Why don't pentodes require aligned grids?
 

As I understand, an important part of beam power tube technology is that the screen grid is aligned in the shadow of the control grid. This keeps enough electrons off the screen grid so it doesn't exceed its relatively limited dissipation rating.

On the other hand, true pentodes rarely (never?) have aligned grids. Why not? Why don't they suffer from cooked screen grids? Is there some fundamental difference in the spacing between control grid and screen grid (pentode vs. beam power tube) that makes the lack of alignment irrelevant?

The answer may be in your post.

-> power

The pentodes without aligned grids probably are low power pentodes or maybe a screen grid resister is used in the circuits.

Someone correct me if I'm wrong...

Pentodes and Beam-Tetrodes (which I assume you are mentioning) were both created to solve the problem of secondary emission (cause of the kink in anode characteristics in Tetrodes).

The beam-tetrode avoids this problem focusing the electron stream using beam anodes that generate narrow streams of electrons which go straight through the two vertically aligned grids. When secondary emission accours the rouge electrons get swept back up by the dense electron flow, thus solving the problem.

Where as the Pentode uses a third grid to absorb all electrons with low velocity and transfer them back to the anode. Hence no need for grid alignment, even though it would be more efficient.

The beam tetrode was invented primarily to avoid the Philips patent of the Pentode.

Hope this helps...and is correct!

Will :)

I'm specifically thinking about EL34 vs. 6CA7. They're both firmly in the same dissipation and output power class. I'm probably also considering 6BQ5 (pentode) vs. 6AQ5 (beam tube).

Assuredly so.


I'm not sure the mechanism is exactly the "sweeping up" as you describe, but basically I think that's it. Perhaps the secondary emitted electrons are repelled by the relatively dense negative field presented by the flowing electrons.


Not exactly. The suppressor (third) grid is held at a low potential, just like the beam formers. It repels electrons, not absorb them. The repelled electrons get collected back at the anode.


I believe that to be true.


So my question is still this - presented with the positively charge screen grid and a relatively large current of electrons, why doesn't the pentode exceed it's G2 dissipation rating? It's got to be some kind of grid spacing thing...

"So my question is still this - presented with the positively charge screen grid and a relatively large current of electrons, why doesn't the pentode exceed it's G2 dissipation rating? It's got to be some kind of grid spacing thing..."

I think you will find, after checking thru some data sheets, that the non-aligned pentodes cannot operate with plate voltage swings as far below the screen voltage as the aligned beam tubes can. This is due to the screen current picking up heavily there. So this alignment/g2 dissipation issue works itself out as higher g2 efficiency allowing larger plate voltage swing (which ends up producing higher efficiency plate power output) when using beam tubes. So each kind is operated up to its max diss. specs, but the aligned ones produce more output.

Don

Ty,

I have some RCA 6005/6AQ5Ws. Their anodes are circular in cross section. That suggests that they are power pentodes, not beam power tetrodes. Elliptical construction is what one expects in a "beamie".

The EL34/6CA7 situation is highly illustrative of the fact the the 2 categories are "equivalent". Look for aligned g2 and radiators in power pentodes, as is found in beam power tetrodes. One way or another, screen grid power handling has to be correct.

BTW, the single big difference between the 6AQ5 and the prototypical 6V6 lies in a more fragile screen grid.

I have many beam tubes with knife-edge anode structures, and decidedly *NOT* elliptical construction. Good ole TV sweeps they are. When in doubt, check for the third set of grid posts to confirm pentode construction.

I also need to go and say that none of youse has answered the question originally posted yet.
cheers,
Douglas

"I also need to go and say that none of youse has answered the question originally posted yet."

"Why don't they suffer from cooked screen grids?"

You have to operate non-aligned pentodes at higher plate voltage (relative to the g2), in order to keep g2 current safely down, so that the g2 doesn't get cooked. This causes the NA pentode to waste more DC power in the plate. So non-aligned pentodes are less efficient when comparing AC power output to rated plate dissipation.

On the other hand, many g1 frame grid beam tubes do not have aligned grids. Like 12GN7, 12HL7 and others, and they aren't so efficient either.

Don

Good stuff. Keep it coming. I'm starting to think about how ultralinear operation fits into all this. At idle, isn't the screen grid sitting at a higher potential than the plate?