Hi,
what is the screen grid voltage requlation requirement for a tube such as 6P42S when used in class AB? That is, how much the screen voltage can vary before it starts affecting the performance, is it 10%, 1%, 0.001%?
I will attempt to build such a regulator, but would like to know when it's "good enough".
what is the screen grid voltage requlation requirement for a tube such as 6P42S when used in class AB? That is, how much the screen voltage can vary before it starts affecting the performance, is it 10%, 1%, 0.001%?
I will attempt to build such a regulator, but would like to know when it's "good enough".
I can not say any particular percentage, but with high gm tubes like 6P42P, the regulation of BOTH screen- and control grid voltage should be good. And especially when fixed bias is used. I studied this subject when I built a high power PP-amplifier with a pair of similar 6P45P tubes. At first I regulated the screen voltage with a 0D3 glow tube only, but later added a NFET to get higher screen current capacity. Also the G1-voltage needed a bit special stabilizing circuit. The best overall stability against supply voltage variation required a 2k2 resistor in series with the 75 V zener.
Attached the schematic.
Attached the schematic.
Attachments
Some others here will have more idea than I do, but I would have thought it depends on your design, IE how far are you pushing the valve, what power OP.
Not familiar with the 6P42S but like all pentodes, beam tetrodes screen voltae and dissapation max is usually a percentage of the anode often found by calculation rather than from a figure on the datasheet.
It's also complicated by the fact that when designing an amp, we design for sinewave power rather than music power, which is less.
So finally in answer to your question, if your running the OP stage right at maximum then regulation wants to be as tight as poss, say 1%, but 5% would ok if not. Most regulation of screen's in amps is done by a zener and dropper resistor, so this would be ok in the latter case.
However all the above is concerned with the safe running of the valve and not anything to do with distortion or other parameters. Your load line and biasing conditions (fixed/cathode bias - above/below the knee etc) will have a bearing on the answer; more info would help us, help you.
Andy.
Not familiar with the 6P42S but like all pentodes, beam tetrodes screen voltae and dissapation max is usually a percentage of the anode often found by calculation rather than from a figure on the datasheet.
It's also complicated by the fact that when designing an amp, we design for sinewave power rather than music power, which is less.
So finally in answer to your question, if your running the OP stage right at maximum then regulation wants to be as tight as poss, say 1%, but 5% would ok if not. Most regulation of screen's in amps is done by a zener and dropper resistor, so this would be ok in the latter case.
However all the above is concerned with the safe running of the valve and not anything to do with distortion or other parameters. Your load line and biasing conditions (fixed/cathode bias - above/below the knee etc) will have a bearing on the answer; more info would help us, help you.
Andy.
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I will try to modify my Priboi 50um-204s to use 6P42S for output, accrding to this post.
6P42S is very similar to 6P45S used there, but has a different pinout.
It says there that the g2 voltage has to be 175V and regulated, so I thought I could use a VR tube with a triode or pentode (and if that is not good enough, use a transistor). But I have to know when it's good enough to use. I currently do not yet have some of the parts I need for the mod, but I can build and test the voltage regulator separately.
6P42S is very similar to 6P45S used there, but has a different pinout.
It says there that the g2 voltage has to be 175V and regulated, so I thought I could use a VR tube with a triode or pentode (and if that is not good enough, use a transistor). But I have to know when it's good enough to use. I currently do not yet have some of the parts I need for the mod, but I can build and test the voltage regulator separately.
"Stabilized" and "Regulated" are synonyms, in different countries the same thing is called differently. It means, the voltage is stable, and does not depend on load current, temperature, and the outlet voltage, to the certain degree.
My main goal was to get negligible idle current variation against supply (230 VAC) voltage variation. The 2k2 resistor in series with the bias voltage zener completed this. It allows the bias voltage to follow (slightly) supply voltage variations.
The 0D3 glow tube alone was sufficient screen voltage regulator for up to some 120 W, but above this power level the regulation was lost due to high screen current. That problem was solved with series NFET.
Basically, the answer to your question is 'nobody knows'. Everyone around here likes to talk a lot, and measure very little. No one has ever bothered to hook a pentode up to a distortion meter and examine the change in distortion caused by different degrees of screen un-regulation. It could be a little, it could be a lot, it could do something weird under certain conditions. Nobody knows the details. You'd think this was an obvious thing to test, but no; it takes up too much precious time that could be used for waffling on the internet.
