Screen Resistors

[C Moore]

Can you guys explain the importance of these? I put together a Fender 5E3 Ceriatone kit, and there are no screen resistors with the kit, nor did the original Fender amp have them. I have read there is wisdom in adding a 470 ohm resistor between each screen and the 16/450 cap that leads it to ground. Obviously the amp works without them, but what would be the advantage of adding them?
Thank You

[drhulsey]

It helps prolong the life of your tubes

[C Moore]

I changed the cathode resistor from 250 to 470 ohms. That brought the plate current from about 41 MA's to 26. I thought that would help tube life a bit. How do the screen resistors prolong tube life?

[FunkyE9th]

Just like there is max plate dissipation, a pentode has max screen dissipation. You don't want to exceed these max ratings to ensure longer tube life. When a tube amp is driven hard, the plate voltage will swing to a very high voltage and to a very low voltage. When the plate voltage is very low compared to the screen, the screen tends to draw more current. This happens because when the screen is more positive than the plate, more electrons are drawn towards the screen. So if screen current increases significantly, you can exceed the max screen dissipation. Adding a resistor helps prevent/minimize this risk.

[jaysg]

I believe when tubes were made in the US, UK, and Western Europe, they were more robust than what you get now. An early 60's amp w/out screen resistors was probably a safer bet than today.

[Alexo]

...plus, screen resistors only really come into play when you're slamming the output stage to the point that it's breaking up, which of course, no self-respecting musician would ever do! ...in 1960.

[Andy Le Blanc]

tubes evolved over time to achieve more power with less distortion.
they did this by first adding a screen grid to a triode to make a tetrode.
then adding a supressor to a tetrode to make a pentode. The tetrode was first
developed to eliminate the capacitance between the control grid and plate and
forms a shield so that the plate poteintial has no effect upon the emission of the cathode.
and the space between the cathode and screen grid is not affect by plate voltage.
this makes the control grid more effective and the cathode emission linear.

when you push a tetrode, the limiting factor become the relationship between the
screen grid and the plate. Electrons knocked off the plate (secondary emission)
are attracked to the screen under demanding conditions and limit the tubes characteristics.

this secondary emission current is eliminated by the suppressor in a pentode.
which improves the operating curves and power handling efficiency.
the suppressor forms a virtual cathode that returns the secondary emission to the plate,
with the electron flow being accelerated past the virtual hump by the screen grid.

when you push a pentode, the signal swing drops the plate potiential,
to where the current flow through the tube cant make it past this "hump".
the virtual cathode becomes a barrier which leaves the screen grid to
deal with the current. This is called "Bottoming" in the old text books.
so you use a screen grid resistor to limit current caused when the tube bottoms.

[skyboltone]

tubes evolved over time to achieve more power with less distortion.
they did this by first adding a screen grid to a triode to make a tetrode.
then adding a supressor to a tetrode to make a pentode. The tetrode was first
developed to eliminate the capacitance between the control grid and plate and
forms a shield so that the plate poteintial has no effect upon the emission of the cathode.
and the space between the cathode and screen grid is not affect by plate voltage.
this makes the control grid more effective and the cathode emission linear.

when you push a tetrode, the limiting factor become the relationship between the
screen grid and the plate. Electrons knocked off the plate (secondary emission)
are attracked to the screen under demanding conditions and limit the tubes characteristics.

this secondary emission current is eliminated by the suppressor in a pentode.
which improves the operating curves and power handling efficiency.
the suppressor forms a virtual cathode that returns the secondary emission to the plate,
with the electron flow being accelerated past the virtual hump by the screen grid.

when you push a pentode, the signal swing drops the plate potiential,
to where the current flow through the tube cant make it past this "hump".
the virtual cathode becomes a barrier which leaves the screen grid to
deal with the current. This is called "Bottoming" in the old text books.
so you use a screen grid resistor to limit current caused when the tube bottoms.

