Screen resistors - please weigh in

[iknowjohnny]

[img:399:126]http://i24.photobucket.com/albums/c39/dazco/screen.jpg[/img]

[roberto]

iknowjohnny,
if we are talking about voltage drop, you know V=IxR, so if you double the current flow the voltage drop is doubled, same as doubling the resistance with a single screen. So 1k is like 2k on each, plus 1k you already have, that's equal to 3k on each.

[klingo]

1K/both=2K/each that's the way i see it too

[iknowjohnny]

Ok, so if i used two 3k resistors, one on each tube, that would be the same as what i have now?

[iknowjohnny]

Ok, heres what i found. with 3k verses the original 1k per tube, the tone changed in the following ways.

PROS: smoother top, more compress top.

CONS: loss of volume, loss of highs

It's a tradeoff, and i suppose there might be a happy medium for me between the 2. But i felt that with the higher resistance the cons outweighed the pros because my amp has what i feel is extrordinary versatility in it's ability to change the tone fairly radically for a tube amp and get a lot of different and all usable and cool sound. this is mainly accomplished thru a lot of different kinds of high end manipulation thru a variable NFB pot, presence control, and combined with the treble and mid controls i can get a ton of great tones. But adding resistance to the screens seemed to dull the high end and the various controls did much less thus allowing me far less versatility. I feel i can also cop a pretty close approximation of what i got with the more screen resistance by turning the variable NFB down. So i think i'm going to stick with the original 1k's. It just seems to give me a lot more room to tweak and get a much better range of tones, not something i'd want to sacrifice. i can however see where some people would like it and that it is very dependant on the amp itself as to whether it would benefit that particular amp's sound or not.

[iknowjohnny]

Ok, heres what i found. with 3k verses the original 1k per tube, the tone changed in the following ways.

PROS: smoother top, more compress top.

CONS: loss of volume, loss of highs

It's a tradeoff, and i suppose there might be a happy medium for me between the 2. But i felt that with the higher resistance the cons outweighed the pros because my amp has what i feel is extrordinary versatility in it's ability to change the tone fairly radically for a tube amp and get a lot of different and all usable and cool sound. this is mainly accomplished thru a lot of different kinds of high end manipulation thru a variable NFB pot, presence control, and combined with the treble and mid controls i can get a ton of great tones. But adding resistance to the screens seemed to dull the high end and the various controls did much less thus allowing me far less versatility. I feel i can also cop a pretty close approximation of what i got with the more screen resistance by turning the variable NFB down. So i think i'm going to stick with the original 1k's. It just seems to give me a lot more room to tweak and get a much better range of tones, not something i'd want to sacrifice. i can however see where some people would like it and that it is very dependant on the amp itself as to whether it would benefit that particular amp's sound or not.

[Andy Le Blanc]

going further......... the distortive qualities change....... towards the nonlinear
its not a bad sound.... very colorful...... theres a range from 470 to 4.7K ohm
that provides a wide range of textures....... allowing the amp to be used for
tone shapeing ......

has anyone tried to switch on the fly yet..... does it affect bias point ?.....

[iknowjohnny]

ya know, i just went back to the original resistors but as i was discussing in my thread about chokes vs resistors, i went back to the 1k resistor in place of the choke. I'm still undecided, but i must say that these 2 different hings are similar in thier effect. using that 1k in place of the choke does similar things to the sound and feel as increasing the screen R's, but without the loss of volume or highs. I just need to play keep trying the different resistors and choke and combinations of choke and R till i decide i've found the best way for me. ni still may play with some added screen resistance, tho i'll try less next time.

[Alexo]

Hi all, fascinating thread!

I find it very interesting that so many builders associate more compression with more touch control, while I feel the opposite is true.

Now, this is just an opinion, but to me, touch response means the amp has a strong ability to translate the commands given by the player, i.e. a wide dynamic response and the capacity to reproduce sharp transients without excessive sag or smearing of the tone.

To me, more compression means the amp will tend to put out the volume that it wants to put out, as opposed to the volume the player asks it to put out. I can certainly appreciate compression in an amp, but more is not always better - it's those sharp transients that can let a sound cut through the mix and be heard, too much compression and you'll lose punch and clarity.

