So the 50E is not a better version of the 303, in fact it's a pity Quad didn't develop a 50E with the famous Triplet output configuration.
The 50E has a regulated power supply for the driver stages and a well filtered supply for the output stages. A choke is very good in deleting HF rubbish. It is a technique from the tube era. The 50E will give 50W into almost any load, but not at the same time! In fact the power bandwidth concerning loads is relative small, it uses an output transformer instead of an output capacitor like the 303.
It is idiot proof though, it is funny, for professionals you design an idiot proof amplifier, for normal people.........well. There is an optional 600 ohm balanced input transformer. There also is an input potentiometer to cope with large voltage, 1966, input signals.
The first thing you should do when you buy vintage audio equipment, is to collect information about it. Never connect it to the mains supply in the first place, in most cases there will be an expensive noise! Start by cleaning the unit, 40 years of dust-collection will cause problems. As Quad used standard components, replacing the electrolytics and trimmer pots is easy. Check for suspect looking solder joints as well in the first place.
It's a pity the replacement capacitor for the power supply looks a little lost in the engine bay, although it is supposed to be twice the capacity. That should be a progress! I used a NOS, New Old Stock, capacitor from BC, with a projected life of 200000 hours. It appears to fit between the original brackets. I use a Hameg component tester to check the components within the circuit. Once everything looks ok I connect the unit with a Variac to the mains supply. If this gives a good result, I connect the unit directly and I can start testing & adjusting.
First we set the Emitter voltage of Tr3 to 5.5 volts with trimmer Rv1, then the bias currents of Tr8 and Tr9. Instead of breaking the transformer links, we measure the voltage in R30 and R31. The bias current should be 30-40 mA. Some people think this is also the correct value for a 303, well it is not. This means the voltage across R30 or R31 is .5 x 30-40mA = 15-20mV. You will set this voltage with pot Rv2 and Rv3. Purists would say that I made a mistake. I forgot the base current of Tr8 and Tr9 flowing trough R30 and R31 indeed, but that does not really matter.
The output of a 50E consists of eight (4 x 17V and 4 x 8.5V)independend transformer outputs, you can combine these outputs in any way, as long as you keep Ohm's law in mind. Most people will use the 50E with the 17V output configuration. This means in fact six 17V windings in parallel. The 50D has a different output combination, it has only the four 25,5V windings (17V and 8,5v in series)! So compared to a 50E you will miss the important 17V (6 Ohm) option, you can rewire the 50D to this specification. In the 50D there is no option for placing an input transformer and the netsupply voltage must start at a minimum of 200V AC. Also the netconnector is an odd Cannon connector. The output connector is a 11 pin version of the 19 pin 50E connector.
An interesting feature occurs: biwiring!
You can connect four windings in parallel for the woofer and two in parallel for the tweeter section.
Because all the electrolytics are used for coupling or decoupling we can change (increase) the value of the caps without worrying about frequency problems. Because capacitors are a lot smaller, you can easily tenfold the value with the same outer dimensions. Sometimes I almost did.
Well, almost all the capacitors, C6 should stay 50uF, because it forms a high pass filter in combination with R10. By changing the value of R10, the gain can be adjusted. Increasing the value of R10 means reducing the overall gain of the amplifier.
This is far more preferable (lower noise) than reducing the input signal by the input pot meter on the front panel of the amplifier. Remember that the product of C6 and R10 should stay constant, when you change the value of R10, otherwise the low frequency behaviour of the amplifier will be changed.
So with Tr1 the gain is set, feedback is applied and the high pass filter is implemented, an combination we also find in later Quad designs.
The protection of the 50E (and 50D) is based on a current limiter circuit formed around Tr10. This circuit is triggered by two parameters: the current flowing trough the emitter resistors of Tr8 and Tr9 and the temperature of the heat sink sensed by R18.
The peak output can be increased by changing (increase) the value of R26 and R27. Remember that this amp was designed in the days of constant RMS output.
With modern speakers, peak power (and coping with low impedances) is very useful. The two output circuits are identical, that’s the advantage of using an output transformer with two primary windings.