Hola, MEF! Long time no see. I’ve been pondering a project that I’d like to resurrect and need a sanity check. I’ve been doing the design work for a push-pull GU-50 amp. I’ve never really done the whole “design an output stage from scratch” thing before, so I figured this would be a good time to learn. I’m trying to work out what reflected impedance I need for a pair of tubes operating in a push-pull AB1 output stage. After I get impedance down, I’ll then ask questions about musical output power and power consumption. Unfortunately since this is my first time doing this, I’m a little fuzzy on the rules and customs for selecting an OT impedance, and the research I’ve done is somewhat confusing, so I’m looking for some comment. For the sake of discussions, assume I’m designing around:
800v anode
250v screen
Load line selected for maximum power
Fixed bias
Here’s how I approached drawing this load line:
Start at 800v, 0ma and draw a line directly through the middle of the knee of the 0v grid line. This is the Class B load line (1/4 Za-a) and reflected impedance during the Class B portion of the operating cycle - I name it Blue.
Calculate the Za-a, then calculate 1/2 Za-a
Plot a load line for Class A portion of the operating cycle - I name it purple.
And finially, shift the Class A line up to the desired idle bias point to construct the full AB load line - I name it green.
Now here’s where I start to have questions. I understand that in Class AB we’re able to gain efficiency because the tube is not conduction during the full 360 deg cycle. Now as I look at these load lines, I see that for the most part, they’re violating the 40w max dissipation curve – significantly. Further, it looks like I’d need to drop the B+ to ~450v to cross the same point on the 0v grid line while not violating the 40w max dissipation curve. These two observations seem to contradict one another, so could someone help me out here? From my reading violating the max dissipation curve is allowed, but something tells me that this is too much. If it is indeed too much, are there any negatives to increasing the reflected impedance load to reduce the slope of the load line? If I eyeball what I think is acceptable (just a SWAG), this is what I come up with:
800v anode
250v screen
Load line selected for maximum power
Fixed bias
Here’s how I approached drawing this load line:
Start at 800v, 0ma and draw a line directly through the middle of the knee of the 0v grid line. This is the Class B load line (1/4 Za-a) and reflected impedance during the Class B portion of the operating cycle - I name it Blue.
Calculate the Za-a, then calculate 1/2 Za-a
Plot a load line for Class A portion of the operating cycle - I name it purple.
And finially, shift the Class A line up to the desired idle bias point to construct the full AB load line - I name it green.
Now here’s where I start to have questions. I understand that in Class AB we’re able to gain efficiency because the tube is not conduction during the full 360 deg cycle. Now as I look at these load lines, I see that for the most part, they’re violating the 40w max dissipation curve – significantly. Further, it looks like I’d need to drop the B+ to ~450v to cross the same point on the 0v grid line while not violating the 40w max dissipation curve. These two observations seem to contradict one another, so could someone help me out here? From my reading violating the max dissipation curve is allowed, but something tells me that this is too much. If it is indeed too much, are there any negatives to increasing the reflected impedance load to reduce the slope of the load line? If I eyeball what I think is acceptable (just a SWAG), this is what I come up with:
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