Quick question about screen resistors

[martin manning]

I think pdf is right- a huge screen resistor will colapse the grid curves and all but shut down the current flow. Tying the screen to the plate, even through a resistor (as several amps with "tiode mode" switches do) makes it act like a triode.

[jdw3]

How do you measure plate voltage/screen voltage? If I get the readings at idle, how do you know where they are under load? A signal generator?

[jdw3]

My approach would be to increase the 470 ohm Screen B+ node supply resistor to 1k or 1k5, and use smallish individual screen grid stoppers, mounted on the tube bases, eg 220 or 470 ohms.
That minimises the local negative feedback, gain / power compression.

What difference does the amp "see", compared to 470 on the board and 2.2K at the tube?

My amp sounds good, without measuring anything. Should the voltages be checked?

[martin manning]

How do you measure plate voltage/screen voltage? If I get the readings at idle, how do you know where they are under load? A signal generator?

Just measure plate-to-ground and screen-to-ground. If the screen resistor is selected to get screen voltage a bit below the plate voltage at idle it will be fine under signal conditions.

My approach would be to increase the 470 ohm Screen B+ node supply resistor to 1k or 1k5, and use smallish individual screen grid stoppers, mounted on the tube bases, eg 220 or 470 ohms.
That minimises the local negative feedback, gain / power compression.

What difference does the amp "see", compared to 470 on the board and 2.2K at the tube?

pdf answered your question already: "That minimises the local negative feedback, gain / power compression." If you drop the screen node voltage on the B+ string, then you'll minimize the compression effect, but also drop all of the preamp voltages (unless you can reduce another resistor downstream to bring them back up). Other people might prefer the screen compression.

My amp sounds good, without measuring anything. Should the voltages be checked?

I'd measure the plate and screen voltages to see where you are, and if the screen is a few volts below the plate, then it's all good.

[tubeswell]

If you use massive Rg2 (i.e.; like 1M), the pentode or tetrode will act like a triode (because the screen will no longer be acting as an effective electron accelerator)

That doesn't sound quite right to me; my understanding is that pentodes can be used as triodes when their screen grids are strapped to their plate.
Rather a massive (unbypassed) screen grid resistor will tend to hamstring the tube, it won't be able to operate as intended due to all the NFB created.
As Vsg tends to zero, so does plate current, ie the tube gets turned off.
It would just act as a really weak tube, rather than a triode equivilant.
Pete

Actually yes you are right - the resistance would need to be very high indeed in order to block screen current.