Screen grid resistors

owen291 · Post by **owen291** » Thu Jul 29, 2004 1:18 am

I posted this one few days ago @ Marshall side but no response. I figured I'd give it a try here.

"I was looking around my 2555 and notice that the screen grid resistors (pin 4 and pin 6) on each power tubes are all 2K 5W. Does that look right to you guys? I checked the schematics online and it says 1K 5W. Will I fry anything if I change it to 1K 5W? How does the increased value of the screen grid resistors affect the operation of the tubes and/or the overall sound?"

rjgtr · Post by **rjgtr** » Thu Jul 29, 2004 3:09 pm

I don't see why changing them back to 1k 5 watt should do anything but imporve the bottom end of your amp, as long as the rest of the amp is stock.

Someone will correct me if I'm wrong, but I believe those resistors are added to protect the power tubes and keep them from oscillating at high volumes. Marshall has used these resistors (1st 470ohm then 1K) in all their power amps. Early Tweed era Fender amps, like Pros and Twins didn't have these (but no one expected to turn them up to 10!). Maybe the amp has some lead dressing problems that a tech "fixed" with the higher value resistors?

Good Luck

owen291 · Post by **owen291** » Thu Jul 29, 2004 4:32 pm

Thanks for the info. I checked all the other values (as per schematic) and they all seem to be stock (except the screen grid resistors). I will change them to 1.5K them to 1K and see. My only concern @ the moment is that I don't fry anything.

Post by **VelvetGeorge** » Fri Jul 30, 2004 4:15 pm

The screen grid resistors are in place to limit and to some degree regulate the current flowing through the screen.
This is very important because it directly effects the "feel" of the amp. 2K seems pretty high for EL34's. Does the amp seem stiff or unresponsive?
If so, using the correct 1K value should help. Make sure they are 5 watt resistors. And if you experiment, don't go less than 470 ohms.

George