Discussion in 'Amp Tech Center' started by peteb, Apr 9, 2018.

1. ### petebFriend of Leo's

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If you don't like

AC:

Ground-cathode resistor-12ax7-plate resistor- ground

Then consider

DC:

Ground-cathode resistor-12ax7-plate resistor- DC power supply- ground

If AC going to ground and coming back from ground is a problem, remember that the plate and cathode are alternatingly going positive and negative with respect to ground, so true to AC, the current is traveling both ways.

Ground is zero. The plate is going + or minus. Electrons are alternating being drawn to ground, then to plate

Last edited: Apr 30, 2018
2. ### petebFriend of Leo's

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RLee, I think you may see there is more to it.

Just as a steel string vibrating in the presence of a magnetic field produces AC current, the fluctuating grid voltage in the presence of significant DC current produces significant AC current flow. The tube produces AC current flow from an AC voltage.. The DC current played its role too, but the AC current is the product of the tube.

The tube produces the AC current, the plate resistor then develops the AC voltage from this current. The AC current is the product of the tube itself, the AC voltage is the product of the tube stage.

Last edited: Apr 30, 2018
3. ### petebFriend of Leo's

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If you don't like ground, consider this AC equivalent circuit:

Or this. I think a natural grounding reference point would establish itself between the two equal loads.

Or this. An AC power supply connected to two load resistors in series:

4. ### SSL9000JTele-Meister

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I think we may have reached "po-TAY-to, po-TAH-to" in this discussion...

5. ### BendyhaFriend of Leo'sSilver Supporter

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...............and I think otherwise...............

6. ### elpicoTele-Afflicted

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The plate and cathode never go negative wrt ground and the electrons never flow "both ways". It's physically impossible. I thought we covered that already, it's one direction only for the electrons and the plate and cathode voltages are always > 0V.

It can be helpful at a times to set aside the DC offsets for a moment and consider only the AC voltages, but that's a mental tool, a situation that exists only in the mind. You can't say that in the real world the cathode goes negative wrt to ground. It's just not true.

Give me a minute and I'll make a chart.

7. ### elpicoTele-Afflicted

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The plate and cathode voltages do swing up and down by almost 20V, but even at the lowest point of their swing they are well above ground. At no time do they ever go below ground. A sufficiently large input signal could make the cathode follow it right down to ground, but never below. It would simply stop following the input at that point and clip.

Last edited: May 1, 2018
8. ### BendyhaFriend of Leo'sSilver Supporter

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9. ### elpicoTele-Afflicted

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I should put the current on there too. It's in yellow:

You can see it never drops below 0.3ma. It certainly never stops being a positive value and drops below zero to "go the other way".

10. ### elpicoTele-Afflicted

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Yeah I looked back a few pages and you did understand this at one point. You talked about how a tube is like a diode because current can only flow one direction remember? Now you're back to saying current flows both directions, I don't know why.

You did say something at the end about making the AC larger than the DC, okay let's look at what happens when you drive this circuit with a larger input signal than it can follow:

The signal we're applying to the grid drops below zero volts, almost to -20V, but the cathode doesn't follow it, it stops at 0V. The current can't go below 0mA because like you said, it's like a diode. Current can only flow one way. So the current drops all the way to 0mA and the cathode falls all the way to 0V, but then that's it. We've reached the limit of the tube's ability to follow the input signal and it clips. The plate signal doesn't go above the 200v supply voltage, the cathode doesn't go negative wrt ground, and current does not start flowing backwards.

Last edited: May 1, 2018
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11. ### BendyhaFriend of Leo'sSilver Supporter

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12. ### LudwigvonBirkTele-Holic

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Great screenshot and explanation in post #150, elpico.

I believe it also is correct to say that this ^ signal would measure the same on the scope if it were not connected to the tube at all. Meaning, the tube has nothing to do with the below-zero values seen in the scope green waveform. That green line is labeled "V(grid)", but the measurement represents incoming signal we're applying to the grid.

13. ### elpicoTele-Afflicted

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Yep, that's true. It's coming from the preceding tube.

I feel like it's only a matter of time before someone notes that the blue cathode voltage trace doesn't stop perfectly at 0V, it appears to curve under it a bit. Fear not, this is not current flowing backwards through the tube or confirmation of that incorrect idea. What it's showing there is that the input signal is always driving some microscopic amount of current through the 1meg grid resistor to ground. In a cathodyne phase splitter the bottom 56k load is in series with the grid resistor so a tiny, unintended wobble is introduced to the bottom output. It's one of the ways it's not a perfect phase splitter. But again fear not, it's the imperfections in these old circuits that make them sound interesting. If we wanted perfection we'd just DI into the mixer.

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14. ### elpicoTele-Afflicted

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It must be true that a picture is worth a thousand words then. >150 posts, tens of thousands of words typed, and all it took to kill it was a picture? Huh. Wish I knew that on page 1.

Last edited: May 6, 2018
15. ### Modman68Tele-Holic

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Fear not, it is only a matter of time before the forces of chaos come to misinterpret said photo.

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16. ### LudwigvonBirkTele-Holic

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Straight-arm dropping a (presumed?) once-licked ice cream cone is an insult to the crafter of the ice cream, the city that maintains the space it was dropped on, and the tax-paying citizens who have to step over that litter.

Further, I bet you knew you were going to drop the thing in dramatic fashion before you even tasted it. You could have spared us the drama and instead have given it to a kid, or an an unfortunate person before you licked it and dropped for somebody else to clean up.

Dairy, sugar, flour, spices, refrigeration, those all have a big environmental footprint that you really shouldn't be not-concious of.

(Note: I'm commenting on the b/w picture of the guy dropping the ice cream cone. The scope photo I already said I liked)

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17. ### SSL9000JTele-Meister

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You mean... he's not levitating the ice cream cone with his mind?

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18. ### LudwigvonBirkTele-Holic

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Interesting theory. Under what conditions will ice cream flow from ground?

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19. ### clintjFriend of Leo's

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Wait, what if you dropped a vanilla ice cream cone on one side, and a chocolate one on the other?

Or to go even further, what if you juggled chocolate, vanilla, and strawberry cones? When you dropped them, would they keep their identities or become a Neapolitan mess?

20. ### BendyhaFriend of Leo'sSilver Supporter

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Under normal urban conditions, any dropped ice cream will flow ACROSS the ground, not through the ground.

Some may leak into ground, but much slower than you might think it would.

Gelatiere scoop - cone - ground

Although the action of dropping is the added impulse, that needs a dropper, it is not the dropper creating the ice cream, or the supplyer of energy required by the scoop to fall to ground.

Gravity is the force, all the dropper can do is help accelerate the first part of the fall with an initial inertia opposing input

.

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