19-05-2024, 11:17 AM
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 The idea is obsolete.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.
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.
