Secondary electrons will therefore be repelled back to the anode where they belong, as illustrated in the image on the right. It is positioned between the screen-grid-and anode, and is made much more negative than the screen grid (usually by connecting it to cathode). The suppressor grid exists to stop tis from happening. In a triode they are simply attracted back to the anode again with no harm done, but if there is another positive electrode close by –such as a screen grid– then they may be captured by its electric field instead. Once free they will be captured by whatever positive electric field happens to be nearby. This effect is known as secondary emission, and the dislodged electrons are called secondary electrons. When electrons hit the anode with enough force they may dislodge one or more other electrons from its surface. The screen-grid and suppressor-grid are auxiliary electrodes that play a secondary role in the operation. The cathode, control-grid and anode in a pentode serve the same purpose as in a triode, and the connections to these are the same as for a triode. The situation is made worse if the valve is in a combo amp where it is subjected to strong speaker vibration.Īs mentioned already, this is a good reason not to use a pentode at the input stage, but to use it later on and with less gain (less than 100, say). However, pentodes do normally suffer more acutely from microphonics because of their extra grids and more airy construction. Grid-stopper resistors are normally much greater sources of noise than the valves. In reality the difference is usually around 2dB, and rarely more than 6dB. This is due to random fluctuations in the way current splits between anode and screen grid In any discussion of pentodes it is inevitably pointed out that petodes produce more noise (hiss) than triodes. If tone is what we want then it makes more sense to use a pentode in a later stage of the amp where it can be driven over a wider range (or overdriven),Īnd this will minimise microphonic problems too. In other words, the main reason for using a small-signal pentode today is for the tone, not the gain. In some ways they can be thought of as simulators of power-amp distortion, but at lower volume,Īnd they have begun to creep back into boutique designs. When overdriven they can also produce an expressive compression effect as the average screen current increases. However, pentodes still offer a different quality of tone to triodes, with a little more odd-harmonic content and a 'glassy' sound. Many specimens simply couldn't handle the speaker vibrations, so it was replaced with a trusty ECC83 in later models. You don't often see small-signal pentodes in guitar amps, where the ECC83/12AX7 is king.Īn EF86 was famously used as the input stage for the early Vox AC30s,īut it acheived such high gain (about 200) and was used in such a sensitive position that microphonincs was a major problem.
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