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In Reply to: RE: Class A2 Tube Currents posted by Triode_Kingdom on October 09, 2016 at 14:34:06
Going to have to disagree on perhaps a bit of hair splitting.
As grid voltage increases incrementally, the plate current increases. I see no mechanism that disconnects this transconductance around the transition to a positive grid.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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Not sure I understand your meaning. Grid voltage always swings symmetrically around the static DC bias voltage. That means average grid voltage never changes. My question is whether average plate current behaves the same, even when the grid is driven hard enough to draw current on positive peaks. Extracting from this, if an amplifier exhibits low distortion when operating in the A2 region, can it be assumed that average anode current is essentially the same as in A1?
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The average plate current should be a constant, within the linearity capability of the tube. So yes, to your question. I thought it was clear that the plate current is going to be dependent on grid-cathode voltage.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"I thought it was clear that the plate current is going to be dependent on grid-cathode voltage. "
But that's the rub. No?
The driver stage has to handle the grid current without changing the grid-cathode voltage beyond that of the music signal.
I hope I said that right.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The usual way of hooking up an A2 tube is by grid drive. If it is not in the 'always positive' region, the driver now needs to deal with a step-change in load when it goes positive. Driving the cathode offers a much more consistent load, but it is usually an order of magnitude lower than the grid when positive...LOL
The HY51 is a power tube just dying for cathode drive. I have enough for an amp project, and some 3k:500 output/interstage iron that will be brought to bear when I have the time.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
> > Grid voltage always swings symmetrically around the static DC bias voltage. < <
Your answer lies in this sentence. It's evident from the plate curves of the 211 tube, that the grid lines on either side of the 0V point are fairly symmetrical, so that along a 10K load line going from say -25V --> 0V --> +25V the increase in plate current appears to be fairy linear. The attached graph shows an OP at 60mA and 900V. Of course, at this operating point there will be no swinging past about +35V on the grid - you're already approaching zero volts at the plate.
Thanks, I do understand load lines and such. I'm just not sure the extent to which I can trust curves for this particular purpose. Your comments got me thinking about all this from a different angle; I think there's no reason I can't use grid voltage as a monitoring point rather than anode current. If distortion is low, they should follow each other. The only difference is that one lets me stabilize bias voltage, the other stabilizes bias current. Neither is necessarily better, as long as there's an adjustment. The OPA445 is a high voltage opamp that can run directly from my -80V bias supply. So, monitoring the grid itself is probably the simplest approach.
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Buy Chinese. Bury freedom.
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.
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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