Question for Marshall experts

[fusionbear]

This may be a silly question, but why do the 100 watt models have a 5K6 resistor between the PI output and the grid input of the EL34 and the 50 watt models have no resistor in between the PI output and the grid input of the EL34?

Thanks!

[Cliff Schecht]

With 2 power tubes each PI pushes only one grid but with a push-pull quad the two grids connected together can crosstalk and cause potential oscillation problems. Grid stoppers are good practice on all amps but some amps are fine without them. 5.6k is standard for any modern Marshall and even newer vintage models.

[fusionbear]

With 2 power tubes each PI pushes only one grid but with a push-pull quad the two grids connected together can crosstalk and cause potential oscillation problems. Grid stoppers are good practice on all amps but some amps are fine without them. 5.6k is standard for any modern Marshall and even newer vintage models.

Makes sense. Thanks!

[JamesHealey]

the 50w'ers didn't have the 1k screen resistors either on some models.

The earlier plexi's had 1k5 grid stoppers rather than 5k6, my 69 superbass was like this.

Personally I think using 10k Carbon Composition in this position really helps old marshalls sound like they've got NOS glass in when using modern production tubes.

[paulster]

And with the tendency to exhibit parasitic oscillation in the output stages of Marshalls, using grid stoppers on all tubes (not just two of the four in a 100W) is a very good idea, and 10K is a good choice too.