I may have been conflating the effect of shunt capacitance on the VR tube vs. zener diode - but it's quite possible for a basic zener shunt reg to oscillate. I know it for a fact, as it was an issue I discovered (and corrected) in some of the gear I used to help produce.
Holding the capacitance as fixed, start increasing the value of the bias resistor and watch the zener take off.. is it due to the resistor value, the cap, or both? You'd have to agree that once it takes off, the cap will have a strong effect on the frequency of oscillation.
Again, what I previously stated is true for VR tubes and I may have confused / conflated this with zener behavior.. it's worth reviewing, and easy enough done.
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No, it is "quite impossible"
Zener diodes don't have any negative resistance region, so can't oscillate.
Zener Diode I-V Characteristics Curve
VR tubes don't ignite on the "working" voltage, they are ignited on a voltage higher, by some radioactive for example particle, after that voltage on them drops. It is what causes oscillations, when the tube discharges the capacitor below a working voltage, then it needs to be ignited, and so on.
https://en.wikipedia.org/wiki/Voltage-regulator_tube
Zener diodes don't have any negative resistance region, so can't oscillate.
Zener Diode I-V Characteristics Curve
VR tubes don't ignite on the "working" voltage, they are ignited on a voltage higher, by some radioactive for example particle, after that voltage on them drops. It is what causes oscillations, when the tube discharges the capacitor below a working voltage, then it needs to be ignited, and so on.
https://en.wikipedia.org/wiki/Voltage-regulator_tube
I'm not sure how to respond to this, as it doesn't reconcile with my real-world experience in the matter. Here's the specific case I ran into, with a zener reg:
A fully-balanced audio amplifier has an input stage composed of a differential cascode, which is fed with a tube-based CCS. All tubes are 6SN7. The grids of the upper cascode tubes are held at a fixed DC point, which was originally 47V but later upped to 94V to accommodate circuit changes.
Previously, the zener reg was a 499K supplying a 47V zener, which was bypassed with a 470n - 1u film cap. When the change was made to 94V (2x 47V zener), neither the cap nor the bias resistor were changed, and at first things seemed fine.
On further investigation, I found that the zener reg was oscillating somewhere in the low AF range.. it wasn't immediately obvious due to the behavior of the differential section. Point was, I needed to drop the value of the bias resistor to 220K to stop the oscillation. And yes, that fixed it - no more oscillation.
What to make of that? Any comments?
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It was not a Zener oscillating. It was most probably a tube stage. Zener most probably even was not in the avalanche breakdown region due to the current that was too low. Changing the resistor's value you increased the current, so the Zener started conducting and shunted your oscillator.
What to make of that? Datasheets specify minimal current that guarantees that the Zener works and has specified dynamic resistance.
What to make of that? Datasheets specify minimal current that guarantees that the Zener works and has specified dynamic resistance.
I'm willing to listen.
Thing is, the correct DC voltage (~90) was found on the upper grids. It was only when I scoped the circuit, looking for another potential issue, that I found the oscillation.
I admit that you've created enough doubt that I'm willing to back off of my previous statements.. but this is the path to knowledge. I'm no engineer, and never claim to be one - I'm just a technician - and do my best to convey my experience. Unfortunately, I no longer own any chassis with this circuit in it, so it would be a real chore to set it up simply to test out the possibilities.. not to mention duplicating the PSU that ran it all.
So that's what to make of it, from my end.
> the line voltage varies between 108 and 122.
+/-6%. That's not even large.
Rural Maine, like rural Idaho, is not suburban Nashville. Long lines and low load density make for less-steady voltage.
The voltage at my house varies from 125V to 108. Depending on _load_.
When the well pump kicks on/off, the lights flicker.
If I turn-off unessential loads, I always have 125V except 124V the hottest day of summer.
The power at the pole is quite steady. But I am 500 feet back from the street, and for whatever reasons, power is brought in at "240V" (like 250V at pole).
When I bought the house the "home inspector" noted lamp-flicker on pump and on oil burner start, and said there could be "a problem".
I've checked all connections. I have done my math. The sag is essentially (95%) accounted by the impedance of 500 circuit-feet of #2 Al conductor. Two inline breakers account for another %. Recent digs suggest my last "50 feet" underground feeder runs the long way, maybe enough more to make the math exact.
The hard math is that upgrading to 0000 wire or 13KV and a transformer at the house costs a LOT more than the value of a little lamp-sag. I have not had ANY trouble with clocks or PCs re-booting, and only minor trouble with 120V incandescents eating 123V most of the time they are on. (I'm getting ~~500 hours on garage lamps that should do 1000hrs @ 120V.)
I recently saw a factory schematic for a tube-amp which said "105V-130V only!" That seems a large range. True, it was a guitar amp which might not see long hours at extreme voltage.
