Why 12E1? We had a longer discussion here that some tubes are made to be used only with auto-bias. If you look into the datasheet of EL3 for example:Some tubes like 12E1 can also cause bouncing.
In my case, I have observed that this remark comes usually on tubes with high mutual conductance. These tubes generate increase/decrease of the current at very small changes of the grid voltage. Is 12E1 in the same situation? What do you think about this "only auto bias" requirement? I use here without problems EL41/EL3/12E1.
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