screen voltage

[Geezer]

jbrew73
Senior tube assistant
Posts: 169
(6/9/05 8:51 pm)
 screen voltage
--------------------------------------------------------------------------------
 with all else being the same how does screen voltage effect the sound or performance of an amp?

let's say i have an amp with 500v on the plates and ran the screens at 300, 400, or 500v. what differences would i expect to see?
 
JayB
Senior tube assistant
Posts: 133
(6/10/05 3:49 pm)
 Re: screen voltage
--------------------------------------------------------------------------------
 I'v been wondering this too. I havn't got quite the understanding of it yet. From what I'v played with, it sounds better with just the screens sitting about 10V less. I've tried between 350 -400 volts and it just seamed lifeless to me. Maybe someone with the tech know how can chime in and gives us the details.
 
PHATamps
I only work on tube amps
Posts: 489
(6/10/05 10:28 pm)
 Re: screen voltage
--------------------------------------------------------------------------------
 Hey guys, here's a link to an archived thread about this issue, but pertaining to EF-86 preamps. I beleive the principle applies to power pentodes and beam tetrodes as well.

p210.ezboard.com/fhoffmanamplifiersfrm8.showMessage?topicID=10.topic
 
HotBluePlates
I only work on Fender's
Posts: 1210
(6/11/05 11:39 am)
 screen voltage
--------------------------------------------------------------------------------
 When there were only triodes for tubes, the high frequency response was limited by the capacitance between the grid and the plate. So engineers put a screen grid in between to shield the plate from the grid. It worked to reduce the capacitance, but what they found was that the plate current had more to do with what voltage was on the screen than what voltage was on the plate. What I mean is that changing the screen voltage made a bigger change in tube current than changing the plate voltage.

In fact, the screen is so e

[Geezer]

In fact, the screen is so effective, if you connected it to 0 volts, tube current would reduce to almost nothing.

Also, tube data sheets for pentodes and beam power tubes shows curves with a set screen voltage. Changing the screen voltage would change the curves.

I don't know what tone effect changing the screen voltage will have on its own. The best way to find out is to use a variable power supply (or a power scale circuit) to change the voltage, and then see what you like. If you change the voltage, you'll probably need to rebias, because of the change in plate current. Just using a larger resistor may not be the best idea, because of the effect of the bigger resistor on the sound.

That's a whole new can of worms. Hi-Fi- people would use the smallest screen resistor they can get away with. As the tube passes more current when a signal hits it, the screen will draw more current. That extra screen current flows out through the screen resistor, and would cause a voltage drop, which lowers the voltage on the screen. Since that lower screen voltage will drop plate current some, you get a compression effect. The effect is bigger with a bigger screen resistor.

I think adding sponginess and compression with a big screen resistor is a valid approach. I just say that if you want to listen to the effect of different screen voltages, do it with a power supply, so that you don't confuse the sound with the effect of the bigger resistor. Also, rebias the tube to have the same dissipation with each changed screen voltage. Then you'll know what effect the screen voltage change has.
 
jbrew73
Senior tube assistant
Posts: 170
(6/12/05 9:29 pm)
 Re: screen voltage
--------------------------------------------------------------------------------
 thanks for the response HBP
i'm building a bass amp with about 550v on the plates and was wondering if i needed to regulate the supply for the screens. i guess this becomes more important on amps with 600-700v on the pla

[Geezer]

i guess this becomes more important on amps with 600-700v on the plates(musicman ,hiwatt 201+405).
i remember doug talking about a large screen resistor causing a compression effect. i wonder how it would work on a bass amp?
 
PHATamps
I only work on tube amps
Posts: 491
(6/13/05 12:36 pm)
 Re: screen voltage
--------------------------------------------------------------------------------
 
Quote:
--------------------------------------------------------------------------------
In fact, the screen is so effective, if you connected it to 0 volts, tube current would reduce to almost nothing.
--------------------------------------------------------------------------------



In my Peavey Classic 50/50 power amp, the standby switches open the respective screen supplies, and the amp goes dead quiet.
 
HotBluePlates
I only work on Fender's
Posts: 1217
(6/14/05 10:10 am)
 screen voltage
--------------------------------------------------------------------------------
 If you look at some amps with 6550's (or 6146, KT88, etc.) that use high plate voltage, the screens are typically much lower voltage. I'm thinking mainly about the SVT, where the plate voltage may be high, but the screen is 150v or more lower.

Nothing magical here, but just that the screens have a dissipation rating like the plates. Best bet, your screens will only be rated for 2.5-3.5 watts, and will draw some current, maybe 1/10 plate current or more. You don't want the screens to burn open and cause tubes to fail, especially since screens seem to be a weak spot on some newer tubes.

Using a choke between the plate and screen nodes in the power supply will help keep screen voltage more constant. Using a resistor instead, and increasing the screen resistor itself will increase the compression effect. Which you choose is up to you. While compression for bass is sometimes good, a very fast responding amp is also sometimes good. It all depends on what you want to hear.