# excessive plate dissipation causes redplating - does this power dissipation from AC or DC?

Discussion in 'Amp Tech Center' started by peteb, Jul 20, 2018.

1. ### TeleTucsonTele-Afflicted

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When the plate to cathode voltage is at a low point during the cycle.

The energy dissipated on the plate by an electron is determined by the cathode to plate voltage at the time of the traversal. The grid determines how many, not the energy.

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2. ### robrobPoster ExtraordinaireAd Free Member

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I think we need to clear up what dissipation is. It's a heat limit for the plate expressed as power in watts. The key is to keep in mind it's just a heat limit so we won't melt the plates.

There is no AC current flowing through a tube because there can be no current reversal in a tube. Your quote just adds to the confusion and noise in this thread. The signal coming out of a tube is the DC voltage variation. A downstream capacitor or transformer converts the DC voltage variation into AC. For deep discussions like this of how tubes work you really need to keep that in mind. I know this can be described as an AC voltage riding on a DC voltage offset but even so there is no AC current.

Last edited: Jul 25, 2018
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3. ### robrobPoster ExtraordinaireAd Free Member

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In a pentode the screen voltage will keep the electron pull & velocity relatively steady and like you said the control grid controls the volume of electrons not their velocity.

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4. ### TeleTucsonTele-Afflicted

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The velocity on arrival at the plate is easily determined by knowing the kinetic energy at impact. For an electron leaving the cathode and arriving at the plate in vacuum, that kinetic energy is given by the voltage difference between the cathode and the plate at the time of the traversal, and that changes with "time-varying DC".

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5. ### robrobPoster ExtraordinaireAd Free Member

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Between cathode and screen in a pentode or tetrode, cathode and plate in a triode.

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6. ### petebFriend of Leo's

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Thank you rob

7. ### petebFriend of Leo's

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Rob,

We are in agreement. Right?

Plate dissipation is DC and not AC.

When you say there is no AC current flowing thru the tube I assume you mean no AC plate dissipation as well.

We are on the same page.

8. ### petebFriend of Leo's

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That’s true, and true in class AB too.

My personal opinion is that it does not explain how class A heat dissipation reduces with signal.

9. ### robrobPoster ExtraordinaireAd Free Member

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There's no AC in the tube so yep, no AC dissipation but the varying DC, which is the guitar signal, does change the dissipation. The AVERAGE dissipation in Class A does drop when a signal is applied.

See the excellent moving graph elpico posted in the other thread (post #46) to see how that works. That graph is key to understanding why plate dissipation decreases when a signal is present on the grid in Class A.

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10. ### petebFriend of Leo's

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I agree with this assessment of class A operation.

Thank you Rob

11. ### petebFriend of Leo's

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I think this explains why the AC current component in the tube generates no heat, because there is no reversal of direction, the average amount of electrons passing any one point over any given time is not affected.

12. ### robrobPoster ExtraordinaireAd Free Member

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elpico's post #46 in the other thread shows how Class A operates.

Last edited: Jul 25, 2018
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13. ### robrobPoster ExtraordinaireAd Free Member

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Nope, not even close. There is no AC current component in the tube. Where's that head-banging-brick-wall emoji when you need it?

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Thank you

15. ### robrobPoster ExtraordinaireAd Free Member

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But you're calling the varying DC signal voltage in the tube AC. The DC signal does change the instantaneous dissipation level and in Class A the average dissipation goes down with signal. You're trying to have your cake and eat it too.

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16. ### petebFriend of Leo's

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I don’t want to mis quote you. You say no AC dissipation. Then you say there is time varying DC signal Changing the dissipation. This is the AC and you say that it lowers dissipation.

To the point of the thread, we are both saying signal does not add dissipation.

We are in agreement.

17. ### petebFriend of Leo's

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The plots are fascinating.

18. ### petebFriend of Leo's

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What part? What do you mean? It’s all class A.

19. ### BendyhaFriend of Leo'sSilver Supporter

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You love to say that don't you. Does your ego good I guess.

So very often you only think you are in agreement, due to your miscomprehention.....or as is so very clear, through misrepresentation of what someone has said.

20. ### petebFriend of Leo's

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Really?

Pulsing DC is DC with an AC component.

How can you really say that there is no AC component in the tube?

The grid imparts the AC in the tube. Do not say there is no pulsing DC in the tube, do not say there is no AC component in the tube.

Last edited: Jul 25, 2018
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