Oddball Preamp Circuit?
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VacuumVoodoo
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Re: Oddball Preamp Circuit?
Yes, the name is ITROD (Ivan The Terrible Out Drunk)
Aleksander Niemand
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Life's a party but you get invited only once...
affiliation:TUBEWONDER AMPS
Zagray!-review
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Life's a party but you get invited only once...
affiliation:TUBEWONDER AMPS
Zagray!-review
Re: Oddball Preamp Circuit?
[quote="VacuumVoodoo"]Yes, the name is ITROD (Ivan The Terrible Out Drunk) [/quote]
Ahh - by my second bottle of Stolichnaya it all started making sense!
[/quote]Ahh - by my second bottle of Stolichnaya it all started making sense!
Re: Oddball Preamp Circuit?
As I'm seeing it, the grid return for V2b is going to the top of the cathode resistor of V2a, rather than ground. This does two things - it lifts it up above ground for DC, perhaps explaining the larger than normal V2b cathode bias resistor, and it also puts a signal on the bottom of the V2b grid resistor that is a small, out-of-phase version of that coming off the plate of V2a and being fed to the top of the grid resistor of V2b.
This would apply negative feedback and would also cause the 470k resistor to 'appear' smaller than it is, sort of like an 'anti-bootstrap'. I am not sure of the formula but I think the resistor is shrunk by the ratio of the voltages on each end, which I think would be about the ratio of the plate and cathode resistors on v2a, which would be roughly 20 to 1. so the 470k grid resistor would appear as if it were a 22.5k resistor.
There is another 470k in series with the grid resistor which together with it acts as a voltage divider. I am not sure what the objective of all of this is, though - maybe to scrub some gain and in the process have some cool overdrive characteristic not realized with more conventional designs? I guess it's one of those build-it-and-see deals...
Dunno what it's called, though - maybe it's in RDH4?
regards,
Dave
This would apply negative feedback and would also cause the 470k resistor to 'appear' smaller than it is, sort of like an 'anti-bootstrap'. I am not sure of the formula but I think the resistor is shrunk by the ratio of the voltages on each end, which I think would be about the ratio of the plate and cathode resistors on v2a, which would be roughly 20 to 1. so the 470k grid resistor would appear as if it were a 22.5k resistor.
There is another 470k in series with the grid resistor which together with it acts as a voltage divider. I am not sure what the objective of all of this is, though - maybe to scrub some gain and in the process have some cool overdrive characteristic not realized with more conventional designs? I guess it's one of those build-it-and-see deals...
Dunno what it's called, though - maybe it's in RDH4?
regards,
Dave
jbefumo wrote:I never checked those out - thanks.
I wired this up with a 3PDT switch to defeat that loop, and really like the result. Engaging it cleans things up, reduces gain, and makes the whole thing more articulate.