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Valves / Vacuum Tubes
Armstrong A220 design questions
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<blockquote data-quote="Ampdog" data-source="post: 803538" data-attributes="member: 144"><p>Follow:</p><p></p><p>Too late to add; just noticed that I did not complete commentary regarding R13! (Phone calls in between.)</p><p></p><p>R13 is not part of the described phase inverter. It is simple voltage feedback to V7a input. The amount is not simply calculated as one has to take the effective V6a output impedance into account (too lazy right now to look up component values!).</p><p></p><p>The purpose is to somewhat lower the V7(a+b) impedance feeding the power stage. (Unfortunately the popular ECC83 topology here typically requires highish values of resistors - 100K as anode load resistors not being uncommon.) The designer (Mr. Armstrong?) probably thought it wise to lower the phase inverter output impedance somewhat, fine then.</p></blockquote><p></p>
[QUOTE="Ampdog, post: 803538, member: 144"] Follow: Too late to add; just noticed that I did not complete commentary regarding R13! (Phone calls in between.) R13 is not part of the described phase inverter. It is simple voltage feedback to V7a input. The amount is not simply calculated as one has to take the effective V6a output impedance into account (too lazy right now to look up component values!). The purpose is to somewhat lower the V7(a+b) impedance feeding the power stage. (Unfortunately the popular ECC83 topology here typically requires highish values of resistors - 100K as anode load resistors not being uncommon.) The designer (Mr. Armstrong?) probably thought it wise to lower the phase inverter output impedance somewhat, fine then. [/QUOTE]
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Audio and Video Talk
Valves / Vacuum Tubes
Armstrong A220 design questions
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