Rickenbacker Grid-Leak Bias
Rickenbacker is known for guitars, not guitar amps. Back in the 1930s, however, things were different.
"Probe Rick's early history, though, to way back before its efforts in the 1960s to hold onto third- or fourth-tier status behind Fender, Gibson, and arguably Gretsch, and you find a company at the forefront of electrification, bold voyagers in tumultuous and uncharted waters." 1 -Dave Hunter
The first stage of the 1937 "Rickenbacher" M11 uses grid-leak bias, an old technique that has disappeared from the repertoire of most builders today.
It uses an octal 6SC7, a dual triode with the cathodes connected together internally. Grid-leak bias has three distinct features:
- a grounded cathode,
- a large grid-leak resistor value, and
- a DC-blocking capacitor at the input.
For a more typical cathode-biased preamp stage, there is a cathode resistor that sets the bias - a large resistor value biases the tube cold and a small value biases it warm. Because of the 4.7MΩ grid leak, the M11's grid-to-ground DC voltage is 0V, at least initially, just like a cathode-biased stage, but the effective cathode resistor value is zero. The cathode-to-ground voltage is 0V, so the grid-to-cathode voltage is 0V. This is hot bias in the extreme!
Making the cathode positive, like in a cathode-biased stage, cannot occur if it is grounded. The grid must be made negative. To accomplish this, the M11 relies on bias excursion.2
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Bias excursion is caused by grid current, which flows from grid to cathode within the tube. This current must come from somewhere and there are only two possible sources: the extremely resistive 4.7MΩ grid leak or the signal input. The input is a guitar, which has an output impedance of perhaps a few kiloohms, or a disconnected jack, which connects the input directly to ground. Whether a guitar is connected or not, the latter signal path is the most lucrative source - grid current flows from the input, through the capacitor, to the grid.
Unless there is a catastrophic breakdown, electrons do not jump the gap between the plates of a capacitor. The only way the capacitor can pass current is to release electrons on one plate and accumulate electrons on the other plate, causing it to charge.
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To see how this works, let's use SPICE simulation to analyze the first stage of the "Rickenbacker" M8E, which uses a 12AX7 for the first stage. When neither of the two jacks has a guitar connected, parallel 68kΩ resistors connect the capacitor to ground via the jack, with an effective resistance of 34kΩ.
Rickenbacker's design is a direct copy from a 12AX7 data sheet, except for a slight difference in resistor values (220kΩ instead of 240kΩ).
The indicated voltage gain of 62 (35.8dB) is for a 2V RMS output. This corresponds to a 2V / 62 = 32mV RMS input (46mV peak), about half the maximum output signal level of a single-coil pickup. The indicated value of 32V for EO is the maximum RMS voltage output for 5-percent distortion. This corresponds to a 730mV peak input signal. Based on the green AC load line shown below, this signal level swings the grid close to the 0V curve.
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A DC operating point SPICE simulation3 indicates that the DC grid and plate voltages are -780mV and 108V, respectively. This is a warm bias, but unreasonable. According to the 12AX7 calculator, the equivalent circuit with cathode bias has an 893Ω cathode resistor.
In a typical 12AX7, significant grid current flows when the grid voltage is greater than about -1V, depending on the particular tube. The threshold voltage is higher for other triodes commonly used in guitar amps, so the resulting bias is hotter. For a single-coil pickup, the 12AX7's grid-leak bias supplies adequate headroom. Hotter pickups or boost pedals may create front-end clipping, which is why this biasing method went out of fashion by the 1960s.
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A transient simulation3 shows that when the initial voltage across the capacitor is 0V, it only takes a few milliseconds for it to charge most of the way to its final DC value of 780mV.
The M8E's second stage is a conventional, cathode-biased 12AX7 voltage amplifier that drives a single-ended 6V6 power tube without negative feedback from the power transformer secondary.
"There's something of the wolf in sheep's clothing about this amp - from a distance it looks cute and polite, but in a flash it can turn into a ferocious, snarling monster that will take your hand off! Played clean, the M-8 sounds mild-mannered and fairly ordinary but when cranked, it delivers the kind of creamy all-valve crunch and tenacious sustain that make it a worthy challenger for any Fender Champ." 4
References
1Dave Hunter, Amped, (Minneapolis: Voyageur Press, 2012), p. 10.
2Richard Kuehnel, Guitar Amplifier Electronics: Basic Theory, (Seattle: Amp Books, 2018), pp. 166-167.
3Richard Kuehnel, Guitar Amplifier Electronics: Circuit Simulation, (Seattle: Amp Books, 2019).
4Huw Price, "Vintage Bench Test: 1960 Rickenbacker M-8 - Silver Service," Guitar.com, August 11, 2017. Available at https://guitar.com/review/vintage-bench-test-1960-m-8-combo/ (Retrieved April 24, 2020)
