU. Do not exceed current rating. Might be 0.5mA to 10mA. We only care about the current, i.e. emission.
5) set grid to -1V and verify anode volts can be increased.
6) Set grid to -30V. Should be no anode current. Increase anode volts to max on data. (100, 200 etc). Verify anode current is still near zero.
7) Keeping anode volts in (6), reduce g1/control grid voltage slowly (less negative) and note grid voltage anode current starts to increase. This is the cut off voltage.
8) Set g1 / control grid to give anode current at half the maximum. Verify that g2 / screen grid voltage reduction reduces anode current.
9) Optionally set g2/screen volts to recommended. Reduce anode voltage towards zero. The Screen current should increase. The amount depends on kind of valve.

This testing applies to parts inside other valves, such as EBF, DAF, EBL, triode-hexodes (ECH), triode-pentodes (ECF) etc. Just leave the other electrodes open circuit, though leakage test to grid against other electrodes is worthwhile.

This doesn't fully test hexodes, heptodes, octodes, nonodes etc, but putting each grid to 0V or g2 volts as per data will at least test emission.

Make sure the heater/filament is within +/- 2.5% of rated 0.65/0.7, 1.15/1.2, 1.35/1.4, 1.8/2.1, 6.3, 12.6, etc voltage or that if a series valve that it's within +/- 2.5% of the 100, 150, 200 or 300mA series rating. Actual use is ideally at +/- 10% but you want it more accurate for tests.
Battery valves may be better tested at the lower number and octode/heptode mixer/osc should be also tested at 1.1 for 1.4V types and 1.8 for 2V types.·

[Mike Watterson]

HT Voltages

You can use about 120V for all HTs if you have the full datasheet. A 120/240 (or 110/220)V shaver outlet transformer is actually about 1:1 with a centre tap on the secondary for the US socket. It's got bathroom grade isolation and core power limiting (15 to 20W) and often a thermistor.

For smaller valves the screen volts can be a rectified power from centre tap to a wirewound pot to feed g2. The main anode supply can be via a halogen lamp and a shunt FET with gate driven from a 100K pot on a 9V PSU.

A 5V PSU with a fresh D cell in series makes a temporary 6.3V 300mA PSU for a heater.

Next I'll do some simple circuits. I'll test them first, so there will be a delay.

90V battery valves can actually be tested with 60V.

Power output devices are best tested in circuit.

[Mike Watterson]

Quick sketch of idea using a shaver transformer/outlet.
   
I think the raw HT is about 300 to 325V depending on your supply and model of shaver outlet.
All six HT rectifiers can be 1N4007.

The doubler gives about 600V or maybe 900V the way I've wired it. Could be a 3 way switch, 300, 600 and 900. The small resistors are only maybe 200V rated so maybe 4 x 1 M in series for the leakage PIV. The main smoothing caps can be 200V as there are 2 in series each with 2 x 220K in series across it.
The lamps should be 10W pigmy, filament/tungsten/halogen max power as the transformer is 20W limited.

Two separate heatsinks that are probably live on the fets, mounted on plastic. Ideally a robust plastic mesh box for them.

The neon is for leakage. Flashes or if really bad looks steady. If suitable series resistance you can touch the PIV or leakage test 900V, but a charged capacitor tested on it could be lethal, so arrange a discharge for Device under test.

Likely regulation is gilding the lily. I will think about it.

Not shown is a negative supply for grid (a regular isolated LW PSU with + to 0V) or FET bias or supply for heater / filament.

Series is nominally 25 or 50mA for simple battery valves (set to 23 and 46) and nominally 100, 150, 200 or 300mA for indirect valves. But set it lower.
Parallel needs 0.5V to 14V.
The 2, 6.3 and 12.6V valves on battery are really 2.5, 7 and 14V max and about 1.8, 5.4 and 10.8V minimum.
The 1.4V valves are really 1.35V nominal, 1V minimum and 1.6V max, though really 1.1V is often minimum for DKseries at shortwave. There are 1.2V battery valves (rechargeable) and 0.7V types (hearing aids). Some Russian indicators might be really 0.6V. They are a little like a DM160.

Each of the two HT supplies could be switchable with optionally different lamps to the 150V centre tap.

I'd not put large electrolytics on the output of lamps/HT. Maybe 1uF or 2uF poly foil or motor start.

This is just some thoughts.

[Kalee20]

A good and useful post!

I'd take a slight issue with this though:

A diode has only an anode and cathode. It only needs a small positive voltage and the current rise with voltage is exponential until the filament or cathode is damaged or reaches saturation.

Actually, it's usefully considered to be a simple power law, Ia is proportional to Va raised to the power 3/2 - definitely not exponential like silicon diodes.

[Mike Watterson]

OK, not exponential in a mathematical sense. But a similar idea.

There may be other mistakes as it's all written off the top of my head!