How do I check the Bias and Plate Dissipation of my valve amp?

[FAQ Team]

This is easy with a calculator if you have previously noted all the voltage measurements on WAD`s test sheet.

Notes follow or go straight to the end for the Calculation

All the WAD amps are designed with an "automatic" bias system, either cathode bias or a cathode and fixed grid compound arrangement, and, in theory, need no adjustment. They are run using the maximum power dissipation that the output valve plate (anode) allows to get the most possible class A output. This is usually fine when using the valves recommended or supplied by WAD and at the same mains voltage WAD designed for. If you are using different output valves, or are in a high mains voltage area, the amp may over run it`s valves and cause a shortening of their life. You may want to lower the plate dissipation, at the expense of losing a little class A output (though not less overall output), to extend valve life.

UK Mains Voltage can vary minus 6% to plus 10% of 230v = 216v to 253v.

"Bias" is the voltage difference between the grid to ground and the cathode to ground measurements. It`s this voltage difference that determines the amount of current flowing through the valve. Here we are only concerned with the output stage where the bias may cause the output valves to be destroyed, or have their life shortened, if not correctly set.

"Plate Dissipation" is the amount of power, in Watts, that the valve plate has to dissipate (lose mostly as heat) whilst the amp is idling. For a class A amp this will effectively be full power. To calculate we find the voltage and current passing through the plate.

Although not applicable to WAD designs, it is worth noting when using a Pentode or Tetrode as a Triode an extra 10-20% dissipation will be allowable, as the screen grid, G2, will be acting as an additional anode plate.

The relationship between bias and dissipation is that changing the bias voltage will change the amount of current flowing through the plate and hence the number of watts dissipated by the plate.

All valves have maximum dissipation(s) specified for the plate (and grid). Operating lower than this will extend life a lot. A couple of volts bias change as an adjustment will not matter but we must bear in mind that we if we make large changes we will change the operating conditions of the valves.

The Calculation

Vk/Rk x (Va - Vk) = W

This assumes no losses and first calculates the current flowing through the valve by dividing the cathode voltage drop, Vk, by the cathode resistor value it’s dropped over, Rk. This current is then multiplied by the the plate voltage, taken as the HT voltage, Va, less the cathode voltage, Vk, to find Watts.

This just means using the voltages on the WAD test sheet as follows,

Cathode voltage (cathode pin to ground) divided by cathode resistor value in ohms multiplied by

HT voltage (anode pin to ground) minus cathode voltage.

An example using KiT88 circuit diagram is,

10v/150ohms x (510v - 10v) = 33.33W

Evaluating WAD`s design result

We find it’s fine but yours may not be the same. Measure cathode voltage carefully as a little difference here makes a large difference to the result.

WAD`s JJ`s are pretty tough but valves vary between makes with GEC KT88`s and Svetlana 6550C`s both having design max of 35w. Even NOS GE6550A`s with a design absolute max 42w can be seen overheating in a dark room at 38w. Therefore, if the dissipation did work out to 38w it may cause red spotting and shorter valve life with some valves (one year instead of several) or a short life (months) ending in catastrophic failure with other makes. Remember these are maximum figures and are not to be regarded as in the same way as the usual 10-15% voltage variations within a valve amp. Although the example is the KT88 valve these variations will apply to other types including 6L6, EL34 and 300B so it may be prudent to find the data and err on the safe side if unsure.