Bias current settings in Quad 303 and 50E
Quad produced two vintage transistor power amplifiers with a bias current, the 50E and D variant, and the 303. In the service manuals the following currents are mentioned: 30 to 40mA for the 50 and 5 to 10mA for the 303. Maybe this is the reason for a lot of discussion on this subject; the 50E is a professional amplifier, so 30 to 40mA is better than the 5 to 10mA specification for the 303.
Why a bias current?
Most analogue power amplifiers operate in class A, B or AB.
In class A the complete input signal is used to drive the output stage. In its basic form the output stage consist of a single transistor, FET or Tube. To generate the full output signal the bias current should be high enough to let the output voltage swing from the positive supply rail to ground when a signal is applied. In practice the bias current is more or less half the maximum output current.
In class B only half the signal is used for the drive of the output stage with no bias current applied. For the amplification of the positive and negative halve of the sinus you need two output devices. This led to the popular push pull design.
The problem with class B is the non linearity of the output devices at low voltage. A transistor starts conducting at 0.6V base to emitter voltage. The non linearity at higher voltages (currents) is a problem in both class A or B amps. The advantage of a class A amp is the smaller signals involved, but with the lower output associated with this.
In class AB the class B push pull circuit is combined with a low bias current. For small signals the amplifier operates as a class A amplifier. The bias current, solves the low voltage problems of the output devices.
Class A is still used in pre amplifiers and driver stages of power amplifiers; efficiency is not a problem and quality the goal. There are still class A power amps, but mostly in the High End (Cost) territory.
How much bias current?
Here the mystification and Voodoo starts.
Some people think: the higher the better. It depends; most of the class AB amplifiers start their life in the design stage as a Class B circuit, including local and overall feedback. You cannot convert such a amplifier to semi class A by increasing the bias current. You have to redesign the driver stages and the feedback regime to make a proper class A amplifier. In a well designed class AB amp, like the 303, the output transistor is part of a circuit with local feedback, in this case the famous triplet circuit. So the linearity of the output transistor is not a big problem anymore, only the cutoff and start behavior of the output transistor.
So to prevent the transistor operating in this non linear fashion a small current is applied. In most datasheets the cutoff current (this is the current where the transistor stops conducting) of the transistor is specified. In the case of the 2N3055 output transistor of a 303 this cutoff current is 5mA. So the bias current specification for a 303 of 5 to 10mA seams reasonable. The 50E’s 40411 transistors have probably a higher cutoff current, unfortunally I could not find a proper datasheet anymore of this transistor to proof this. The often used replacement transistor, MJ802 has a lower cutoff current than a 2N3055, 2 mA. In both the designs a minimum DC voltage is applied to the base emitter junction to force the transistor in conducting mode. The result of all this is a certain collector current. This is design dependent! Because it is easier to measure the collector current than the very tiny voltage difference at the base emitter junction, this is still the way to do it. So only if you have access to very sophisticated distortion measurement equipment you can play with the values of the bias current. But you will still face the problem: which bias current will sound the best? So we stick to the manufacturers specifications.
Actual settings in a 303 and 50E
In practice you can still measure the DC bias current indirectly by measuring the DC voltage across the emitter resistance of the output transistor.
According to Quad 5-10mA current trough Tr1 of a 303 is enough. But....through R124 flows also the DC current from Tr109 and Tr105. You can measure the DC voltage across R120 and R123 and calculate the current. These currents are 0.8mA and 8.6mA. So the total DC current should be between 14.4 - 19.4mA, giving a DC voltage across R124 of 4.3 - 5.8 mV to meet the Quad specification. More accurate is to measure the voltage across R124 and R125 (leads 4 and 6 on the driver boards). It doubles the value of the voltage (8.6 -11.6mV), but with some voltmeters this give better results. So a good value in practice is 10mV across R124 and R125 (pin 4 and 6 on the driver boards).
Instead of breaking the transformer links in a 50E, we measure the voltage across R30 and R31. The bias current should be 30-40mA. This means the voltage across R30 or R31 is .5 x (30-40mA ) = 15-20mV. You can set this voltage with pot Rv2 and Rv3. I made a mistake, correct, I forgot the base current of Tr8 and Tr9 flowing trough R30 and R31, but that is only a small error. The same fault applies to the measurement of the 303, but also a small error. I advise to set the bias in a 303 at 10mV ( pin 4 and 6) and in a 50E at 18mV (across R30 and R31), after the amplifier has reached his operational temperature, without any signal drive.