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Currently Available Triode Valves
Not surprisingly, the availability of valves of any sort is extremely limited these days, since there are so few applications for them (and for those that are available, the quality can be a bit suspect), Forgetting about the high-power applications such as radio transmission and industrial eddy current heating sets, for which they are ideally suited, the audio field offers the major application area now, especially for power amplifiers, In this case the first stage is likely to be a pentode, as will be the power output stage also (operating in push-pull),
Triodes are used for the intermediate amplification, and the most usual (and useful) configuration is the double-triode, that is two triodes of the same type in the same envelope (glass tube). There have always been three firm favourites in these stakes, and these are the ones still readily available today. They are the ECC81, ECC82 and ECC83 also known alternatively as the 12AT7, 12AU7 and 12AX7 respectively.
In the latter nomenclature the first part. the number 12. indicates the heater voltage (actually 12.6 V AC at 0.15 A). and this implies that a 12 V supply is required to power the heaters. In fact this is not so since the heater is actually centre-tapped. so making it possible to parallel the two halves and energise them from a 6.3 V AC supply at twice the current, namely 0.3 A (If powered in series the higher voltage is actually 12.6 V) The pin connections for all these three types are the same and are shown below.
Base connections for the ECC81, ECC82 and ECC83 double triode valves.
This makes it easy to swap the different types around in the same valve holder while experimenting with them.
Principle parameters of ECC81, ECC82 and ECC83 double triode valves.
The table above shows the principal data for the three types for comparison. This data includes typical anode and grid voltages. and a corresponding anode current value. The values of the triode parameters. μ ra and gm are also given. This table indicates that the ECC81 is a medium gain valve with moderate values of ra and quite high values of gm; the ECC82 is a low gain valve with low values for both of these parameters but the ECC83 is a high gain valve due to its having a much higher value of ra even though its gm value is very low.
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