I had 900 Watts of tube-amp which, in a very stable environment, caught fire. (Factory non-solder.) After repairs I used them in touring, including on some really awful generators. Not a problem.
_I_ think a tube amp can be designed for 10% high at high line voltage. Normal line variations will not put it 10% low at low line voltage. The real limiting factor is life of INCANDESCENT bulbs, which are far-far fussier than tubes, and were (until recently) used by far more customers.
+/-6%. That's not even large.
Rural Maine, like rural Idaho, is not suburban Nashville. Long lines and low load density make for less-steady voltage.
The voltage at my house varies from 125V to 108. Depending on _load_.
When the well pump kicks on/off, the lights flicker.
If I turn-off unessential loads, I always have 125V except 124V the hottest day of summer.
The power at the pole is quite steady. But I am 500 feet back from the street, and for whatever reasons, power is brought in at "240V" (like 250V at pole).
When I bought the house the "home inspector" noted lamp-flicker on pump and on oil burner start, and said there could be "a problem".
I've checked all connections. I have done my math. The sag is essentially (95%) accounted by the impedance of 500 circuit-feet of #2 Al conductor. Two inline breakers account for another %. Recent digs suggest my last "50 feet" underground feeder runs the long way, maybe enough more to make the math exact.
The hard math is that upgrading to 0000 wire or 13KV and a transformer at the house costs a LOT more than the value of a little lamp-sag. I have not had ANY trouble with clocks or PCs re-booting, and only minor trouble with 120V incandescents eating 123V most of the time they are on. (I'm getting ~~500 hours on garage lamps that should do 1000hrs @ 120V.)
I recently saw a factory schematic for a tube-amp which said "105V-130V only!" That seems a large range. True, it was a guitar amp which might not see long hours at extreme voltage.
I had 900 Watts of tube-amp which, in a very stable environment, caught fire. (Factory non-solder.) After repairs I used them in touring, including on some really awful generators. Not a problem.
_I_ think a tube amp can be designed for 10% high at high line voltage. Normal line variations will not put it 10% low at low line voltage. The real limiting factor is life of INCANDESCENT bulbs, which are far-far fussier than tubes, and were (until recently) used by far more customers.
As said, I see no point in regulating a tube amp.
I cringe when someone regulates G1. G2 still has large effect on current. Plate voltage times current is dissipation. Regulate everything, or don't regulate anything.
Zeners hiss. Everything hisses. It is not huge. In a G1 bias application (I assume no-current) this is totally moot because you can R-C filter it cleaner than needed. But G2 also wants regulation (and is an input for alleged hiss) and that is not no-current and not even a steady current. R-C filtering will cost some bucks. Pentode plates are fairly hiss-resisting so the multiplied-up hiss of its Zener may not matter.
Ferro-resonant (Sola) transformers with Sine correction will turn 100V-140V into steady 117V. I worked with one in a radio station. Not a nice thing: acoustic hum and heat. (Un-corrected Solas make a rounded square wave which is great for lamps but bad for most AC-to-DC supplies.)
I cringe when someone regulates G1. G2 still has large effect on current. Plate voltage times current is dissipation. Regulate everything, or don't regulate anything.
Zeners hiss. Everything hisses. It is not huge. In a G1 bias application (I assume no-current) this is totally moot because you can R-C filter it cleaner than needed. But G2 also wants regulation (and is an input for alleged hiss) and that is not no-current and not even a steady current. R-C filtering will cost some bucks. Pentode plates are fairly hiss-resisting so the multiplied-up hiss of its Zener may not matter.
Ferro-resonant (Sola) transformers with Sine correction will turn 100V-140V into steady 117V. I worked with one in a radio station. Not a nice thing: acoustic hum and heat. (Un-corrected Solas make a rounded square wave which is great for lamps but bad for most AC-to-DC supplies.)
Good grief, this is my last comment on this..
1) Q: Do zener diodes generate noise? - A: Yes, they do, this is an established fact. Are you actually going to argue this?
2) Q: If there is any noise, is it an "issue" for our purposes? - A: No, not in my experience. It can be dealt with, and I never made any claims to the contrary.
3) Q: Are there any other issues with zener regs? A: I claimed they were capable of oscillation if misconfigured, but have accepted that it may have had as much or more to do with the tube itself. This might well be an error on my part - as the tube(s) may have formed a large component of the oscillation (which was very real, and observed on multiple chassis).
Once again - does my pointing out a noise source mean even for a moment, that zener regs are too noisy for our use? Seems like some folks are kinda twitchy on here.
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I don't specialize in reading datasheets aloud, sorry. I advise you to read them each time you wish to solder something as complex as a tube based OPAMP.
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