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Blackbird Bias Settings

Biasing

The new model of the Blackbird tube preamp is fitted with a ‘PRE-EMPHASIS’ (previously labelled ‘PRESENCE’) toggle switch on the rear panel. This replaces the ‘BIAS’ switch on the older model. Both models feature an internal ‘BIAS’ trim-pot which allows full control over biasing of the first gain stage of the drive channel. The brief article below explains how make the adjustment.

The internal trim-pot allows the resistance value of the cathode bias resistor of first tube stage in the drive channel to be decreased or increased. The dual triode tube in the middle position is the input stage and second gain stage of the drive channel. A trim-pot adjustment tool or precision screwdriver can be utilised to adjust the trim-pot. Rotating the trim-pot clockwise increases the bias voltage, ‘hot’ biasing the tube stage and rotating anti-clockwise ‘cold’ biases the tube altering the voltage gain, sensitivity and linearity of the first gain stage. In laymans terms this affects the biting point and ‘feel’ of the drive channel, that is, how it responds to playing dynamics. For a more detailed explanation of this check out the article Guitar Preamp Tone Explained

According to the RCA Receiving Tube Manual—the same book Leo Fender used when he designed his amplifiers in the early 1950s—each tube type needs to be biased so that it’s operating in the middle of its linear region. The book contains charts with values for bias resistors used in resistance coupled amplifier test circuits—the same type of circuitry as used in tube preamps. These circuits utilise a test 100KΩ anode resistor and the cathode bias resistor is chosen to set the anode current for different types of tubes according to the table below.

Tube Type	mµ	Resistor (Ω)
12AX7	100	1K8
5751	70	1K6
12AT7	60	3K
12AY7	45	1K6
12AU7	20	4K7
6189	20	2K7

A digital voltmeter can be used to confirm the resistance of the bias resistor is technically correct for the first gain stage, however the ‘BIAS’ trim pot is really there for guitarists who wish to experiment with moving the bias point up (‘Hot’ biasing) or down (‘Cold’ biasing) to achieve more non linearity to affect the character of the Blackbird’s drive channel. Note: when measuring resistance of the cathode resistor make sure the pedal is disconnected from the power outlet. Place test probes between ground and wiper of the trim-pot.

blackbird_bias_trimpot_adjustment_640px — A digital voltmeter being used to confirm the resistance of the cathode bias resistor.

Bias resistance can be adjusted from about 10KΩ (cold bias – trimpot fully anti-clockwise) to 0Ω (hot bias – pot fully clockwise). Note: you cannot damage a preamp tube by hot biasing it.

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Blackbird Bias Settings