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Old 1st July 2005, 09:39 PM   #11
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I would make the selection on a case-by-case basis. I'm doing a design using 807s, and went with fixed bias. As you can see from this snippet from the STC application report, fixed bias has a significantly lower THD, output impedance, and somewhat more power out for identical voltage and current. In this case, fixed wins for multiple reasons. As for deriving bias voltage, that's NBD these days since a solid state regulator isn't that difficult to build, and these are much quieter than the method from the "good ol' days": half wave diode/ripple capacitor/pot from one half of the main plate xfmr.

It isn't quite as easy, but then again, best to design for good operation before applying the global feedback. That way, you just might need less of it. Since this is a one of a kind project, and I'm not anticipating building these by the thousands, it doesn't matter if it requires a few more parts.
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Old 1st July 2005, 09:52 PM   #12
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Quote:
Originally posted by andyjevans
Since the subject of a clean negative bias supply has been raised, could the brains on the forum suggest a compact way of getting such a clean supply?
LM337 with a HV PNP transistor playing standoff duty (the complement of the Maida circuit) will give you noise down in the microvolt range and rock-steady DC.
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Old 1st July 2005, 10:22 PM   #13
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Quote:
Originally posted by Miles Prower
I would make the selection on a case-by-case basis. I'm doing a design using 807s, and went with fixed bias. As you can see from this snippet from the STC application report, fixed bias has a significantly lower THD, output impedance, and somewhat more power out for identical voltage and current.

But the operating voltage for the tube (plate to grid) is not identical. In the first case, the tube is idling at 32 watts plate dissipation and in the second it's at 29 watts (similar ratio for the screen). Would the two results be identical if you raised the B+ to compensate for the 22v rise in the cathode voltage?

sheldon

This raises a novice question. Is plate voltage considered from plate to ground or plate to cathode? Plate to ground doesn't seem logical to me, as ground can be whatever you want it to be. the tube doesn't know where ground is. On the other hand, the tube does know where the cathode is relative to the plate. In other words, plate voltage seem like a different thing than B+.
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Old 1st July 2005, 10:50 PM   #14
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But the operating voltage for the tube (plate to grid) is not identical. In the first case, the tube is idling at 32 watts plate dissipation and in the second it's at 29 watts (similar ratio for the screen). Would the two results be identical if you raised the B+ to compensate for the 22v rise in the cathode voltage?

Don't know; don't care. Take another LQQK at those distortion figures: 1.8% with fixed bias vs. 4.0% with cathode bias. That, alone, is a good enough reason to prefer fixed bias.

This raises a novice question. Is plate voltage considered from plate to ground or plate to cathode? Plate to ground doesn't seem logical to me, as ground can be whatever you want it to be. the tube doesn't know where ground is. On the other hand, the tube does know where the cathode is relative to the plate. In other words, plate voltage seem like a different thing than B+.

Always measure from plate to cathode. Now, there are a great many characteristic charts that'll tell you that they are measuring B+. Of course, this would apply with the cathodes connected to ground. If they are not, then it's plate-to-cathode anyway. Many tube designers will talk about "B+" since, unlike solid state, the B+ to ground will be significantly greater than any positive cathode bias voltage. After all, 20V against 400V is less than a 10% difference. Nevertheless, the cathode is your reference point, just as the emitter/source is when dealing with solid state.
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Old 1st July 2005, 11:42 PM   #15
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Quote:
Originally posted by Miles Prower
But the operating voltage for the tube (plate to grid) is not identical. In the first case, the tube is idling at 32 watts plate dissipation and in the second it's at 29 watts (similar ratio for the screen). Would the two results be identical if you raised the B+ to compensate for the 22v rise in the cathode voltage?

Don't know; don't care. Take another LQQK at those distortion figures: 1.8% with fixed bias vs. 4.0% with cathode bias. That, alone, is a good enough reason to prefer fixed bias.

This raises a novice question. Is plate voltage considered from plate to ground or plate to cathode? Plate to ground doesn't seem logical to me, as ground can be whatever you want it to be. the tube doesn't know where ground is. On the other hand, the tube does know where the cathode is relative to the plate. In other words, plate voltage seem like a different thing than B+.

