4W Valve amplifier

I am not much of an audiophile; amplifiers aren't of any interest to me at all, if I am after one I couldn't care about how it's made whether it uses valves, IC's or transistors. I am especially not a purist; I don't use parts like gold plated connectors or valves for the 'sound' of it. The reason why I chose to use valves is that: they are cool. They are just some bits of metal in a vacuum and somehow they can amplify a signal, isn't that cool?

I found the plans to build this in an old article of Radio & Hobbies when I was flicking through the articles. The article is from February 1950.

Schematic and about the circuit

Starting with how this amplifier works, we should begin with the power supply. We are presented with the power transformer set up with: 240V, primary; 225V-0-225V 50mA; HT secondary; 6.3V 2A, heater supply. The HT is rectified in a full-wave arrangement using a centre-tapped secondary. The centre-tap is tied to earth at the chassis. The end of the windings are connected to both plates of the rectifier. The rectifier used is a 6X5GT; however, a 6X4 could be used instead but, the designers chose a 6X5. The output from the rectifier is filtered through a C-L-C PI-network made of: 2*8µF capacitors and a 14H DC choke. The HT voltage for the pre-amplifier has been dropped by a 50kΩ resistor and filtered by an 8µF capacitor.

The next stage; is the pre-amplifier. The valve chosen for the pre-amp is type 6N8. This and also the power-output valve is the only miniature type valve used in the whole circuit. They are part of Philips Miniwatt's 'innoval' range. The 6N8 is a duo-diode-pentode but the diode plates are useless as they are both earthed just to use the pentode section. The grid is just fed off the wiper of the 500kΩ volume control where the signal is fed in one end and the other being earthed. The screen dropping resistor used is 1MΩ which is tied to the HT supply of the pre-amp. The plate load is a 250kΩ resistor. The valve is being self-biased with the use of a 1,5kΩ resistor and a 25µF bypass capacitor. The output is capacitively coupled from the plate to the grid resistor of the output valve via a 50nF capacitor.

The power output valve used, 6M5, was very popular in Australia for use in radios and Televisions. The signal is fed in through the 50nF coupling capacitor and is fed into the control grid of the 6M5 via a 10kΩ series resistor. The screen grid dropping resistor is 100Ω. The plate load for the 6M5 is a 7kΩ load which is on a set of terminals to be connected to speakers with a 7kΩ impedance. The 6M5 is self-biased with the help of a 175Ω resistor and a 25µF bypass capacitor. There is a feedback network consisting of a 2MΩ resistor and 1nF capacitor in series across the plates of both 6N8 and 6M5.

Modifications

There are some modifications to make to the original designs. Some are nessecary and some are all volentary.


			Some values are no longer produced anymore so have to be tweaked to modern values:
			
				2MΩ - 2.2MΩ
				500kΩ - 560kΩ (not potentiometer)
				250kΩ - 220kΩ
				50kΩ - 56kΩ
				175Ω - 180Ω
				25µF - 22µF
				8µF - 8,2µF
				50nF - 56nF
                Bass control


                To add a bass control add a 1MΩ across the 1nF capacitor in the feedback network.
                
                Trebel control


                To add a trebel control, wire a 500kΩ potentiometer in place of the 6M5 grid resistor. Connect a 5.6nF capacitor from the potentiometer's wiper and earth.
                
                Stereo


                 To add stereo sound; you need to wire two 1kΩ resistors in series and connect the outer ends to each sound channel and connect the input to the centre point of the resistors.
                 
                 Stereo Balence


                 If the stereo modification has been done it is attractive to add a balence control. To add this; instead of the two 1kΩ resistors; wire in a 2kΩ potentiometer and connect the wiper to the input of the 6N8's grid. 
                 
                 Output


                 The orginal circuit calls for the output being 2 terminals where a 7kΩ load is to be connected. This can be dangerous as the HT voltage is on one post. This modifiaction adds an output transformer with a 7kΩ primary impedance. In my build I did add a internal loudspeaker; however, this is optional as an external speaker can be wired in.

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4W Valve amplifier

Schematic and about the circuit

Modifications

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