Tube/valve (active gain stages)
An active gain stage is another term for an amplifier and the fact that the amplifier raises the gain of the input signal means that there is an element of distortion associated with the process and result. Valves, or tubes, fall into this category. In this instance, the way active gain stages work are actually not that complex; the output is controlled by the power (via the power supply) that is driven through the valve (grid) by the input signal. The higher the input signal value the more power is driven through the valve via the power supply. This will invariably account for variances, no matter how small, between the input and output stage.
Tubes have always been associated with ‘warmth’ because of this process but don’t let this fool you as some poorly designed systems work against you instead of for you.
Generally, the most common valves used are triodes (12AX7) which are mainly used for mic and line level gain stages, and pentodes which are mainly used for amplifier stages. However, this is not set in stone as they can be alternated and used in any system and are dependent on the topology design and application.
Triodes tend to produce both even and odd harmonics whereas pentodes tend to produce odd harmonics. The choice of valve is important when dealing with distortion as the harmonic content is reliant on the type used. Of course, the design of both the circuit topology and powering comes into the equation but I don’t want this to be an epic journey into the world of electronics. I want to simply explain the basic differences between the types of valves and how the gain stages work as you will come across tube/valve selections in some vst plug-ins and it helps to know what you are dealing with and how the plug-in will behave.
Suffice to say that running any audio into a gain stage device will create its own character at the output stage and whether the choice comes down to using valves, solid state etc is purely dependent on what works best. As this chapter is about tube/valve I will stay with using valve emulation plug-ins and reserve solid state for another day simply because we won’t be using any solid state processing. However, it is important to state that running audio through a solid state device will add its own sonic character onto the output signal and this is why it is common practice to run audio through pre amps and the like .
Running audio through valves can be a very subtle effect unless the valves are driven.
Let us explore how valves behave when used both subtly and in anger.
Excerpt taken from Creative Effects book at Samplecraze.