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10-11-2019, 05:42 PM
Actually, I tried DC power scaling, so I have a 22k low limit resistor on a 1M + 100k pot Vref divider (which is exactly what you predicted in 120k, lol). As of now, this makes ~50V the lower limit.
I think I figured out what is going on; it's two things in combination. First, for whatever reason I used a 68k resistor in the bias regulator feedback instead of the 47k. Coupled to this, I added a 100k to ground on the -Vbout node. I can't find any notes as to why I did either of these. After changing to 47k and removing that resistor, things appeared much better on the scope and there's about 3-4mA range now at the lowest power scale setting (this may not matter but it's a difference). Most importantly, things sound much better, everything is spankier and more lively.
I'm not sure which resistor made the difference, but my best guess is one of them was choking the bias regulator when the screen voltage dropped? This would explain why I'd would hear it at full power sometimes and when the second set of tubes is added, the screen voltage would sag.
Second, is that my testing is atypical in that I'm using a homemade load box and into an IR loader. In this case, it's no big deal to play at full power cause it will be completely silent. It's also no big deal for me to turn the power scale down to drive the power tubes much harder, which is when that distortion is at its worst. However, with a speaker cabinet, you'd never play at full power, and would likely be more sensible as to how much signal you're throwing at the output tubes. So this is likely just due to me using these circuits in extreme ways.
I'm thinking this could be alleviated somewhat by semi-scaling the phase inverter like that graph in TUT6 and/or increasing the grid stoppers on the tube grids.
And one more note, the off tubes don't seem to affect the on tubes as much. Triode still does noticably, but this is likely normal...according the the datasheet the plate resistance in triode mode is about the same as the primary impedance seen by the tubes, whereas as a pentode its around 10-20x higher.
I think I figured out what is going on; it's two things in combination. First, for whatever reason I used a 68k resistor in the bias regulator feedback instead of the 47k. Coupled to this, I added a 100k to ground on the -Vbout node. I can't find any notes as to why I did either of these. After changing to 47k and removing that resistor, things appeared much better on the scope and there's about 3-4mA range now at the lowest power scale setting (this may not matter but it's a difference). Most importantly, things sound much better, everything is spankier and more lively.
I'm not sure which resistor made the difference, but my best guess is one of them was choking the bias regulator when the screen voltage dropped? This would explain why I'd would hear it at full power sometimes and when the second set of tubes is added, the screen voltage would sag.
Second, is that my testing is atypical in that I'm using a homemade load box and into an IR loader. In this case, it's no big deal to play at full power cause it will be completely silent. It's also no big deal for me to turn the power scale down to drive the power tubes much harder, which is when that distortion is at its worst. However, with a speaker cabinet, you'd never play at full power, and would likely be more sensible as to how much signal you're throwing at the output tubes. So this is likely just due to me using these circuits in extreme ways.
I'm thinking this could be alleviated somewhat by semi-scaling the phase inverter like that graph in TUT6 and/or increasing the grid stoppers on the tube grids.
And one more note, the off tubes don't seem to affect the on tubes as much. Triode still does noticably, but this is likely normal...according the the datasheet the plate resistance in triode mode is about the same as the primary impedance seen by the tubes, whereas as a pentode its around 10-20x higher.
