Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on February 12, 2011, 03:30:32 pm

Title: Screen resistor values?
Post by: firemedic on February 12, 2011, 03:30:32 pm

Hi all,
I'm getting ready to build a Pro Reverb AA165, my last build probably for a while. I'm getting used to putting Pentode/Triode switches in my builds & I use the Torres layout from "Inside Tube Amps". According to Dan Torres, the screen resistors must be changed from 470R to 1K. He deems it to be a Very Important Step that must be done.
Is this true, & if so why? I'd like to keep the values as close to the original schem as possible but I like my 1/2 power switch too much to give it up.   

Title: Re: Screen resistor values?
Post by: FYL on February 12, 2011, 07:26:07 pm

Torres recommendations are always to be taken with a grain of salt. 470R is a minimum with 6L6s, you may use 1K or 1K5 with no ill-effects and get slightly better screen protection - could be useful at stage levels with the amp fully cranked. Use at least 3W-rated FP resistors.

Title: Re: Screen resistor values?
Post by: jjasilli on February 12, 2011, 08:42:38 pm

See:  http://archive.ampage.org/threads/0/ampdiy/003858/DR_screen_resistor-3.html

In triode mode the screen is connected to the plate.  But the plate draws much more current than the screen can handle.  The larger resistor limits current and protects the screen.

Title: Re: Screen resistor values?
Post by: firemedic on February 12, 2011, 08:48:20 pm

Thanks guys, that clears it up.

Title: Re: Screen resistor values?
Post by: GuitarPilot65 on March 11, 2011, 05:48:25 pm

I swapped the stock 470Ω screen grid resistors for 1kΩ/ 3w flame-proof ones in my DRRI (R62 & R63).  Sounds great.

Title: Re: Screen resistor values?
Post by: firemedic on March 11, 2011, 06:53:40 pm

I did the same thing, 1k 5w. It sounds real good.