Motorboating question - help me figure this one out!
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frequencycentral
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Motorboating question - help me figure this one out!
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Last edited by frequencycentral on Mon Apr 12, 2010 5:55 pm, edited 1 time in total.
Re: Motorboating question - help me figure this one out!
Perhaps a shorting jack is in order?
In any case, in a three stage amp, the first and third stages are in phase, so you should have decoupling between them (i.e., some series resistance/impedance and a decoupling cap).
In your circuit, maybe you can solve the issue by picking the B+ for the first stage off of the filter cap before last.
Without decoupling, this is a textbook case of a motorboat waiting to happen.
Maybe the cable capacitance creates a shunt for AC that is enough to short out microsignals which can prime the motorboating, building up through positive feedback to become audible. With an AC short on the grid, not much signal is gonna get through that tube.
In any case, in a three stage amp, the first and third stages are in phase, so you should have decoupling between them (i.e., some series resistance/impedance and a decoupling cap).
In your circuit, maybe you can solve the issue by picking the B+ for the first stage off of the filter cap before last.
Without decoupling, this is a textbook case of a motorboat waiting to happen.
Maybe the cable capacitance creates a shunt for AC that is enough to short out microsignals which can prime the motorboating, building up through positive feedback to become audible. With an AC short on the grid, not much signal is gonna get through that tube.
Re: Motorboating question - help me figure this one out!
Do you have a 1Meg resistor to ground on the input?
Tom
Don't let that smoke out!
Don't let that smoke out!