wintercept
Well-known member
I know, I know. This is my second tube amp post on a pedal forum. This is a new low for me guys, I’m desperate. I posted a bit about this on Ampgarage and was blessed by a response from R.G. Keen, but the thread has lost traction.
I’m working on a VHT Pitbull Hundred/CL (yes the same one from March or whatever) and have working great, but I am struggling to bias it with the so called “Dual Class” mode. As I shared on the other forum:
In "Class AB" mode, all four output tubes are run in fixed bias, easy peasy. But in "Dual Class" mode one tube in each pair is switched entirely to cathode bias, with no negative voltage on the grid, while the other tube in the same pair is run entirely with fixed bias and the cathode grounded. The cathode biased tubes share a single resistor/bypass cap arrangement between them. The cathode biased tubes were red-plating in Dual Class mode while the fixed tubes were not. So, I increase the cathode resistors from 150R to 280R and brought the bias from ~120% to ~90% dissipation.
The issue I have now is if I set the fixed bias to say ~15 watts per tube in Class AB mode, switching the amp to Dual Class mode increases the voltage drop between B+ and the plates, from which I calculate that the fixed bias tubes are running at ~20 watts. If I set the bias colder to say ~17 watts per tube in Dual Class mode, they go down to around 8 watts in Class AB. So, I must be missing something here. Anyone here know how to bias this thing? What sort of idle dissipation should I look for in Dual Class?
For all the weird and novel tube ideas out there, I can't find a single reference or schematic for this kind of arrangement. This is not like Mesa's Simul-class, nor is it the hybrid bias like some late 60's Fenders where all output tubes have cathode resistors and negative voltage applied to the grid. It's literally one cathode biased tube next to one fixed bias tube on each end of the push-pull arrangement.
I’m working on a VHT Pitbull Hundred/CL (yes the same one from March or whatever) and have working great, but I am struggling to bias it with the so called “Dual Class” mode. As I shared on the other forum:
In "Class AB" mode, all four output tubes are run in fixed bias, easy peasy. But in "Dual Class" mode one tube in each pair is switched entirely to cathode bias, with no negative voltage on the grid, while the other tube in the same pair is run entirely with fixed bias and the cathode grounded. The cathode biased tubes share a single resistor/bypass cap arrangement between them. The cathode biased tubes were red-plating in Dual Class mode while the fixed tubes were not. So, I increase the cathode resistors from 150R to 280R and brought the bias from ~120% to ~90% dissipation.
The issue I have now is if I set the fixed bias to say ~15 watts per tube in Class AB mode, switching the amp to Dual Class mode increases the voltage drop between B+ and the plates, from which I calculate that the fixed bias tubes are running at ~20 watts. If I set the bias colder to say ~17 watts per tube in Dual Class mode, they go down to around 8 watts in Class AB. So, I must be missing something here. Anyone here know how to bias this thing? What sort of idle dissipation should I look for in Dual Class?
For all the weird and novel tube ideas out there, I can't find a single reference or schematic for this kind of arrangement. This is not like Mesa's Simul-class, nor is it the hybrid bias like some late 60's Fenders where all output tubes have cathode resistors and negative voltage applied to the grid. It's literally one cathode biased tube next to one fixed bias tube on each end of the push-pull arrangement.
