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RE: biaing my Scott 340 receiver questions...

Posted by Jim McShane (D) on December 11, 2016 at 17:59:00

In Reply to: biaing my Scott 340 receiver questions... posted by gkargreen on December 11, 2016 at 15:49:37:

You must have a 340A model since the 340B had bias adjustment pots - one for each channel's output tubes - along with the DC balance adjustment.

You need a matched quad for you amp. The pair in the first channel is in great shape running 34 ma and 14.5 watts dissipation. The second channel tubes are either weak or a very cool biasing pair (meaning less current at a given bias point).

Regarding the bias voltages at the tubes - here is a section from my FAQ on "Tube Problems" that should explain that:

Keep one last thing in mind - often you see a voltage "spec" on a schematic at the control grid of a tube. DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO THAT SPEC! The negative voltage at the tube grid should be whatever value is required to set the tube's cathode current to spec. If the tube is passing the correct amount of current and the voltage at the grid is -20 volts then the hell with the "spec" of -22.5 volts or whatever it is. Making the grid voltage more negative with respect to the tube's cathode is the equivalent of stepping down harder on a garden hose to restrict the flow of water. And if you let up the pressure on the hose and more water flows - that's the same thing that happens if the tube grid is less negative!!

Since you have no way to set the bias voltage differently in each channel you need to have tubes that bias properly at the voltage you have. Or change the bias supply to have adjustable bias for each channel.

• http://www.audioasylum.com/audio/tubes/messages/25/253369.ht   (Open in New Window)

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