Shared screen resistor

[andrew]

Hi, would it be safe for KT66s in pp to share a 2K 5W screen resistor instead of each tube having its own 1K resister? Thanks.

[Firestorm]

Safe? Well, it's safer than no screen resistor at all, and lots of amps have done it (hi-fi mostly), but a shared screen resistor means directly connecting the screens of two tubes together so a catastrophic screen failure in one tube can take the other out more easily. In a push-pull amp, you also can have one tube modulating the screen voltage of the other (something hi-fi amps sometimes did on purpose). But -- my opinion -- it's safer and more stable to give each tube its very own screen resistor.

[Cygnus X1]

And at less than a dime each it is cheap insurance.

[Smitty]

470 ohm 10 watt for common and then 100 ohm 1 watt on each tube. You can use the Xicon metal films for the 100 ohm:

mouser 273-100-RC

[andrew]

My parts order shipped out a few days ago and I forgot to order the screen resistors! However I do have a 500ohm 10watt and plenty of 100ohm 5watt in my pile. Thanks for the help guys!

[Firestorm]

470 ohm 10 watt for common and then 100 ohm 1 watt on each tube. You can use the Xicon metal films for the 100 ohm:

mouser 273-100-RC

I guess. Dave Reeves did it, so you're in good company. The 100Rs are supposed to act like fuses, but may not open up in time except with an actual short.

Some of early 100-watt Marshalls have 1K screen resistors that look an awful lot like 1/2W. If only they were flameproof ...