But that's just my opinion.

I'll also say that there are a lot of factors that will determine how a given screen resistor value will affect the power amp. I'm sure many people here know this already, but for the sake of those who don't, it should be mentioned. Depending on the load and the screen supply voltage, a screen resistor may do a heck of a lot or it may do nothing. If the amp isn't slamming the screens and drawing a ton of screen current (i.e. if it is running into a relatively low load or has a low screen voltage), even a large screen resistor won't have much effect. Also, as has been said, if you're playing the amp in its clean range, (as Leo presumed most musicians would) then the screen resistors aren't going to come into play either.

Throwing 1K 2W screen R's into an amp that has no screen resistors is probably always a good idea if you want to keep your tubes happy. But it won't have the same effect in every amp, it's kind of like saying you should inflate every car's tires to 34 psi - it's a very generalized statement and there are a lot of factors that can be taken into account to optimize the figures for the specific amp or car you're working on.

As with all things, use your ears and follow what sounds good to you, just thought I'd throw that out there.

[iknowjohnny]

I DO associate compression and touch. Maybe a better term would be "limiting". But in any case, what i feel is a great touch sensitive amp goes from clean to quite distorted and with more hi end depending on pick strength. But when you DO hit it hard it tends to feel squishy and compress or maybe limit. So in other words, there a range of picking hardness that is quite dynamic, but past a certain point it distorts compresses. It takes a very large range of clean to distorted to do this this, and not all amps have that much range of clean to dirty just with pick attack. Mine does, and that was one of my main goals.

[benoit]

I DO associate compression and touch. Maybe a better term would be "limiting". But in any case, what i feel is a great touch sensitive amp goes from clean to quite distorted and with more hi end depending on pick strength. But when you DO hit it hard it tends to feel squishy and compress or maybe limit. So in other words, there a range of picking hardness that is quite dynamic, but past a certain point it distorts compresses. It takes a very large range of clean to distorted to do this this, and not all amps have that much range of clean to dirty just with pick attack. Mine does, and that was one of my main goals.

+1. When I think touch sensitive it's less about actual loudness (though more distorted will sound louder) and more about the amount of dirt I can get depending on whether I pick hard or not.

[Andy Le Blanc]

the function of a screen grid in a pentode is that it is raised to a positive
potential to attract electrons from the cathode and get the current flowing
between the cathode and suppressor..... otherwise the space charge would
make current flow difficult...... so by increasing the screen grid resistor your
reducing the potential of the screen grid.... changing the potential distribution
and allowing the space charge to have a greater effect......

now I dont have the gear to check it out.....but.....

how does this affect the tubes constants?.....

is what your hearing simply more distortion?.......

[greiswig]

I tried upping from the 600 ohm screen grids in my D'Lite to 2k resistors, and I actually didn't care for it. It seemed to take some of the life out of the response to my pick attack. If I tried to put it in terms of compression, I'd say that it did increase the compression, particularly at higher volumes, but not in a way that I cared for.

[iknowjohnny]

Thats how i felt. kinda wraps a blanket around the tone and kills the punch a bit.

[novosibir]

[img:399:126]http://i24.photobucket.com/albums/c39/dazco/screen.jpg[/img]

iknowjohnny,
if we are talking about voltage drop, you know V=IxR, so if you double the current flow the voltage drop is doubled, same as doubling the resistance with a single screen. So 1k is like 2k on each, plus 1k you already have, that's equal to 3k on each.

This is the static situation at idle, because the left 1K you also may regard as two 2K's in parallel - so it adds up to each screen to 3K

But when the power stage is operating, especially so far, that always one of both output tubes is driven into cutoff, the situation is changing. The dynamic situation is, that the tube, which in its half duty cycle is 'resting' in cutoff, in this half duty cycle it also doesn't draw screen current - so the first 1K now is completely in series to the screen grid R of the tube 'in operation' - and is adding up to 2K, each tube is seeing dynamically.

Larry