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Screen Resistors

2/21/2004 4:19 PM
Dan Lurie
Screen Resistors
What is the effect of changing screen resistor values?? Fender uses 470ohms, Marshall uses 1K and I have seen as big as 5K+. Could I expect a different sound, feel or both?? Thanks!
2/21/2004 5:31 PM
Chris ( CMW amps )

They're used to limit the G2 current . Most of the time I'm using 470/5W for 6V6s and 6L6s and 1k/5W for EL34s , 6550 and KT66 .  
I did experience loss of "punch" with 1k for 6L6s in a pretty stock Bandmaster .  
Hope this helps ,  
2/22/2004 3:35 AM
Yes, screen resistor values affect sound
Hi Dan  
I made the mistake once of putting 10k resistors on the screens of a push-pull 6V6 amp. It sounded so bad that it prompted me to experiment with screen resistance by using a 5 watt potentiometer, varying the resistance to see what happens. The amount of screen resistance made a big difference. I posted my notes from that test some time ago - here they are again.  
6V6 screen grid resistance:  
1) 10k ohms: sounded terrible.  
2) 1k ohms: not nice at all, artificial sounding, harsh distortion.  
3) 470 ohms: some improvement, but still harsh and ugly sounding.  
4) 220 ohms: starting to sound better, good clean tone with a smoother transition to better-sounding distortion.  
5) 100 ohms: nicer still, guitar and amp sounding great, good clean sound, really nice distortion, still has some amount of compression.  
6) 10 ohms: loving it  
7) no screen resistor (like the 5F2 schematic): loving it  
As long as the screen grid doesn't exceed maximum dissipation there should be no problem (although a resistor can offer protection in the event of a tube failure). Even with zero or minimal resistance the screen dissipation in my amp is within working levels.  
2/22/2004 8:27 AM

According to the Radiotron Designers Handbook, beam tetrodes donīt need high value screen resistors, 1K is too much, 100-470Ohms is recommeded. I have also tried various vealues and found the same, bigger values sound like doodoo.  
2/22/2004 4:43 PM

I'm waiting a hammond 378cx pt which I ordered from Angela Instruments. Now I find myself thinking that can I use it with 6L6GC's/kt66's? I mean that using this PT in a trad marshall style circuit there is gonna be around 540V right after the choke going to screens. How 6L6's are taking these higher voltages, and how about in ul and/or triode mode too?  
My plan was to use a 378cx, four 6L6GC's and take 100w out of 1650N. Perhaps it would be better (safer?) use here two kt88's instead? Which combination will be "tighter" with 1650N - quad of 6L6's or a pair of kt88's?  
2/25/2004 2:56 AM
Jack Koochin

Good post, danf!  
Turn over another rock and look what data pops up! Glad to see stuff like this coming up.  
I've tried the 1K screen R on my Fender and didn't like it much. Prefer the stock 470R. But never tried a lower value...  
How do you measure the screen current without a resistor, just a low value R in there to measure across?  
2/25/2004 4:14 AM

Hi Jack  
I have an accurate 1 ohm resistor, which is close enough to zero - so that would work. But with these tests I plotted the results, resistor value v. current, using Excel (or was it graph paper - I did this approx. 2 years ago). With values plotted from 10k down to 10 ohms you can get a good indication of what the current is at zero ohms.  
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