Increasing the grid stopper resistors on output valves?

[Mark]

I have a post going in the Fender section where I am rebuilding a Bassman 100. I have been thinking that the amp may distort and generate high frequencies.

This amp will be used as a Bass amp so I'm not looking for that kind of content. I'm thinking if I increase the 1.5K grid stopper to the Marshall value of 5.6K. This will slow down current flow during forward conduction and reduce the higher frequencies generated. The SVT amps use 47K grid stoppers, so it shouldn't really be an issue increasing the resistance. Thinking aloud, 5.6K seems a bit tame when looking at 47K.

What are your thoughts?

https://www.thetubestore.com/lib/thetub ... ematic.pdf

Thanks for your time and assistance.

[danlad]

From what I read in Valve Wizard articles and books you can go surprisingly high on pentodes with little to no audible effect.

Think (some at least) Supro amps had 47k on the power valve grids too.

Don't have access to my book just now to confirm though!

[Mark]

Thanks for your reply.

I found a bit of info here. Nothing really solid though.

https://el34world.com/Forum/index.php?topic=21489.0

https://www.aikenamps.com/index.php/gri ... -they-used

[martin manning]

The example Blencowe has used is the Marshall 4001 Studio 15 2x 6V6 amp where the power tube grid stoppers are 100k. Going to high values will certainly delay the onset of blocking distortion, so probably a good idea for a bass amp.

[xtian]

What's the drawback of high value screen grid resistors? I think you will get stiffer, less compressed response from overdriven power tubes. True or false?

[martin manning]

Larger screen resistor value means more compression. Screen voltage goes down when the drive signal goes up, and the plate curves get squashed.

[xtian]

Larger screen resistor value means more compression. Screen voltage goes down when the drive signal goes up, and the plate curves get squashed.

OK. Similar function to anode resistors on preamp stages? Like if you raise from 100K to 220K plate resistor?

[martin manning]

No, changing Ra on a preamp stage would move the load line. Dragging the screen voltage down on an output tube doesn't change the load line, it compresses the plate curves and reduces the transconductance.