What are Matched Valves and do I need them?

[FAQ Team]

Each type of valve works within a set of electrical parameters. These include anode voltage, screen voltage (if applicable) and grid bias. When a signal is applied to the grid it is amplified by a designed amount. These values are termed the valve's "characteristics" and are different for each type of valve. There is a tolerance in manufacture, which means that although they meet their type specification, they will vary somewhat between samples.

Valves are normally checked on a valve tester, such as the AVO or Hickock range, against a set of "characteristics". The fixed settings for the heater, anode voltage, screen voltage, and grid bias are set on the machine.

A signal measured in mA current is applied and the resultant amplification read as mA/V. Thus 2 variables are recorded for each valve, current, in mA, and "slope" in mA/V. Although the valve is being measured at only 1 point on it's response curve this usually gives a good indication of it`s performance for the purpose of establishing good/bad and matching with other valves for use in pairs or quads.

If the operator is willing, or is using more sophisticated matching equipment, the valves can be compared at more than one point and a response curve built up which can be compared to the valves published "characteristic curve".

In an amp the first stage amplifier valves can be matched between channels to provide a similar level of amplification between channels and so help the stereo performance. The main and most essential use of matching though is when using pairs of output valves in "push pull" output stages. Here the 2 valves must share the work equally. If one draws more current than the other under the amps operating conditions it will cause an imbalance in the output transformer, which will partially saturate it, and upset it's response. If the valves are very badly matched one may draw increasing current until it fails catastrophically.

In practice, we should aim to match our 2 variable values of mA current and mA/V slope to within 10% to make push pull pairs. This is more critical in some designs than others but is a usual tolerance for cathode ("automatic") bias amps. Match current as priority, to maintain transformer balance, then slope to match amplification.

It is here, incidentally, that we see the special need for matched quads of OP valves in parallel push pull amps like KEL80. To share the work equally each valve of a pair of valves on each "side" of the transformer need to be matched, then each pair needs to match the pair on the opposite side.