Vox (Thomas Organ) tube Berkeley without screen resistors

[seveneves]

Hi there,

Have an early Thomas Organ Vox Berkeley I'm working on.

Have recapped the electrolytics and am using a 150-ohm/3W cathode resistor.

Plate dissipation is coming in around 8.25 (317 plate voltage - 10.3 cathode voltage = 307 x 27 mA) and 10.75 (316 - 10.3 = 306 x 35mA) watts for each tube.

I noticed the amp does not have screen resistors on the EL84s. Both screens just go straight to the 240V power supply node.



Although this amp is absolutely bone stock, I'm thinking it could be better to install screen resistors.

However, this is a learning experience for me so I'd like to know what y'all think? What is the disadvantage of leaving the amp without scren resistors? Advantage?

Thanks!

[Stevem]

Screen grids are not that hardy in 84s and that’s why they short out and fail way more when pushed to being over driven then a similarly wattage tube like a 6V6.

I would add a resistor that sets the screen voltage at 10 volts less then the plate.

Since it’s really the screens that have the most effect on the idle current you may then need to change the common cathode resistor to idle the tubes back up.

[ChopSauce]

Idle the tubes back up?

[Stevem]

Wait!
I did not even look at the schematic.
There already at a lower voltage potential then the plates so your fine.