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Can someone explain what's going on with the parallel triode input stage on the Tweed Twin? One input uses one triode of the 12AY7, and the other input uses both triodes. Why is there a gain boost going through two triodes in parallel. Voltages don't add when they're in parallel, so where is the gain boost coming from?
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Originally posted by Diablo View Post
because if you bias with a resistor, then the typical cathode resistor is about half. Instead of the nominal 1.5K, it becomes 820 ohms @ approx 2 milliamps of cathode current, giving you a bias voltage of 1.6 volts. But the gain of the combined stage is double ; or an increase of 3 dB because the ratio between the plate resistor to cathode resistor doubles from 100K/1.5K to 100K/820.
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Where do you get 820R from? The cathode resistor for the 4 x 12AY7 triodes in V1 is 470R in the 5E8A.Originally posted by mooreamps View PostBuilding a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- EnzoComment
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just read what I wrote. do the math.
-gComment
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So if I redesigned the input stage and used a separate cathode resistor for each triode - four 1880 ohm ressistors instead of a shared 470 ohm, then the high input (parallel triodes) would have the same gain as the low input (single triode)?
http://www.schematicheaven.com/fende...5e8a_schem.pdfComment
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Originally posted by Diablo View Post
yes ; assuming 4 milliamperes of total current flow through a 470 ohm cathode resistor calculates to 1.88 volts of grid bias ; for each gain stage.
and 1 cathode resistor and 1 bypass cap ; are cheaper "then 4"..
-gComment
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