Filtering and Screens

[mick]

Hi all
as the title says I’m trying to find out how these two interact, I’m working on my EL84 SE and I have 1k on the screens, when I first put this together I just went with another 1k dropping resistor (no choke) between the OT and the screen in the filtering, my intention was to lower the 1k dropping resistor later when I could see what all my voltages were. However when I fired it up I had 8-10 volts more on the screens than the anode, so straight away I upped the 1k dropping resistor to 2k that put the screens about 2 volts less than the anode, because the output TX is the SE from west labs (he states his TX’s are uncollected matchless stock) I took a look at the badcat minicat and according to the schematic I’ve seen the voltages on the EL84 have the screens about 8 volts higher than the anode, so my questions are:

1) Is it ok to have the screens higher than the anode ? is there advantages / disadvantages ?

2) If I have the 1k on the screens and a 2k dropping resistor ( with a filter cap between them ) does the EL84 screen see a 1k load or a 3k load ?


Thanks Mick

[Super_Reverb]

Screen grid does several things in a pentode. Screen accelerates electrons boiled off the cathode into the plate. Screen also isolates the control grid from the plate, reducing C(grid-plate) Screen is between the control grid and plate in the physical makeup of the tube. If the screen voltage is too high, you'll get too much screen current and exceed it's power dissipation ability. To keep screen voltage more stable, some designers add a resistor from screen to ground, forming a voltage divider with the screen limiting resistor. Check out the Valve Wizard's comments:

http://www.freewebs.com/valvewizard/pentode.html


rob

[mick]

Thanks Rob
I’ve always read that screens should be less than the anode but still people did and still do make amps that seem to cross that line, I’m reading that it’s bad for the tubes life as much as any other reason (if I’ve got that right), what about the second question does the screen see the dropping resistor as in series with it’s (in my case) 1k resistor or does the reservoir cap stop that ?

Thanks Mick