Auto/Self Bias

 


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Last Edited:
12-Sep-2004


Self Biasing (a.k.a auto-bias) uses a high-current resistor and cap between the output tubes' cathodes (and ground) and a constant negative grid voltage. As the name implies, in self-biased amps, the tube itself determines the bias, based on condition and operating circumstances.

The only popular H.H. Scott integrated amplifier to use the non-adjustable, self-biasing method was the 6BQ5-based, Type 222(A). (Note: It had good company, the VOX AC30 guitar amp, used by The Beatles, also featured self-biasing.)

The Type 222B  (also 6BQ5-based) featured an adjustable self-bias design.

To adjust a 222B's bias:  place a DC voltmeter across resistor R-201, 1.2k-ohm, located across the preamp filament heater supply. (Note: Remove the bottom plate cover to gain access below the chassis). Adjust the Bias Control (located between the power transformer and electrolytic cap until the meter reads 38.5 volts DC.

Typically, Self-biased amps tend to run hotter and deliver slightly less power than their fixed-bias siblings. Consequently, they are a little harder on tubes. As the tube itself is providing the self-bias, it is more important to used matched output tube duets.

 

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