Values of the screen grid resistors influence sound

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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

musicheals wrote:
Thu 05/13/21 5:15 pm
..well I think I will keep the 100 ohms because the difference in sound without resistors is negligible.
It's really worth a try to keep the resistors lower. Vox and Marshall also did it for a better tone in IMO
Thinking that the screen resistor value is magic by itself is not accurate. It simply means that within the circumstances of that amplifier, you like the resulting effect of lowering the value.

In GUITAR AMPS (Hi-Fi aside 😉😉) a larger gap between anode and screen voltages sounds better to my ears. I've experimented with different values designing amps with EL84, EL34, and EF86 tubes. And I've consistently found that to be the case.

Thanks,
Josh
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

I agree that every amp is different (even same models a bit) but it's really worth to tweak the screen resistors. In my case the difference was significant. Because after a long "iron and listen session" you cannot really trust your ears. So I did a second test after a few hours with 1k and 100 ohm and I confirm that the 100 ohms give a more "in your face sound". Very good for humbuckers
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

musicheals wrote:
Thu 05/13/21 5:56 pm
I agree that every amp is different (even same models a bit) but it's really worth to tweak the screen resistors. In my case the difference was significant. Because after a long "iron and listen session" you cannot really trust your ears. So I did a second test after a few hours with 1k and 100 ohm and I confirm that the 100 ohms give a more "in your face sound". Very good for humbuckers
If you want a real comparison, you should make recordings and compare them. Now, stop thinking about the screen resistor and start thinking about the relationship of the voltages. Changing the screen resistor is only one way to change the relationship. There are better ways to do it.
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Thanks a lot for your input. I'm afraid I'm not far enough in amp design to understand the whole thing about voltage relationship. My amp is sounding fantastic, but I always try to improve the tone and feel a bit.

Thanks
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Re: Values of the screen grid resistors influence sound

Post by TriodeLuvr »

JMPGuitars wrote:
Thu 05/13/21 5:59 pm
Changing the screen resistor is only one way to change the relationship. There are better ways to do it.
That might not be true, Josh. Dropping screen voltage with a simple series resistor allows screen voltage to wiggle around and to drop even further when the tube is overdriven. I'm convinced this contributes to a smoother breakup. Other methods, such as a series Zener or a voltage regulator, or even just bypassing the screens with a capacitor after the resistor, won't produce the same effect. This is one area in which I think you and I are in agreement, i.e. larger screen resistors improve the overdrive sound.

Ironically, my current hi-fi project with 6L6 types will apply 100V less to the screens than the anodes. I'm using a gas VR tube in series from B+ to produce the drop. The screens won't be stiffly regulated at a fixed voltage, but they will be locked to the anodes over variations in B+. NOT a good technique for a guitar amplifier. :lol:

Jack
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

TriodeLuvr wrote:
Fri 05/14/21 8:37 am
JMPGuitars wrote:
Thu 05/13/21 5:59 pm
Changing the screen resistor is only one way to change the relationship. There are better ways to do it.
That might not be true, Josh. Dropping screen voltage with a simple series resistor allows screen voltage to wiggle around and to drop even further when the tube is overdriven. I'm convinced this contributes to a smoother breakup. Other methods, such as a series Zener or a voltage regulator, or even just bypassing the screens with a capacitor after the resistor, won't produce the same effect. This is one area in which I think you and I are in agreement, i.e. larger screen resistors improve the overdrive sound.

Ironically, my current hi-fi project with 6L6 types will apply 100V less to the screens than the anodes. I'm using a gas VR tube in series from B+ to produce the drop. The screens won't be stiffly regulated at a fixed voltage, but they will be locked to the anodes over variations in B+. NOT a good technique for a guitar amplifier. :lol:

Jack
lol, we do agree on stuff. Hell, I agree with your HiFi stuff too...when we're talking about HiFi. 😉
And when I eventually have enough free time to make a HiFi amp, I'll definitely pick your brain on it.

You and I dig the higher screen resistor sound, but in this case, we have no idea what any of his voltages are. My guess is the screen resistor play is making up for something else.

To be clear, my intent is for him to use a higher screen resistor value, and correct the tone through other means.

Thanks,
Josh
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Sorry, I posted the voltages before. Here it is again

308V on the screen grids incl. 100 ohm resistors

314V on anodes

11 V on the kathode

The kathode resistor is 150 ohm with a cap 2200uF

Thanks a lot
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

musicheals wrote:
Fri 05/14/21 11:15 am
Sorry, I posted the voltages before. Here it is again

308V on the screen grids incl. 100 ohm resistors

314V on anodes

11 V on the kathode

The kathode resistor is 150 ohm with a cap 2200uF

Thanks a lot
Ah, I didn't see them, they got buried behind other posts.

What's your B+? What amp circuit is this? What power transformer are you using?
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

My B+ is 324V
The PT got 250V 110mA. SS bridge rectifier.
I got a kind of trainwreck related preamp. The PI and PA is straight 12 Watt. I use VVR for PA and PI.....

THANKS A LOT
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Sorry, not 12 watt. 18 watt
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

What's the brand/model of the PT? That B+ is pretty low for EL84s... Do you have a schematic of your build?
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

PT is a TBT from Germany 250V 110mA, 6,3V 5A
Yes 324V is a bit low, but I didn't want to much because I want to have the option to have classA also. I'm a big fan of vox
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

... here's the schematic

Thanks a lot
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Re: Values of the screen grid resistors influence sound

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musicheals wrote:
Fri 05/14/21 1:09 pm
My B+ is 324V
The PT got 250V 110mA. SS bridge rectifier.
I got a kind of trainwreck related preamp. The PI and PA is straight 12 Watt. I use VVR for PA and PI.....

THANKS A LOT
In what aspect is your PI and PA straight 18 watt? The NFB loop is contraproductive to the 18W vibe...
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

That's not the typical 18W circuit, but it looks like typical Marshall. I would try it without the NFB and see if you like it better. It should be more touch sensitive, and less sterile sounding.
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