Screen Resistors?

[Structo]

I have often thought about screen grid resistors in my amps.

I notice on my 100w ODS amp (6L6GC) that the screen voltage is the same as the plate voltage at idle.

I have never measured it when the amp is being played.

I remember Andy Le Blanc writing that thread about how he felt that if the screen voltage was not a bit lower than the plate it can cause fizzy artifacts in the sound.
Sometimes I hear that and sometimes not.

Why do we typically use a 470R-500R screen resistor on 6L6's and 1K-1K5 on EL34's?

Andy came up with a simple voltage divider resistor network to correct that but I have been a bit unsure to try that.

I believe on high end audio amps they sometimes use discrete power supplies for different nodes, but not on less expensive guitar amps.

Have any of you experimented with different screen resistor values and voltages and what have you observed?

Thanks!

[Structo]

I was reading Norman Crowhurt's Basic 3 book this morning and found this section about Screen Biasing, very similar to what Andy has talking about using the voltage divider.

Then below that is the article about using a screen resistor then going from that to ground with a .1uF cap or so.

The cap maintains almost a steady charge thereby regulating the voltage to the screen.

Anybody try that?

[Colossal]

Then below that is the article about using a screen resistor then going from that to ground with a .1uF cap or so.

The cap maintains almost a steady charge thereby regulating the voltage to the screen.

Anybody try that?

Same set up for an EF86 preamp gain stage!

I like the idea of restricting the screens to stay below plate potential by using a resistive divider; like the method outlined by Andy DeBlanc a while back and shown by Crowhurst. But I'm wondering if we actually like the sound of that negative feedback given by the traditional fixed resistor method of 470R or 1k for 6L6 and EL34 respectively. In other words is that single screen resistor part of the mojo formula for giving us the sounds we've come to call classic. Is a fixed screen voltage going to stiffen the amp in a good or bad way? Time to go warm up the iron...

[Structo]

Not sure Dave, when I built my D'lite I experimented with different screen resistors but I don't recall the tonal changes.

Plus that amp originally didn't have a choke, which I added later.

[Roe]

higher values will give a softer response in my experience

[talbany]

Tom
I remember reading somewhere that different screen resistors can effect tone (something about how they breakup) I've tried this and damn if I can tell a difference.. I can tell you that in my experimental days I was playing with the OC3 as a regulator for the screens and those experiments yielded some interesting tonal results..I think it has more to do with how the tube distorts ( so some harmonic content is involved) more than anything..

Tony

[Gaz]

Tom, interesting post. I think for amps designed on the edge of tube limitations (most guitar amps) would do bad things with a stabilized screen voltage. I'm talking bad for the tube not necessarily tonally.

I'm not sure how effective Crowhurst's method would be, but you better believe I'm gonna try it tomorrow and take some measurements in a couple different amps.

[Structo]

Tony, I remember when you posted info about the OC3 tube regulator.

Gaz, that would be great if you could experiment with this.
I don't think it stabilizes the voltage, but to keep it slightly below the plate voltage so that the screen is not more positive than the plate.
I believe it is when the screen conducts too much positively is when we get the nasty sounds and glowing screen grids.

I have a note from Dogears back when I was tweaking my D'lite that said the larger the screen resistor on a 6L6 the mushier the bass will be.

He said it was more of a feel thing than hearing it.

At that time I was running 1K screens and eventually I went down to 470R.
I believe I left that at that value because it sounded best.

[roberto]

higher values will give a softer response in my experience

Yes, that's was the reason why larry novosibir used a rheostat in series with the screen resistors as an attenuation on its 100W amps:
[img:800:533]http://www.larry-amplification.de/briti ... pur091.jpg[/img]

In part this is due to the fact the the screen current changes with the volume of the amp, so an higher resistance is more audible at higher volume levels, when you have more voltage drop across the screen resistor, so a lower voltage on the screen, so an higher difference between plate and screen, and then more compression.

The dynawatt has done something similar with a different approach.

[mick]

Anyone had the chance to start experimenting with this yet ?

Mick