Hey Andy, I'm poking around on another forum where they are talking about converting amplifiers from field coil speaker use to perminant magnet use. This involves removing the field coil connections from 1.) The screen side of the screen dropping resistor to 2.) the cathode side of the cathode bias resistor. The fields draw 115ma !!!. So he's eliminated the field coils, changed the cathode bias resistor from 40 ohm to 130 ohm (4ea 6L6 (no suffix) and a 5K screen resistor (!!??!!). I'm new there and have been kinda pushing a different approach to no avail. Given our understanding of the purpose of the screen and screen resistor being a current limiting device and that the new issue 6L6 or 5881 tubes have no restrictions on screen voltage; can you add append what you've just written to reflect the error of a 5K screen on a 360 Plate supply for a pentode? I told him his bass would be much improved but he says he likes the bass like it is. ? I don't think those tubes are acting in pentode mode at all. I don't know what they are doing. The screen voltage must drop to near 0 on any moderate grid swing.

[C Moore]

tubes evolved over time to achieve more power with less distortion.
. This is called "Bottoming" in the old text books.
so you use a screen grid resistor to limit current caused when the tube bottoms.

Andy -
Thanks for explaining all of that, I appreciate your effort. It looks like "skyboltone" has another fire for you to put out.
Thanks Again

[mhuss]

Yes, simply put, when the tube is clipping, the screen draws a lot of current.

EL34s are the worst in this regard, and their screens will light up in time with the guitar chords if they are not adequately protected. You get very few hours out of a set of tubes this way!

6L6/6V6 tubes are not nearly as bad, but adding resistance to the screen circuit is still a good way to extend tube life.

--mark

[CaseyJones]

I'm new there and have been kinda pushing a different approach to no avail.

"YOU CAN LEAD A WHORE TO KNOWLEDGE BUT YOU CAN'T MAKE HER THINK!!!"

[Andy Le Blanc]

Ive tried 4.7k resistors. But I lost more than I gained. What Ive noticed
is that you begin to reduce the tubes function.

Again, you have look at the most basic decriptions of how a tube works.

So, in a diode, when the current is flowing, the positve "potential" of the plate
exerts a force of attraction on the electrons of the space charge around the cathode.
The electrons "fall" from the cathode, with increasing velocity, through the space between, to the plate.
In a triode the control grid modifies this potentials effect on the cathode and
space charge. Just like a valve, it controls the potential gradient close to the cathode.

In a pentode the screen grid takes the place of the plate in this relationship.

It creates the potential distribution nessesary to move the electrons from
the space charge (to be controled by the grid) and accelerate them past
or over the potential of the suppressor grid or virtual cathode in a beam tube.
When the tube "bottoms" you need a screen grid resistor to limit current as the screen grids acts as a plate,
or it gets cooked trying.

With really large screen grid resistors you will begin to see the performance
of the tube become limited because the potential gradient nessesary
to overcome both the space charge and the virtual cathode is reduced.
Which is exctly what you hear, the compression of dynamics and tone.

There is a line where the performance of the device will suffer.
I have tried 4.7k resistors, it worked but I didnt suffer it for long.
The amp lost power noticablly, and lost the color that gave its break up nice tone.

[Structo]

Right now I have 470R on my 6L6 D'Lite.
3K3 on the grids.

Is that safe and valid value for 6L6's?

[Andy Le Blanc]

470 on the screens? 3.3k on the control grids?
should be fine. You need screen grid resistors for the application.
If the amp was for HIFI you'd not operate it at clip so you could use smaller
resistors for grid stops and screens, wouldnt want to lose the transient detail.
but for for guitar you want to hear the highs trimmed and encourage
compression to a point. D'lite centers around the fender values?
You could make the arguement to raise those values but the object was to
build the amp because of how that choice of those values sounded, right?

[Structo]

The D'Lite stock configuration centered around 6V6 tubes has 1K8 on the grids and 1K on the screens.

Since I never intended to run 6V6's the 470R screens and 3K3 grids were recommended for 6L6 setup by Dogears.