Hello Again,
I have recently become the owner of a Mesa Boogie Bass 400+ amplifier. Planning on adding an adjustable bias circuit for the power stage, I took a look at the only free schematic I could find for it online, Power Supply.
After reading about notoriously cold biasing of power tubes in Mesa amps, I decided to modify the circuit, rather than just sticking in a pot of the same value as the original bias resistor. This led me to take a look at the datasheet for the 6L6GC power tubes in the amp, to get an idea of the typical operating circuit for these tubes, as well as their max plate dissapation. Comparing the operating parameters listed on the datasheet with the circuit of the Power Amp in my amp, I was very surprised. The schematic is of poor quality, but I'm fairly certain it lists the plate voltage as 480VDC and the screen grid as 465VDC. This screen voltage is off the charts on the datasheet, with 400VDC being the upper threshold they bother to graph. The bias voltage is an icy -54VDC, also on the extreme low end/unplotted on the datasheet charts. Looking at a the schematics of a few of the other non-adjustable bias 6L6GC amps I owned, the same scenario presented itself:
Fender Silverface Quad Reverb
Peavey Butcher
I've read that independently, excessive control grid bias causes nonlinearites, same for high screen grid voltages in relation to plate voltage. When the two conditions are combined, would this distortion be increased? Also, does the high bias voltage affect the slew rate of the tube, reducing the "tightness" of the amp? Would decreasing the screen voltage while moving the bias voltage closer to zero improve the sound of these amps? Would this be a "healthier" operating condition for the tube/amp (assuming proper dissipation parameters were observed)?
- Jack
I have recently become the owner of a Mesa Boogie Bass 400+ amplifier. Planning on adding an adjustable bias circuit for the power stage, I took a look at the only free schematic I could find for it online, Power Supply.
After reading about notoriously cold biasing of power tubes in Mesa amps, I decided to modify the circuit, rather than just sticking in a pot of the same value as the original bias resistor. This led me to take a look at the datasheet for the 6L6GC power tubes in the amp, to get an idea of the typical operating circuit for these tubes, as well as their max plate dissapation. Comparing the operating parameters listed on the datasheet with the circuit of the Power Amp in my amp, I was very surprised. The schematic is of poor quality, but I'm fairly certain it lists the plate voltage as 480VDC and the screen grid as 465VDC. This screen voltage is off the charts on the datasheet, with 400VDC being the upper threshold they bother to graph. The bias voltage is an icy -54VDC, also on the extreme low end/unplotted on the datasheet charts. Looking at a the schematics of a few of the other non-adjustable bias 6L6GC amps I owned, the same scenario presented itself:
Fender Silverface Quad Reverb
Peavey Butcher
I've read that independently, excessive control grid bias causes nonlinearites, same for high screen grid voltages in relation to plate voltage. When the two conditions are combined, would this distortion be increased? Also, does the high bias voltage affect the slew rate of the tube, reducing the "tightness" of the amp? Would decreasing the screen voltage while moving the bias voltage closer to zero improve the sound of these amps? Would this be a "healthier" operating condition for the tube/amp (assuming proper dissipation parameters were observed)?
- Jack
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