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Questions about tubes and gear that glows. FAQ

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Covering a few more bases...on Bias

Posted by Sector-7G on June 3, 2002 at 17:00:57:
For the ST70 it might be helpful to have a second voltmeter for each channel. Ground is easy to connect to the ground terminal for the speakers. The grids---g1-- need to be negative compared to the cathode by a specific amount to set how much current is flowing per tube. It is important to have matching tubes or one will conduct more current than the other. The factory spec is for 50 mA per tube. Like the previous posts said, check the value of the common cathode resistor, or what ever has been put in its place. For the initial set, it is best to use a Variac to warm things up slowly the first time, don't want to overheat things...turn the AC voltage up slowly while watching cathode current, and adjust as necessary. CHeck often as things stabilize and then for a bit as they break in, if things aren't changing, slack off a bit on use of the voltmeter...if you still are not sure, ask for clarification, they don't make those things anymore...
regards,
Douglas