screen-plate resistor and grid leak resistor

metebalci · 25th February 2004, 02:45 PM

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 · 25th February 2004, 04:40 PM

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 · 26th February 2004, 02:00 AM

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 · 26th February 2004, 03:23 AM

Quote:

Originally posted by Sch3mat1c
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 · 26th February 2004, 04:18 AM

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

Tim

25th February 2004, 02:45 PM	#1
metebalci diyAudio Member Join Date: Feb 2004 Location: Istanbul/TURKEY	screen-plate resistor and grid leak resistor 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

25th February 2004, 04:40 PM	#2
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	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 __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

26th February 2004, 02:00 AM	#3
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

26th February 2004, 04:18 AM	#5
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	I mean voltage drop in normal conditions. A shorted tube isn't normal Tim __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

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