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In the Forum: Audio Discussions
In the Thread: SET amplifiers
Post Subject: SEP loading and feedbackPosted by JJ Triode on: 3/24/2016
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Novygera,
If you are really going to run a single-ended pentode as a pentode (not triode-strapping the screen grid) then you will almost certainly need to use some negative feedback, whether global or local. It is partly needed to reduce distortion, at least that is "common wisdom" but even more needed to reduce the output impedance, which is very high for a pentode running as such; like roughly 10-20 times higher than a triode, or triode-strapped tube. Feedback gives you another variable to play with, and the subjective effects are somewhat similar to what you get with what Romy calls reduced or lighter loading: more feedback = more transients, "tighter" sound but eventually more hardness. At least this is true for voltage feedback, such as globally from the output positive tap to the input tube cathode. There is also such a thing as current feedback, which may sound differently.
SEPs are kind of strange animals. There is a lot less information about them out there than for SETs. You can find some discussion on local (like the RH amplifiers) versus global feedback, and screen power supplies (gas tube regulation at significantly lower voltage than the plate is advocated by some.)
Good Luck,
JJRerurn to Romy the Cat's Site