screen resistors and grid stoppers

[Bluefinger]

Hi,

I wonder what the general reason for using these is. Would somebody care to explain? There is hardly a voltage drop across them because their values are usually very low. All I know is that installing grid stoppers on my new tweed style build cured a motorboating problem I had but I don't understand the theory behind them or the screen resistors.

thanks!!!

BF

[defaced]

Here's a quality link on grid resistors:
Grid Resistors
I'd recommend checking out the rest of Randall's site as well. It covers alot of good information.

And a discussion on screen resistors:
screen grid resistor questions [Archive] - The Gear Page

Also, don't forget about the search function here. Both of these topics will have plenty of prior discussions if you do some digging.

[Wilder Amplification]

Hi,

I wonder what the general reason for using these is. Would somebody care to explain? There is hardly a voltage drop across them because their values are usually very low. All I know is that installing grid stoppers on my new tweed style build cured a motorboating problem I had but I don't understand the theory behind them or the screen resistors.

thanks!!!

BF

Screen resistors serve to stop screen grid oscillations as well as limit screen current. Under zero signal conditions there's not much screen current and hence the screen resistor will have a low voltage drop. However, once the valve is driven with signal, screen current will increase, which will also cause the voltage drop to increase across the screen resistor. Pop your meter across the resistor to measure voltage while you're playing through the amp and you'll see what I'm talkin' about.

The grid swamper resistor is to suppress grid oscillations. Under clean signal output no control grid current flows so there's no voltage for the resistor to drop. It works via miller capacitance that exists between cathode and grid. The resistor along with the valve's internal miller capacitance forms a high frequency low pass filter (the corner frequency is well and above the frequencies that matter). This attenuates higher frequencies to suppress any control grid oscillations in the upper register of frequencies.

[Bluefinger]

Thanks!!!!

I did a search but I got a little too much information which still was a bit confusing to me ...

[kg]

It works via miller capacitance that exists between cathode and grid.

small very technical caveat: the miller effect only exists between g1 and anode--because they are out of phase with one another the effective capacitance is multiplied by the in circuit mu. it's the out of phase-ness that makes it "miller."

everything else i completely agree with!

[Wilder Amplification]

small very technical caveat: the miller effect only exists between g1 and anode--because they are out of phase with one another the effective capacitance is multiplied by the in circuit mu. it's the out of phase-ness that makes it "miller."

everything else i completely agree with!

Whoops...that was actually a typo on my part. Good catch!