Twin Reverb problems

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Re: Twin Reverb problems

Postby ricochet » Mon Oct 06, 2008 2:13 pm

Yes, it likely needs a good servicing with replacement of the filter caps and probably some of the resistors, if they're carbon comp sorts. They go bad, too, as do the paper caps.

There will be a pot on the amp for adjustiing the bias voltage, and the amp's service manual will have specs for checking it. Most techs nowadays like to insert a 1 ohm resistor in series with the cathodes of the power tubes and measure the voltage across it to determine the cathode current of the idling tubes and set the bias by that rather than the actual bias voltage. There are a lot of different recommendations for appropriate current ranges or percentages of maximum plate dissipation to set the idle current at, most of which are way too "hot" IMO. (Because many players prefer the sound of a "hot" biased amp, which starts distorting sooner and does so more gradually.) What's appropriate will vary with your amp, the tube types and plate & screen grid voltages. Manufacturer's recommendations are good guidelines to follow.
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