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Some relevant data:
As the ARRL Handbook points out, “The power output from a tetrode is very sensitive to screen voltage, and any dynamic change in the screen potential can cause distorted output. In a linear amplifier, the screen voltage should be well regulated for all values of screen current." But how well regulated does the screen voltage need to be? The answer will depend partly on the type of tetrode that you’re using, but mostly on the standards you’re setting for low intermodulation distortion (IMD).
Taken from: Power and Protection for Modern Tetrodes
As the ARRL Handbook points out, “The power output from a tetrode is very sensitive to screen voltage, and any dynamic change in the screen potential can cause distorted output. In a linear amplifier, the screen voltage should be well regulated for all values of screen current." But how well regulated does the screen voltage need to be? The answer will depend partly on the type of tetrode that you’re using, but mostly on the standards you’re setting for low intermodulation distortion (IMD).
Taken from: Power and Protection for Modern Tetrodes
50W 6L6 (6P7S) how to replace tube ccs for solid state?
Here is a comparison of screen regulation between tube and solid state, modem solid state device (series shunt) is far superior than tube counter part, down to 1uV ripple is possible. Have a look if interested.
Here is a comparison of screen regulation between tube and solid state, modem solid state device (series shunt) is far superior than tube counter part, down to 1uV ripple is possible. Have a look if interested.
The answer to the OP's original question has to be "how much distortion can you tolerate?".
As I and others have tried to say, there are basically two issues:
1. bias stability: g1 and g2 bias need to be either both stabilised or both allowed to vary with mains voltage
2. distortion: g2 should not vary with signal (except deliberately, as in UL) - the less variation the less distortion
As I and others have tried to say, there are basically two issues:
1. bias stability: g1 and g2 bias need to be either both stabilised or both allowed to vary with mains voltage
2. distortion: g2 should not vary with signal (except deliberately, as in UL) - the less variation the less distortion
How much should it vary with current is largely up to the head engineer of the project, namely you.
If the screen current is low, it's pretty easy to get the sag down really low, but I've always had a hard time figuring out just how stable it should be. Oftentimes designers get it to a point that seems good enough, the mosfet follower with a zener or gas tube reference is really very good, but can benefit from some enhancements to get the variation even lower, but works very well with tetrodes/pentodes that draw low screen current through their operating range, with a simple zener or gas tube shunt supply being good enough in other circumstances.
I've never bothered to do any very serious design and testing for the lowest possible variation, as it seems even basic regulation makes a very large difference in distortion measurements, so the mosfet follower types are my favorite.
If the screen current is low, it's pretty easy to get the sag down really low, but I've always had a hard time figuring out just how stable it should be. Oftentimes designers get it to a point that seems good enough, the mosfet follower with a zener or gas tube reference is really very good, but can benefit from some enhancements to get the variation even lower, but works very well with tetrodes/pentodes that draw low screen current through their operating range, with a simple zener or gas tube shunt supply being good enough in other circumstances.
I've never bothered to do any very serious design and testing for the lowest possible variation, as it seems even basic regulation makes a very large difference in distortion measurements, so the mosfet follower types are my favorite.
Define your peak load current.
Pick Vg2 so it will cover that current, with some margin.
Re-figure for high and low wall-voltage. Allow for the fact that, at the same load, low plate voltage will not require as much load current. (So basically if your wall is steady enough that incandescent lights work fine, it is more than steady enough for a pentode.)
Unbypassed series screen resistance *spoils gain*! As well as limiting peak current.
Bypassed series screen resistance reduces peak load current on *sustained* signals. May play unclipped speech/music just fine. Will sag-off on bench tests. Can be an "interesting" tonal effect on hard-played guitar amps.
I do think many folks over-think this. It has always worked well to not-regulate, and let both voltage and current vary as they will. Maybe not if your wall varies from 140V to 95V! But even doing tours in good places, bad places, and on generator, my big Bogens played well with little trouble.
Pick Vg2 so it will cover that current, with some margin.
Re-figure for high and low wall-voltage. Allow for the fact that, at the same load, low plate voltage will not require as much load current. (So basically if your wall is steady enough that incandescent lights work fine, it is more than steady enough for a pentode.)
Unbypassed series screen resistance *spoils gain*! As well as limiting peak current.
Bypassed series screen resistance reduces peak load current on *sustained* signals. May play unclipped speech/music just fine. Will sag-off on bench tests. Can be an "interesting" tonal effect on hard-played guitar amps.
I do think many folks over-think this. It has always worked well to not-regulate, and let both voltage and current vary as they will. Maybe not if your wall varies from 140V to 95V! But even doing tours in good places, bad places, and on generator, my big Bogens played well with little trouble.
So, no stabilising but not varying with signal, use a capacitance multiplier to feed the screen.
Mona
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