Amplifier Tubes for Beginners – a quick guide
By Dorian Stonehouse
This article follows from:
THE old saying ” you can’t get something for nothing” is of- course (to all intents and purposes) true.
An amplifier is all about a small varying voltage (at the input) controlling a large varying voltage or current, or both, at the output – and not about creating something out of thin air!
The Mullard EL84 Audio valve (tube) amplifier is sought after by audio purists everywhere
First, let’s take a step back:
A mechanical relay will control a large output voltage, with a relatively small voltage on the input.
Consider the arrangement below:
A working low frequency mechanical amplifier, with no valve (tube) or transistor in sight
The relay coil shaft moves back and forth horizontally, according to the small voltage going in to the coil.
The shaft has a very light spring, which under zero input voltage (left of diagram) holds the voltage selector pointer (right side of diagram) on zero volts.
The input voltage (zero to 10 volts wave on the left) begins to increase, reaching 2 volts.
This moves the pointer (right side of diagram)⇑⇑ to the 10 volts position on the stud selector contact, channelling 10 volts to a bulb (not shown).
Typical example of relays showing coils and contacts
As the input voltage reaches 4 volts, the pointer touches the 20- volts stud contact, and the bulb burns brighter.
When the input reaches 10 volts, the output reaches 50 volts.
This arrangement is, therefore, a times 5 (x5) amplifier; and it is possible to see the bulb start dimly, then get brighter, before sinking back again to darkness.
I used to use these mechanical amplifiers up to an input frequency of around 5 cycles per second input, but beyond that frequency, the output could not follow the input, so the device failed!
An early triode valve
The UY-227 is an early triode valve. It has an anode, control- grid and cathode (tri) and a filament, which is isolated from the cathode. This allows the valve to work on alternating current (AC) heater voltage, which is used in, for example – the UK.
Showing the internal parts of the UY-227 valve (tube)
Please follow the current path in the diagram
The current goes round in a big circle: through the high voltage (ht) supply; back to the cathode of the valve (tube); through the valve (tube) envelope; out through the anode resistor and back again through the ht voltage source.
Valves (tubes) come in all shapes and sizes
The radio frequency valve: Osram Z77
Something very important to understand (for Tom Davis in particular):
- The cathode of a valve (tube) is negative and is rich in electrons.
- The electron-rich cathode is heated up (and its electrons are excited) by the heater.
- The anode of a valve (tube) is positive and is poor in electrons and wants to attract electrons from the cathode to balance things out.
- When studying valves (tubes), electrons travel out from the cathode, through the valve and in to the anode (current flow).
- The flow of these electrons is governed by the control grid.
- The control grid is like a garden gate: loads of people want to come through it, but you control how many pass!
- In effect, the valve (tube) acts like a relay – small grid voltage controls big anode voltage (see description of relay amplifier above).
So how does voltage amplification arise?
Looking at the current path picture⇑⇑, you will see a resistor stuck between the HT (high tension voltage supply) and the anode of the valve (tube).
This resistor is called a load resistor (called RL).
When RL passes a certain current through it (I), then it must have a certain voltage produced across it. This voltage change (across RL) will be higher than the voltage change across the control grid⇑⇑, possibly giving a change of 50 volts across RL for a 1 volt change at the control grid (for an EF80) – known as the amplification factor.
I hope this very brief introduction to valves (tubes) has been of use to you and that you will take the hobby to the next level and strive to find out much more about these wonderful devices and pass on your knowledge to others
Remember, if you would like any technical help on valves (tubes), and providing that I am able to answer your question/s, I should be very pleased to help.
In the meantime, why not visit the tube gallery that I am setting up?
Do keep an eye out for the link that I will be posting here shortly: