In contrast, early transistor equipment used a supply voltage of around 20V, and 5V is one quarter (1/4) of the supply voltage. With such a small relative change, the overall linearity will be quite good. There will be exceptions as always, but in general this is a fair guess.Ī valve stage with a supply voltage of perhaps 250V and an output of 5V means that the signal level is only one fiftieth (1/50) of the supply voltage. Audio signals haven't changed much in amplitude since the beginning, so the AC signal voltages after amplification are generally within the range of 1-5V RMS. I expect that this myth came about in the early days of transistors, and there's a simple explanation. This is false! Valves are actually quite non-linear, and some are much worse than others. One of the claims you might come across is that valves are inherently linear, while transistors are not. This will be looked at later in this article. All active electronic devices make noise, but valves introduce a noise source not experienced by transistors or opamps. There are also a few tricks that one needs to understand if noise is to be kept to a minimum.

As a result, gain is often much lower than expected because this has not been considered. This means that the gain is determined not only by the stage itself, but by any circuitry that follows the stage.

Unlike opamps, valves have a relatively high output impedance.

Not that there's anything wrong with using a test setup to determine the optimum values, but unless you go through the calculations you don't learn anything. Most applications also require a specific gain, but we need to know how to work that out or we're stuck with the 'hit or miss' technique. Having decided that you want to use valves in your preamp project, it is necessary to understand the transfer curves and know how to bias the valve to get the desired result.

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