|One of the design typical design conditions of a Williamson amplifier is the shared cathode resistor of the power tubes. In the diagram above this resistor is composed out of R17, R16, R18, R 21 and R22. R17 and R21 are wire wound potentiometers. Added up to together with the potentiometers in the middle position the shared resistor should be around 300 ohm. The tubes communicate through this shared connection in a way that lowers the distortion in a significant way. If this resistor is bridged by an electrolytic capacitor the amplifier virtues and vices change in a way that it couldn't be called Williamson amplifier anymore.
It is very important that the current through the tubes is equal because if it is not the core lamination of the output transformer will saturate magnetically which results in a lot of distortion and lack of bass reproduction. The smaller the air gap between the E's and I's off the core, the sooner the saturation takes place.
With R21 the current through the tubes can be set and with R17, which regulates a small + voltage to the grids of the power tubes so that the flow of current through each of the tubes can be set .
The current through the tubes X the voltage between cathode and plate should not exceed to the maximum dissipation of the tubes which in case of KT66, KT77. 807, 6CA5 and EL34 is around 25 watts. In most cases (depending on the HT voltage) this means that the current through each tube should be (and not exceed) 62mA.
The difference between the currents through both tubes should be around 0. (zero). Therefore the tubes should be as equal as possible and should be bought as pairs. But even in an ideal situation this equality doesn't last forever! It remains an arranged marriage between the tubes.
Since a real Williamson output transformer is wound in a symmetrical way (check that) the current though the tube can be determined by the voltage drop over each primary half of the output transformer. V : R = I.
First thing you do is measure the dc resistance of each primary half of the output transformer when it is switched off to make shure that these values are equal. Next you connect 2 voltmeters as in the diagram below. Voltmeter 1 should be accurate and set to the 0 - 10 volt dc range. Voltmeter 2 should be able to return whether the difference between the voltage drop over both primary halves of the output transformer is positive or negative. Accuracy doesn't matter.
Connect the input of the amplifier to the ground, connect a dummy load to the loudspeaker connection, adjust R17 to the middle of the range and R21 to the max.