Dunb question alert

[eschoendorff]

What the heck is a "screen resistor?" What does it do? Anyone have a pic of one in an amp?

Sorry for the doofus question, but it has been haunting me for some time.

 

[Rich_S]

Re: Dunb question alert

They are used to limit current and maintain the voltage of the "screen grid" in a pentode power tube, at something lower than the plate voltage. They operate at high voltage and need a large power handling capacity. In some designs, they tend to be a weak link, and when they fail, they do so spectacularly.

As to what they look like, they could be normal (though oversized) striped resistors, but they are more likely rectangular (white of tan) ceramic blocks, or some type of blobby epoxy-dipped thing.

 

[glassman]

Re: Dunb question alert

A screen resistor limits the current flowing in the accelerator grid (also known as the screen) in a pentode or beam power amp type tube...basically any power tube found in most guitar amp is one of these types. Without a screen resistor (screens hooked directly to the power supply), the amp may have problems controlling maximum dissipation at high output levels.

The "screen" acts like a turbocharger; at idle, it has only a marginal effect on the tube but as the input signal increases, it increases the output of the tube dramatically. When a screen resistor opens (burns up or otherwise) the effect is that that particular tube does no work and essentially just sits there.

 

[Gearjoneser]

Re: Dunb question alert

Excellent thread title! lol