It’d only be appropriate to write a tutorial on tube theory as I understand it for synthesizer enthusiasts. I’ll add more circuits as I go. Also, I say “tube” because I prefer the sound of it to “valve”, even though I’m Australian
IMPORTANT! Tubes are usually always designed to operate at a high voltage, and it’s a lot easier to run them off mains electricity. I design my circuits around a 250V DC power supply, which gives quite a bite if you touch it! I initially had them running at 150V, and they could work as low as 75V. The problem with lower voltages is you’ll get less amplification out of the circuit, as I’ll discuss.
The first thing you need to do to get a tube working is apply a 6.3V AC or DC voltage to the filament. This was known as the “A” power supply, going back to antiquity when the only electricity available was from batteries. The A battery powered the filaments, the B battery was a high voltage and powered the positive side of the circuit, and the C battery was used as a negative voltage to bias the control grid of the tubes.
In mains powered tube equipment, the A supply is usually just called the filament winding and is AC straight off a transformer. The B supply comes from a high voltage winding on the transformer, rectified and smoothed with a capacitor that can handle the voltage. Regulation may be used to improve stability. The C supply is also rectified and smoothed to generate a negative voltage.
Transformers with multiple windings for high voltage and filament voltage are available. My synth uses a Hammond 290VEX, which has windings for 342V AC (yikes!), 52V AC and 6.3V AC. I had to fork out for it though. I bought it new from Mouser.
Conducting across a vacuum:
Why do we need to waste all that electricity on making the tubes light up? Because electrons aren’t able to escape the filament when it isn’t incandescent. A cold electrode in a completely evacuated envelope is about as good an insulator as you can think of. Heating the “cathode” so it glows a dull red allows electrons to escape the metal and travel to the “anode”. Remember electrons are negatively charged, so the cathode connects to a low voltage, such as ground, and the anode connects to a high positive voltage (this is contrary to “conventional current” where we think of electricity as flowing from positive to negative, which makes things a little tricky).
A tube like this is simply a diode. It only allows electricity to flow if the anode is more positive than the cathode. If the anode becomes more negative than the cathode, it will not conduct.
When battery radios were common, “directly heated” tubes were used. These used the filament itself as a cathode, and it basically meant you had to design your circuits with all the cathodes across tubes at the same potential (though there were ways around this). “Indirectly heated” tubes are more common. They heat a small cylinder which encases the filament and is electrically disconnected from it. This allows for the cathode to be at any potential and isolated from other tubes.
Controlling the current:
The major leap to controlling the current through a diode is to introduce another electrode, called the
‘control’ grid, separating the cathode and anode. This makes a three electrode device called a triode. The important thing to remember is to slow and stop the flow of electricity from the cathode to the anode, the control grid has to be more negative than the cathode. This is the same thing as depletion mode MOSFETs and JFETs. If you want to compare this to an NPN transistor, the cathode and emitter, the grid and base, and the anode and collector are analogous. Notice there’s no such thing as a P-channel tube!
The problem is the amount of electrons that flow from the cathode to the anode is also affected by the voltage of the anode. This is why tubes need a high voltage to operate. This problem isn’t present in silicon anywhere near as much, if at all. It’s also why tubes have their distinctive ‘soft’ distortion sound - the more current the triode has to conduct, the lower the voltage on the anode (assuming a resistive load) and the fewer electrons flow, reducing the amplification and causing a less abrupt clipping of the signal.
To solve this problem, a second grid was placed between the control grid and the anode, called the ‘screen’ grid. We now have a tetrode. The screen grid is run at a constant positive voltage. I think of it as another anode, because the electrons are drawn towards it through the control grid, but they shoot straight through the screen grid and continue off to the anode. This means the output voltage on the anode doesn’t affect the amplification of the tube, because the cathode only sees the screen grid voltage. This introduces another problem called secondary emission, where the electrons may be travelling with so much velocity they knock out more electrons from the anode when they hit it, which then travel back to the screen grid. This causes instability in certain conditions. The solution again is to add another grid, this time called the ‘suppressor’ grid, which is usually connected to ground or internally to the cathode of the same tube. This makes a pentode. The purpose is to deflect the stray electrons away from the screen grid and back to the anode.
Pentode schematic symbol. The suppressor is internally connected.
I’ll continue this story in further posts.