While restoring/modding my Dual Showman, a tech recommended me to try out 82 ohm screen grids resistors instead of the 470 ohms. I've tried to figure out the pros et cons, but I don't get it (right). Anyone who knows? Thanks in advance.
Maybe ask the tech to explain it more simply then - or with reference to what you might expect to hear. There is no reason why any explanation here should be any more understandable than what you got from him - especially given we have no idea why you didn't understand in the first place.
Your second sentence makes little sense as is - did the second tech warn you not to go below 470ohm
I'm not an amp guru, so this is just what I read on a Google search.
Some people raise the grid resistors to 1000 Ω. Apparently to be able to use tubes like EL34/6CA7.
I didn't find a single recommendation to lower the grid resistors, with several people specifically saying not to go below 470 Ω.
This is primarily a guitar forum, with a general discussion amp sub-forum.
The Marshall forum is a sister site of this forum. It has a sub-forum called "the workbench" where actual amp techs hang out. They know a ton of theory, gladly talk about Fenders, and could tell you the why's and wherefore's of grid resistors.
There are Fender-specific forums with gurus who would have already answered your questions. They aren't affiliated with this site so I don't know if I should mention them.
Here’s my 2 cents as an amp/tube gear tech for 20 years:
My only guess is that an amp will often sound better with no screen resistors. But…the tube not be as stable. Plenty of old amps ran and tun just fine with no screen reissitors, but yes it is an extra bit of protection for your tubes.
So my guess is that he was suggesting improving the tone while still offering a bit of protection.
The tech that said ‘no way’ is probably thinking like many techs do: about reliability only.