Always measure from plate to cathode. Now, there are a great many characteristic charts that'll tell you that they are measuring B+. Of course, this would apply with the cathodes connected to ground. If they are not, then it's plate-to-cathode anyway. Many tube designers will talk about "B+" since, unlike solid state, the B+ to ground will be significantly greater than any positive cathode bias voltage. After all, 20V against 400V is less than a 10% difference. Nevertheless, the cathode is your reference point, just as the emitter/source is when dealing with solid state.
That's what I thought. But the chart in the original post seems to indicate plate to ground, or does it? It has a blank for grid voltage. Does that mean 0 or? If it means 0, then the operating conditions for that tube are different than for the fixed bias example. My question was, could those different operating conditions (plate/cathode, screen/cathode voltages) explain the results, at least partly.

If we assume that B+ is raised 20volts, to keep the plate/cathode and screen/cathode the same for both cases, it seems that the bypass cap is then solely responsible for the distortion. If that's the case, I would think that those results would only apply to the the specific cap used and might well be different with a different cap. Or is there something else?

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Old 2nd July 2005, 12:02 AM   #16
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That's what I thought. But the chart in the original post seems to indicate plate to ground, or does it?

The figures given in the 807 app report do reference voltages to ground. They shouldn't've, but this is quite common when dealing with VT specs. It's a bit of carelessness that couldn't happen when dealing with low voltage, high current solid state devices where a few volts on the emitter/source will make a big difference since those few volts are a significant percentage of the collector/drain-to-ground voltage. Increasing the screen/plate voltage-to-ground to provide the same voltages referenced to the cathode would make the currents more equal in both cases.

It has a blank for grid voltage. Does that mean 0 or?

This is a common convention to indicate that the bias is derived from a cathode resistor, not from an external source. As for the increased distortion, this may very well be topology dependent. A common cathode bias resistor may very well give different results from one resistor per 807. Also, in those days, they didn't have access to the better capacitors we have now.
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Old 2nd July 2005, 12:03 AM   #17
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LM337 with a HV PNP transistor playing standoff duty (the complement of the Maida circuit) will give you noise down in the microvolt range and rock-steady DC.>>

Hello there - I regret to say that I need a bit more information to see this circuit (especially at 1 in the morning...). could you give me some more details? Thanks, Andy
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Old 2nd July 2005, 12:48 AM   #18
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Quote:
Originally posted by andyjevans


Hello there - I regret to say that I need a bit more information to see this circuit (especially at 1 in the morning...). could you give me some more details?
Here's a sketchy outline. The app notes from National Semiconductor go through the design in reasonable detail. For simplicity, I've eliminated bypass caps and protection diodes, but all the stuff you need to understand the circuit is here. Basically, the PNP transistor has its base held at a zener voltage more negative than the output voltage (e.g., if you use a 6.2V zener and the output is -50V, the transistor's base is at -56.2V). The emitter (and thus the regulator input) is 0.7 volts more positive. This guarantees that the regulator doesn't see an excessive input-output voltage differential.

The two resistors attached to the 337 set the output voltage. It's really pretty simple, so much so that it is scorned by the hair-shirt audiophile brigade. Nonetheless, it works and it works damned well.
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Old 2nd July 2005, 01:42 AM   #19
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Quote:
Originally posted by Miles Prower
That's what I thought. But the chart in the original post seems to indicate plate to ground, or does it?

The figures given in the 807 app report do reference voltages to ground. They shouldn't've, but this is quite common when dealing with VT specs. It's a bit of carelessness that couldn't happen when dealing with low voltage, high current solid state devices where a few volts on the emitter/source will make a big difference since those few volts are a significant percentage of the collector/drain-to-ground voltage. Increasing the screen/plate voltage-to-ground to provide the same voltages referenced to the cathode would make the currents more equal in both cases.

It has a blank for grid voltage. Does that mean 0 or?

This is a common convention to indicate that the bias is derived from a cathode resistor, not from an external source. As for the increased distortion, this may very well be topology dependent. A common cathode bias resistor may very well give different results from one resistor per 807. Also, in those days, they didn't have access to the better capacitors we have now.
Thanks,

One final check on my logic assumptions. With one resistor/capacitor per tube, and if we had a perfect capacitor, I assume that self bias would be essentially identical to fixed bias.

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Old 2nd July 2005, 04:14 AM   #20
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One final check on my logic assumptions. With one resistor/capacitor per tube, and if we had a perfect capacitor, I assume that self bias would be essentially identical to fixed bias.

Perhaps. There is one big difference: if the finals are over driven, then an unusually large voltage can build up across the cathode resistor bypass. This can take quite some time to discharge, and until it does, it throws off the operating point. This can lead to excessive distortion until the bias returns to normal. You avoid this with fixed bias.

As for which to select, that goes on a case-by-case basis.
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