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In Reply to: RE: Class A2 Tube Currents posted by Triode_Kingdom on October 10, 2016 at 14:15:02
I know you understand loadlines, but I took what you said at face value and had a look at the plate curves. Think about it, it's the simplest and most obvious place to look - and admittedly it didn't come to me right away. I can understand what you're saying about trusting the curves, but then again why not? While tubes might've been hand picked for the purpose of obtaining and publishing test results, I would think that deviation from linearity within any specimen's performance envelope should be pretty much constant along a large sample of the type.
So, having said that and while I don't know what your proposed design looks like, a suggestion I have (haven't tried it so can't vouch for its results) would be to place a grid leak resistor between the power tube's grid and its cathode. There will be verry little current trickling at negative grid voltages and a bit more when the grid starts conducting, but it will effectively tie the grid voltage to that of the cathode so that when the cathode gets disproportionately elevated from the additional current when it enters A2, it will also pull up the grid thus maintaining the same differential.
Follow Ups:
I forgot to include an image, so here's the scheme as implemented on a SRPP Loftin-White circuit.
Edits: 10/10/16
My circuit is quite a bit different, so that won't work. The goal of this project is to eliminate a warmup drift that takes place during the first 20-30 minutes of operation. The drift is being caused by the driver tube itself. At this point, I've sketched a few preliminary controller designs, but the fundamental concept is the same. Monitor the output stage with an opamp and apply the correction at the driver grid. I guess the only decision to make now is whether to sense the output's grid voltage or anode current. Here's the basic amplifier circuit:
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Buy Chinese. Bury freedom.
Okay, I understand now what you're after. Nice design you have come up with. I had a similar DC coupled scheme a couple of years back that was never built. It had an el34 as the driver with a 5K resistor at the cathode instead of a choke and used a bipolar supply at (+) / (-) 330V.I have one question; is your current 211 amp running at the same OP as this schematic indicates? If so, it appears that it's biased at around 95mA give or take, witch has the tube running at close to max dissipation. I understand you're using NOS tubes but how is their in-circuit longevity so far?
Edits: 10/11/16
In practice, -52V at the grid biases the 211 to approximately 70-75mA. I'm using the common GE VT-4C. Longevity is yet to be fully explored, but this is within the manufacturer's ratings for Class A service.
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Buy Chinese. Bury freedom.
Thank you for the info. 70~75% of max plate dissipation is certainly within the limits for long tube life. These being NOS US made tubes, they should provide many hours of good service under these conditions. I'm sure your new project will turn out great.
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