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Higher Screen Resistor Values - Tried It?

8/27/2000 2:38 AM
Higher Screen Resistor Values - Tried It?
Fellow Ampagers,  
I am going to ask a question that was part of "Blue Lyne's" thread which was mostly ignored. He was asking about increasing the value of the screen resistors in his 6V6 amp and the resulting effect on the sound. The responses he received between others flaming each other seemed to be oriented toward what would happen electrically, meaning "less screen current" and "lower screen voltage". We all intuitively know this will happen, but on this board I thought we are most concerned with good tone as the end result.  
Recently, I completed a custom Bandmaster Reverb chassis (former boat anchor) that is now in a 2x10" combo cabinet very loosely based on a Fender blackface amp with reverb. It is using 2x6L6 tubes, solid-state rectification (New Sensor Blackface Bassman power tranny), a New Sensor Bandmaster-style output tranny, and reissue Jensen 10" speakers. I have repaired, designed, and built many tube amps over the years, but with this one I installed 2.4k screen resistors just as a test.  
I must say that this amp sounds very good at low volume and full band volume. I did not notice any difference in tone really from a Fender amp using 470 ohm screen resistors. This amp is still in the final tweaking stages, so I don't have numbers to share with you all, but I was surprised that the tone was not all that different from other Bandmasters/Bandmaster Reverb I have heard using 470 ohm resistors.  
Has anyone done an A/B tone test with normal Fender screen resistor values (470 ohm) versus increased values (say 1k and up) on the same amp? I am interested to know if anyone thinks I am way off here, or if higher values sound better/different/worse than stock values to you.  
Thanks in Advance,  
8/27/2000 5:38 AM
Peter S

I have done fairly extensive testing with regards to the effect of different screen voltages/resistor values etc. while I was developing the early Kimerik K-50 amps. With lower screen voltages/higher resistor values you will have to set your control grid voltage less negative to draw a given amout of current at a given plate voltage. The amp loses headroom with the lower screen voltage, but the effect on tone is very subtle. With 6L6GC tubes, the screen grid voltage difference between a 470ohm resistor and a 1k-1.5k is very minimal because 6L6GCs don't draw very much screen current. The effect on tone with a 1k resistor on a Fender style amp is IMHO hardly noticable, if even audible at all.  
Peter S
8/27/2000 6:02 AM
Steve A.

    Have you done much testing with the screen and power supply resistors for EL84's? I ask because I see all sorts of different screen and PS resistors used in EL84 amp schematics... I bet that it does make a difference but there are too many combinations for me to try them all.   :(  
    In particular, do you have any observations about using a 470R vs 1K resistor in the power supply (instead of a choke) with EL84's? More power, more headroom, more xxxx?  
Steve Ahola
8/28/2000 5:56 AM
Peter S

I haven't messed around much with EL-84's. I mostly play with 6L6,EL-34, and sometimes 6v6 amps.  
Peter S
8/28/2000 3:05 AM

Thanks for your reply. I know that lots of folks have used 1.5k resistors with 6L6s, especially guys who swap in EL34s. That is why I decided to take my screen resistors up to 2.4k and see what happened. Like you said you saw with 1.5k, I didn't see a whole lot of tonal difference using 2.4k.  
Recently, there has been a lot of talk on this board about glowing screens on KT66 reissues and 6V6 tubes. Higher screen resistor values can protect against this, and my tests seem to show that the protection comes without much or any damage to tone. Nowadays, with new tubes being made that are not as robust as older models, using larger value screen resistors seems like a reasonable safety precaution to take.  
Does anyone else have any thoughts or sound experiments using higher-than-usual screen resistors in their amps? The more opinions the better ...  
8/29/2000 2:49 AM
More Opinions Please!
Come On,  
Somebody else must have tried higher value screen resistors! How about chiming in here?  
8/29/2000 4:16 AM
Carl Z

I've gone as high as 1k in Fender circuits and I couldn't hear a bit of difference either. However I've never gone with as large a value as you're suggesting. No particular reason...just never thought about it.  
Carl Z

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