screen-plate resistor and grid leak resistor

[metebalci]

Hi,

What is the purpose of connecting a resistor between screen and plate in triode connected pentodes ? How to determine the proper value of this resistor ? What is the proper values for KT88, EL34 and EL84 ?

How to determine the proper value of grid leak resistor ?

Thanks in advance...

MB

 

[Sch3mat1c]

Prevents parasitic oscillation. As such, there is no exact value. You'll see them anywhere from zero (preamp tubes) to 1 or 2.2k for smaller outputs (6V6). Large resistors don't go well with large tubes due to higher screen current causing a significant voltage drop.

Tim

 

[SY]

As for the grid leak, you want the highest value practicable within the constraints of the tube's leakage currents and any DC feedback at the cathode. That value varies a bit from tube type to tube type; most power tubes have a max value specified, which is a good place to start. You do have to cross your fingers a bit and hope that modern equivalents show the same behavior as the older tubes; if your tubes show a tendency to run away (i.e., idle current drifts upward and out of control), switch brands or lower the grid leak value and live with the consequences of the increased driver load.

 

[SY]

You'll see them anywhere from zero (preamp tubes) to 1 or 2.2k for smaller outputs (6V6). Large resistors don't go well with large tubes due to higher screen current causing a significant voltage drop.

Actually, that resistor is usually there to cause that voltage drop under hard drive conditions and prevent screen overload. If nothing is called out in a data sheet for triode or ultralinear connection, I default to about 100 ohms.

 

[Sch3mat1c]

I mean voltage drop in normal conditions. A shorted tube isn't normal

Tim