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.
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.
