Screen resistors

[fritz203]

I'm curious about the Fender power tube screen grid function, and why so much power is dissipated by them? They are connected to the main B+ power rail. I've had a second failure in my Deluxe Reverb of the screen grid resistors. Is bias an issue?(mine are at about 35mA), is the rectifier tube an issue?, I'm using a Sovtek 5U4, about 405V on the plates. I just got the 3 watt screen resistors to replace the old ones.

[sluckey]

You're biased at 14 watts! Crank it down to about 25mA and see if the screen resistors last longer.

[pdf64]

For the proper 1W 470ohm screen grid resistors to blow, the 6V6 were almost certainly heavily overdissipating, probably redplating.
By blowing, they typically stop the valves conducting and so may have saved the amp’s transformers.
Your 3W replacements may allow the 6V6 to carry on conducting, thereby putting the transformers in line for the next weakest link.
Beefy screen grid resistors are fine, provided suitable fusing is added to the HT supply. In the absence of that, uprating them may be seen as a bad idea.
Equipment mains fuses are there to protect the building and its occupants from the equipment, not to protect the equipment from collateral damage when there’s an internal failure.

And yes, I suggest around 8.5W should be regarded as the max idle dissipation for 6V6 in fixed bias.

[fritz203]

Ok, thanks for your help.