I am looking at building a 100W 4 x 6L6 amp and am wondering about screen resistors. There are some various discourses to be found on the effect of varying values on the screen voltage, but I was wondering if someone has some input or opinion on their effect on tone.
One highly regarded designer suggested I replace the stock ones in my Twin Reverb with 1K 5 watt devices, without any other comments.
Can anyone enlighten me here? I do have some 820 Ohm 10 watt devices sitting here gathering dust and wondered if I should pop those in the Twin and also use them on the new build.
Thanks,
rmb
Screen Resistors
Moderators: pompeiisneaks, Colossal
Re: Screen Resistors
The old standby on 6L6 tubes is the 5 watt 470R sand box resistors.
The 1K resistors are frequently seen on EL34 and 6V6 tubes.
If you are using a choke in your amp, sometimes that does not lower the voltage to the screens so you can up the resistance there.
I'm not sure how important that is, for example on my 2 x 6L6 amp I have a choke before the screen supply and I have 560R screen resistors.
The voltage is the same as on the plates.
The 1K resistors are frequently seen on EL34 and 6V6 tubes.
If you are using a choke in your amp, sometimes that does not lower the voltage to the screens so you can up the resistance there.
I'm not sure how important that is, for example on my 2 x 6L6 amp I have a choke before the screen supply and I have 560R screen resistors.
The voltage is the same as on the plates.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Screen Resistors
There's a nice chapter on pentodes in Merlin Blencowe's book where he has a lengthy discussion on the effect of changing screen voltages.
There's a cut-down version here on small signal pentodes.
http://www.freewebs.com/valvewizard/pentode.html
There are also a couple of pages on output stages on the same website.
Basically, the screen voltage affects the overall dynamic plate resistance of the tube. Lowering screen voltage reduces transconductance and increases plate resistance. It also increases input sensitivity and lowers gain. Raising screen voltage has the opposite effect.
You usually design pentode stage so that the load line passes through (or slightly below) the knee of the 0V grid curve. Raising or lowering the screen voltage respectively expands or compresses the grid curves, so that you can get the 0V grid curve in the area you want it to be in relation to the load line.
If you clamp the screen voltage with a screen bypass cap, then depending on the value of the bypass cap, you can manipulate the frequencies that get boosted through the stage.
Screen grid resistors are there to eat up surplus screen current. Any value between 100R and a 4k7 will work, with 470R to 1k5 being commonly used. You need to be generous with the power rating of the resistors because screen current increases under signal conditions.
There's a cut-down version here on small signal pentodes.
http://www.freewebs.com/valvewizard/pentode.html
There are also a couple of pages on output stages on the same website.
Basically, the screen voltage affects the overall dynamic plate resistance of the tube. Lowering screen voltage reduces transconductance and increases plate resistance. It also increases input sensitivity and lowers gain. Raising screen voltage has the opposite effect.
You usually design pentode stage so that the load line passes through (or slightly below) the knee of the 0V grid curve. Raising or lowering the screen voltage respectively expands or compresses the grid curves, so that you can get the 0V grid curve in the area you want it to be in relation to the load line.
If you clamp the screen voltage with a screen bypass cap, then depending on the value of the bypass cap, you can manipulate the frequencies that get boosted through the stage.
Screen grid resistors are there to eat up surplus screen current. Any value between 100R and a 4k7 will work, with 470R to 1k5 being commonly used. You need to be generous with the power rating of the resistors because screen current increases under signal conditions.
Re: Screen Resistors
Thanks for the excellent and concise perspective.
rmb
rmb