Mesa Boogie Studio 22 Phase Inverter

[Dave H]

Why is there a whacking big 2.2M resistor (R202 and R201) to ground at the grids of the EL84s? Is it to suppress any oscillations? Those 2M2's set up a huge potential divider with micro-amps. Any idea on how to effect a bias change? The 33K R111 would be ineffectual compared to the 2M2 for a global bias change, and it may upset the PI cathode +2vDC level. Have you ever rebiased one of these PI & Bias configurations, Chuck? Is there a better way?

The 2M2 and 330k are a potential divider to reduce the EL84 bias voltage to -11.6V. I don;t know why they didn't just split the 330k. They could have saved two whole resistors that way. I'd adjust the value of the 220k resistors to change the bias. R112 (15k) sets the tail current. The +2V at the cathodes is the bias voltage needed by the tubes to pass that current i.e. -2V at the grids wrt the cathodes.

[Chuck H]

As far as I know the "whacking big" 2.2M resistors (high impedance for this circuit) and the reference to a relatively high impedance -V supply via a relatively high 550k (220k grid stop series with 330k bias feed/grid load resistor) where the 2.2M is basically parallel to the 550k for a grid load, but again through relatively high impedance paths,... Is part of Mesa's "Dynawatt" system.

As I recall I did away with 2.2M resistors. I just snipped them out. I replaced the 220k grid resistors with 10k value and that was that. At 400+Vp the amp was better off with the additional bias volts anyway and in fact sounded better for the change. There's probably undocumented circuits or errors in the Mesa schematic, but this isn't uncommon.

[christarak]

As far as I know the "whacking big" 2.2M resistors (high impedance for this circuit) and the reference to a relatively high impedance -V supply via a relatively high 550k (220k grid stop series with 330k bias feed/grid load resistor) where the 2.2M is basically parallel to the 550k for a grid load, but again through relatively high impedance paths,... Is part of Mesa's "Dynawatt" system.

As I recall I did away with 2.2M resistors. I just snipped them out. I replaced the 220k grid resistors with 10k value and that was that. At 400+Vp the amp was better off with the additional bias volts anyway and in fact sounded better for the change. There's probably undocumented circuits or errors in the Mesa schematic, but this isn't uncommon.

Thank you Chuck. I will definitely try what you suggested. It sounds way more practical. I might keep an eye out for oscillations with the 'scope and supply the power through the bulb limiter while experimenting...just in case.

[christarak]

The 2M2 and 330k are a potential divider to reduce the EL84 bias voltage to -11.6V. I don;t know why they didn't just split the 330k. They could have saved two whole resistors that way. I'd adjust the value of the 220k resistors to change the bias. R112 (15k) sets the tail current. The +2V at the cathodes is the bias voltage needed by the tubes to pass that current i.e. -2V at the grids wrt the cathodes.

Thanks Dave. I think my "Plan E" was to change that 2M2 to adjust the bias, but I couldn't see the practicality in playing with such high impedances. I will try Chucks theory of removing the 2M2.s. If I end up with 50MHz oscillations I'll come running back!! Thanks mate.

[christarak]

Can I please thank you all for your knowledge and insights into this annoyingly, unneccarily different Mesa Boogie design of PI and power valve biasing? You have expanded my knowledge and I now have a couple of great suggestions on how to bias the unbiasable. Thank you.

[Helmholtz]

Why is there a whacking big 2.2M resistor (R202 and R201) to ground at the grids of the EL84s? Is it to suppress any oscillations?

There seems to be a misconception. While a grid (series) stopper resistor (1k to 10k is sufficient) can avoid oscillation, a large grid (shunt) resistor does not.

[christarak]

There seems to be a misconception. While a grid (series) stopper resistor (1k to 10k is sufficient) can avoid oscillation, a large grid (shunt) resistor does not.

Thanks Helmholtz. Yes, I would normally insert a Leo-approved 1K5 series resistor for oscillation suppression, but I can't see any reason for such a large value resistor. Chuck H said he has even removed the 2M2's in the past. If it is to act as a potential divider, then I expect that any changes made to earlier resistors would be ineffective for bias changes.

[Helmholtz]

Yes, I would normally insert a Leo-approved 1K5 series resistor for oscillation suppression, but I can't see any reason for such a large value resistor.

Well, the mod suggested by Chuck results in 10k grid stoppers - and the original circuit effectively has 220k grid stoppers.

[Chuck H]

el84's seem to behave better with larger grid stops. Notice the value chosen by Vox! I can't speak to the technical reasons, but I think it's because of their higher gain. I chose 10k arbitrarily on this premise and the amp sounded great so I had no reason to try anything different.

[christarak]

el84's seem to behave better with larger grid stops.

17 February 2020: Another new bit of knowledge learned. Thank you, Mr H.