# 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. ### petebFriend of Leo's

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Some how you are trying to turn this against me and I don’t know why?

Would you listen to yourself for a second? The DC signal?

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3. ### LudwigvonBirkTele-Holic

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This thread is starting to get hard to follow.

4. ### petebFriend of Leo's

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Rob,

Let’s just leave it at that.

The signal does not add heat dissipation to the plate.

That’s all we need for this thread.

When the discussion pivots about the DC signal, I feel the discusssion has pretty much run its course.

Thank you

5. ### robrobPoster ExtraordinaireAd Free Member

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If you insist on calling the signal in the tube AC, then yes, AC does lower the dissipation in a Class A amp. elpico's excellent animated graph he posted shows why that is so.

6. ### elpicoTele-Afflicted

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Maybe the piece you're missing still is that the output transformer is in series with the tube and it only reacts to changes in the plate current.

At idle the same strong current is flowing through both the tube and the transformer (because they are in series) but no voltage is being developed across the transformer because it only reacts to changes in current. The current isn't changing at idle so the transformer isn't reacting and nothing is happening. If all of the voltage is across the tube at this moment, and none of the voltage is across the transformer, then all of the power is being dissipated in the tube.

When we start modulating that current the transformer reacts to these changes in current by developing a voltage across it's terminals. The exact same amount of current is still flowing through both the tube plate and the transformer because again, they are in series, but the transformer has now taken up some of the available voltage. That means some of the power is now being dissipated in the transformer+speaker as well. The rest is still being dissipated in the tube, they are starting to divide the voltage between them (remember voltage dividers?) so they must also be starting to divide the power between them.

As we turn up the volume to max we make the changes in the current flow even larger. Larger changes provoke an even larger reaction from the transformer, so now a lot of the voltage is across the transformer, which leaves a lot less for the tube. In a perfect world the tube and transformer could divide the power equally at max output causing 50% of the power to be dissipated in the load and 50% to be dissipated in the tube. The tube was dissipating 100% of the power at idle, so obviously it's share of the power has fallen now and it's not running as hot as it once was. (In the real world the tube doesn't work well below a certain voltage so some of the supply voltage always goes to waste in the tube and we get something less than 50% of the power being dissipated in the load)

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7. ### BendyhaFriend of Leo'sSilver Supporter

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8. ### Modman68Tele-Holic

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Thank you for your post, I agree with what you’ve just said, the sky is clearly green.

Perhaps if you just see my colorimeter measurements performed in the dead of night, you could come to agree that grass is blue. I’m sure of it... especially after smoking it.

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9. ### robrobPoster ExtraordinaireAd Free Member

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Post of the week. Very nicely said elpico. peteb, read this post over and over until you understand it because this is the piece of the puzzle you are missing.

Last edited: Jul 25, 2018
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10. ### TeleTucsonTele-Afflicted

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This is close to the same argument I used earlier, but in my description I was tracing the power - i.e., (# electrons in a time interval) * (kinetic energy of electronics) in real time during a signal cycle in an effort to illustrate the effect of the load, and that the heat indeed comes from electron impact and the net power delivery is reduced with signal present when averaged over a cycle. Except in your description above, you're also now somehow suggesting that the the inductive nature of the transformer load is key to the general phenomenon of reduction in "redplating" with signal. You'd also see a reduction in redplating with a resistive load.

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

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Pentode and tetrode power tubes' screens keep the pull and velocity of the free electrons pretty constant so I don't buy the differing velocity theory. The velocity is pretty constant and the volume of electron flow is throttled by the grid.

12. ### TeleTucsonTele-Afflicted

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I was tempted to respond to your earlier tetrode/pentode comment, but didn't. Maybe we can go offline, but it seems to me that even while the screen does a lot of acceleration (which results in your "pretty constant" velocity), the velocity will still increase more on its way to the plate to achieve its final energy. It has to, that's what voltage is - in this case, energy difference between one conductor and another, save the work function of escaping the cathode. There is no path whereby an electron can go from the cathode to the plate without arriving with the kinetic energy corresponding to the cathode-plate voltage difference, unless it is an incomplete vacuum and there is a gas collision, or it actually strikes the screen. It may have an interesting roller coaster of a ride due all the variations caused by the screen field along the way, etc., but after the integrated acceleration from the spatially-varying electric field along the path - it will achieve that final velocity on impact.

It's interesting to me that you contest this, because it's just the internal realization of what you've lauded as a great external-load-induced explanation at a macro level. The load-induced cyclic voltage variation, even while the average current is the same, is what reduces the heat. The only way heat is reduced when the average current is the same is if the average impact velocity is reduced.

Last edited: Jul 26, 2018
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13. ### LudwigvonBirkTele-Holic

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Can you please start a new thread on this ^? (So isn't bogged down in noise of this here thread)

14. ### robrobPoster ExtraordinaireAd Free Member

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The plate voltage is higher than the screen voltage maybe what, 0 to 10% of the signal cycle in a typical guitar Class A tube amp? Many amps with only a choke between the plate and screen idle with the screen at a higher voltage than the plate.

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15. ### TeleTucsonTele-Afflicted

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Yeah, the art of teasing better and better tube performance with all the geometry, screen and suppressor advances back in the day must have been a fun trip for the designers. You're much more on top of all the tube circuit scenarios than I'll ever be. Today all that innovative talent that used to go into tube design goes into the next generation of 7nm FINFET technology to get 20 billion devices on a chip at low power, and then the world is working vigorously on what's next after CMOS runs out of steam. It's different challenges, not as fun in some ways as the analog art of tube design, but really cool innovations at the cutting edge of physics and materials science nonetheless.

To the topic here, as I've stated above, I believe that for the energy deposited on the plate, it's going to be the potential difference between the launch point and the crash destination. Tracking that energy together with current, and the respective phase of voltage and current throughout a cycle illustrates both how the AC power generated is delivered to the load and how the deposited heat is reduced. As it must - as has been said so many times in this thread.

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