Valve Amplifier Power Supply - barebonesAmps.co.uk

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Valve Amplifier Power Supply

Ever wondered why your valve amp is so much heavier (and more expensive) than a transistor or solid-state amplifier? Well, one the main reasons is the power transformer.
 
Recalling the simplified view of how valves work, it was mentioned that we can not use batteries in valve amplifiers. The reason is that we need a high voltage supplied to the valve’s plates - our Monaco Lite and Monaco amplifiers runs at a DC voltage of over 350Volts - this would require more than 230 batteries all in a row!

 

 

Give Me Power


Depending on where you live, the voltage supply from your wall socket will be anywhere from 110 to 240Volts AC (Alternating Current).
 
Since 2008, the UK, the mains voltage has been 230±10%VAC. The UK mains was 240±5%VAC and in fact it rarely, if ever, goes outside of these old limits. If you measured the mains voltage you would find that the live wire switches from +325V to -325V every 0.02seconds. The value of 230Volts is in fact the RMS (root-mean-squared) value and gives a better indication of the power available to do useful work (such as creating great music!)
 
AC voltage is very limited in its usefulness. It can be used to drive an AC motor at a constant speed but little else. For guitar amplifiers (and almost everything) else we need DC (Direct Current).
 
In order to get DC from AC we use a rectifier. However, each device will need a different DC voltage so we first change the level of the AC voltage.

 

The Power Transformer


(http://en.wikipedia.org/wiki/Transformer)

A transformer is, fundamentally, a large magnet with wires wrapped around it. An AC voltage is applied to the winding on one side. This sets up a magnetic field which induces a voltage into a second winding. By altering the number of turns in each winding, it is possible for the output of the transformer to have a  different voltage to input. Not only can we reduce the voltage from 230VAC to less than 12VAC to power a small household device but we can also step-up the voltage to, say 380VAC to supply our voltage-hungry valves.
 
Transformers have the added benefit of isolating the secondary winding and all the associated circuitry from the mains power supply – a safety feature not present in some early valve amplifiers!
 
It is possible to add several windings to the output side of the transformer. In this way, the same unit can also be used to power cooling fans, heater circuits, lamps or whatever else is required.

 

AC/DC

The power transformer in our Monaco Lite and Monaco amplifiers steps-up the voltage by a factor of around 1.6. On a good day, when we might receive 253VAC at the wall, the transformer output is 405VAC. During half-time of a big football match, when all of the country’s kettles are switched on, we will get just 330VAC. In reality, we get around 380VAC pretty much all of the time.
 
This voltage of 380VAC output from the power winding must be ‘rectified’ and ‘smoothed’ to provide a useful DC supply to the valves.
 

The Rectifier

The first stage is the rectifier. This can be a valve rectifier or a solid-state rectifier using a simple diode circuit. A diode is a nifty device which allows current to pass in one direction only.
 
In the diagram below, an AC voltage is applied to the input (or ‘Primary’) winding of a transformer. A single diode (D) is placed in line with an arm of the output (or ‘Secondary’) winding. When connected to another part of the circuit (shown by the load resistor, R), only the top half of the output voltage is allowed to pass – the bottom half has been blocked.
 

(http://en.wikipedia.org/wiki/Rectifier)
 
We have removed the ‘negative’ voltage with this half-bridge rectifier, but we are losing a lot of power as only half of the signal can pass.

Instead we add another diode to create a ‘full-bridge’ rectifier

(http://en.wikipedia.org/wiki/Rectifier)
 
We are now using the full power available. The supply is now DC, as we require, but is not smooth. In order to smooth this out we add a capacitor across the load. This smoothes the signal, giving something approaching a steady DC output.

(http://forum.metroamp.com/viewtopic.php?t=34551&p=339084)
 
Large capacitors capable of withstanding 380+volts used to be very expensive and if you look inside a vintage amplifier you will see some huge capacitors. Fortunately, we now have access to much smaller and cheaper options, allowing us to fine-tune the level of smoothing and noise filtering that we require.
 

Now We Have It, What Are We Going To Do With It?


It was mentioned earlier that valves need a very high voltage to operate. Valves may operate with as little as 100Volts but the tone improves greatly as this voltage is increased to 300Volts and beyond.
 
To get the maximum voltage, we could connect the power amplifier valves directly to the output of the transformer. However, take a closer look at the diagram above – the voltage is not smooth and in fact has a ‘ripple’. If we
 
 
use this to supply the power amplifier valves we will hear a loud hum at twice the mains frequency – in the UK this means 100Hz (or just above a low G!). Before connecting the valves we need to add some…

 


Filtering

Throughout your guitar amplifier’s circuit there are filters. These are frequency dependent and are made from just two components – a resistor and a capacitor. By changing the arrangement of this RC filter, and the values of the resistor (R) and capacitor (C) we can created a high-pass or a low-pass filter.

 

The High-Pass Filter


A high-pass filter will cut the bass frequencies. The point where the bass frequency is cut is known as the cut-off frequency and is determined by this formula –

 
For example, say we use a resistor of 0.16Ω and a capacitance of 1F we get
        f c = 1/(2*3.142*0.16*1) = 1Hz
 
The components are arranged as follows -


(http://en.wikipedia.org/wiki/High-pass_filter)

 
 
And the output (V out) looks like this

(http://www.play-hookey.com/ac_theory/filters/hi_pass_filters.html)
We can see that the output is the same as the input at 10Hz and above. At 1Hz, the output is 3dB less than the input, which is equal to half of the input.

 

The Low-Pass Filter

For our power supply, we wish to cut the 100Hz hum so we use a low-pass filter to cut frequencies of, say, 1Hz and above.
 
To create a low-pass filter we simply rearrange the circuit like this –

(http://en.wikipedia.org/wiki/Low-pass_filter)
Now we need to choose the values of R and C to get the 1Hz we are looking for. In the example above we suggested a capacitor of 1F (Farad). This is, in fact, a huge capacitance and we more commonly deal with capacitors 1million times smaller, i.e 1µF.
 
With this 1µF capacitor, we need a 160,000Ω (160kΩ) resistor to get the 1Hz cut-off frequency. But now we have another problem – the current for the entire amplifier needs to pass through this resistor.

 

 

We’re Losing Power, Captain


When current flows in a resistor, a voltage drop occurs across the resistor. This voltage drop is given by the formula
 
V=IR
 
Where V is the voltage drop, I is the current and R is the resistance. Our Monaco Lite and Monaco amplifiers need around 0.05Amps of current. With our 160kΩ resistor the voltage drop is 8000Volts!
 
In order to reduce this voltage drop to something more acceptable, we need to reduce R to, say 1kΩ. To keep the cut-off frequency at 1Hz we increase the capacitance, C to 160µF. But now we have a very large and expensive capacitor to fit!
 

All Calm At Sea


The solution is to use a series of filters on the power supply. Our Monaco Lite and Monaco amplifiers use a 2.3Hz filter, followed by a 15.6Hz filter. Each stage of the amplifier is fitted with further filters of 1.6Hz and 0.3Hz.
 
The result is a low-noise amplifier with enough voltage to drive the valves hard for the best possible tone!
 
Matt Green
barebonesAmps.co.uk

 
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