Prompted by afroaudio's awesome Armstrong restored post, I went and retrieved the Armstrong A220 schematic (see below). After having had a look, I have a question for the boffins out there.
So there is global negative feedback via R12 (75K) and C6 (47pF) from the 16 Ω tap of the OPT to the cathode of the first stage, V6a. Seems pretty standard. But there also seems to be local negative feedback from the outputs of the two phase splitter triodes (valves V7 a & b) through the 1M resistors R15 and R14 (there are oddly two resistors marked R15 - I'm talking about the leftmost one). What is the purpose of this? Is it to control the gain of that stage? Given that they are very high resistors, are they doing something other than nfb?
Please excuse the dof question.
I found this info on http://www.audiomisc.co.uk/Armstrong/200/200page3.html
So there is global negative feedback via R12 (75K) and C6 (47pF) from the 16 Ω tap of the OPT to the cathode of the first stage, V6a. Seems pretty standard. But there also seems to be local negative feedback from the outputs of the two phase splitter triodes (valves V7 a & b) through the 1M resistors R15 and R14 (there are oddly two resistors marked R15 - I'm talking about the leftmost one). What is the purpose of this? Is it to control the gain of that stage? Given that they are very high resistors, are they doing something other than nfb?
Please excuse the dof question.
I found this info on http://www.audiomisc.co.uk/Armstrong/200/200page3.html
