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Got a question:
Let's say I use a 76 as a pre-amp tube in one case, and a driver tube in a power amp in a second case. The output impedance straight from the tube is about the resistance of the plate, about 12kohms lets say (let's assume we're using a CCS to load the plate).
Why in a pre-amp do you need a impedance lowering stage, say a cathode follower? When you use the tube as a driver stage, it doesn't need an impedance-lowering following stage. Why the difference?
Whether or not you need a *follower* before the output stage depends on the particular design. Some products manage without, some use it. BUT... the designer always knows EXACTLY what will be connected to the gain stage, so he can control everything, and put there whatever is needed.
A preamp designer doesn't have such knowledge. You can expect a power amp with high input impedance (470k)... one with low (3k), or anything in between. Plus you can have interconnects with completely unknown characteristics.
Given that uncertainty it is always wise to reduce the output impedance, or increase the drive capability... just in case something nasty is conencted there.
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