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Post subject: Hot Rod Deville bias
Posted: Sun Sep 30, 2012 6:55 pm
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When you play thru the amp should the bias increase? It was my understanding it should not move.

My amp is biased at 60 when I play/strum the guitar thru it with the meter on it shows a spike to 100 and then drops as the volume drops. The amp sounds bad.

Any idea's?


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Post subject: Re: Hot Rod Deville bias
Posted: Sun Sep 30, 2012 8:58 pm
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Location: Natural Bridge, Virginia
When biasing an amp, you are measuring cathode current at idle with no signal running through the amp. When the amp is being played, the cathode current is going to change because the current through the tube is changing as the signal is amplified. What doesn't change is the bias voltage being applied to the tube grid. How old are your tubes? How does the amp sound bad?

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Post subject: Re: Hot Rod Deville bias
Posted: Mon Oct 01, 2012 10:23 am
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+1 bluesky636

Only measure bias current at idle, not while playing. The sound might be bad because 60 mV at the test point is too cold. Considering approx 485 volts on the plate, I would go up to at least 70 mV, or up to 80 mV.

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