Screen current question

[SILVERGUN]

Simply,,,please...my tiny brain is almost full....just trying to fill in some of the blank spaces

How does the difference between screen voltage and plate voltage affect screen current in a SE output tetrode?

I'm trying to understand the relation of screen current to cathode current....and if it's a consideration when designing towards a certain sound or to bring out a certain quality of an output tube.....OR can it just be viewed as a "by-product" of cathode current?

If screen voltage is much lower than plate voltage (30-40 volts or more) does that contribute to higher cathode current?...screen current?...both?

If I keep plate dissipation within spec (or slightly over), can I assume that because tube is not red-plating, that the screen current will usually be acceptable too?

Very sorry if this sounds a lot like questions you have answered before.....I've just been reading a lot of confusing points about this topic and can't seem to wrap my head around it.
Thanks

[HotBluePlates]

How does the difference between screen voltage and plate voltage affect screen current in a SE output tetrode?

Let's talk instead about how screen voltage impacts plate current.

Screen (G2) voltage is like a governor for the plate current, with control grid (G1) voltage being the throttle. G1 voltage change causes plate current change, but only as big as the screen voltage allows it to be. So higher screen voltage leads to higher idle plate current with the same G1 bias voltage, and higher possible peak plate current with a given plate voltage.

I'm trying to understand the relation of screen current to cathode current....and if it's a consideration when designing towards a certain sound or to bring out a certain quality of an output tube.....OR can it just be viewed as a "by-product" of cathode current?

Guitarists think everything is designed for TONE!!!  

Really, the percentage of cathode current that diverts to the screen (to become screen current) is a function of the internal geometry of the tube.

The screen grid is physically between the control grid and plate; current flows from the cathode through the gaps in G1 and on to the plate. In most pentodes (ex. EL84, EL34, 6K6), there is a somewhat stable ratio of cathode current that gets split between the screen and plate, because of how likely that cathode current might impact the screen grid itself instead of passing through the gaps.

"Kinkless Tetrodes" and beam power tubes (ex 6L6, 6V6) have screen grids which are aligned with G1 windings, so that as cathode current is split into sheets passing through the gaps of G1, they will likely pass unobstructed on their way to the plate. G2 voltage has an influence on these streams of electrons, but relatively fewer run into G2. The ratio of cathode current that becomes screen current is lower in these types, due to their construction.

So by comparison, as you apply bigger signal to an EL34, the bigger cathode current is more likely to run into the screen (which is not aligned with the control grid) and result in bigger screen current during signal conditions. That's why the EL34 amps tend to use a 1k screen resistor where most Fender amps using 6L6/6V6 don't. The nature of the tube leads to a possibility of bigger screen current during operation, and so a need to increase the screen resistor to keep the screen within its dissipation rating.

So the old amps did that for purely practical reasons; today, we might slap a 3k screen resistor on a 6V6 for tone...

[HotBluePlates]

If screen voltage is much lower than plate voltage (30-40 volts or more) does that contribute to higher cathode current?...screen current?...both?

Half the time, the screen voltage is more like a couple-hundred volts below the plate voltage.

To function as a pentode, a 5-element tube needs to have its screen held at a reasonably constant voltage. Meanwhile, during the input signal swing, the plate voltage is moving both higher and lower, as the plate current works against the plate load impedance (similar to the case of triode preamp tubes and their plate load resistor).

When the input signal swings positive, it lessens bias voltage, cathode current increases, more current has a chance to impact the screen, remaining current goes to the plate, and plate voltage drops (due to plate current, plate load impedance and ohm's law). Peak plate voltage change in an amp with 400v+ on its plates might be near or a little over 300v. If the output tube started with 400v on plate and screen, at peak tube current the screen might be 400v and the plate at 100v. During the negative input swing, the opposite occurs with the screen at 400v and the plate at 700v (all momentarily).

My example assumed screen current didn't increase and cause a drop in screen voltage due to a series screen resistor.

Back to your question: if screen voltage is lowered below some reference point, tube current will be lowered. The relationship to the plate voltage does not matter (unless the plate of an output tube is below maybe 50-70v). That's because that's what makes a pentode a pentode: that plate voltage doesn't impact tube operation, because the "screen grid" is screening the input from the output.

Triodes have different plate curves because their plate current in influenced to a greater degree by their plate voltage. A screen, held at a steady voltage, is what gives pentodes/beam tubes their characteristic plate curves. If you allow the screen voltage to vary (perhaps by connecting the screen to the plate), tube current will vary, and if you connect the screen to the plate the plate voltage will cause plate current changes just like a triode.

If I keep plate dissipation within spec (or slightly over), can I assume that because tube is not red-plating, that the screen current will usually be acceptable too?

Probably. If you start with plate voltage near screen voltage, and plate dissipation stays okay, screen dissipation will probably be okay.

There are ways you could booger this. But adding a series resistor insures if screen current rises, screen voltage will be slightly lowered, loweringthe screen dissipation.

[SILVERGUN]

They are some great (not too technical) answers...
Thanks for taking the time to help......your answers make sense and are easily digestible.

I'm definitely smarter now than I was before I read your responses...

Seriously...THANK YOU!

[HotBluePlates]

You're welcome.

If you scrool down to the bottom 3 pictures on this page deconstructing an EL37 (pentode), you'll see what I'm talking about regarding cathode current being able to hit the screen sometimes due to non-aligned grid windings.