Values of the screen grid resistors influence sound

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musicheals
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Values of the screen grid resistors influence sound

Post by musicheals »

Hello at 18 watt,
I built a few EL84 amps before and that's my absolute favourite tube. About one month ago I found the 18watt site and I have to say, that I'm very impressed by the knowledge of the members. So I decided to join. I'm still learning (who doesn't 8) ) and there's a thing I'm wondering for a long time. The original 18 watt Marshall and vox ac15 and ac30 use screen grid resistors of 100 ohms. Many people suggest to go higher, for example 1k.
I tried different values already and my impression is, that the value of the resistors altering the sound significantly. I even dropped them and I like the tone. But I was afraid to burn the tubes, so I use 100 ohm.
I curious, what the other builders think about this topic.
Thanks a lot

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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

The screen resistor has a couple functions. 1 is to protect the tube, and the other is to keep the screen voltage lower than the plate voltage (which also protects the tube). This isn't the whole story though. You also have to look at the other parts around the power tube, and the resistor between the stages that feeds the specific screen resistors you're referring to. Typically in 18 watters, that resistor is 1.5K to 2.2K. Then the 100Ω or 1KΩ is added after that. I usually use two 1K resistors, 1 for each tube. Zaphod_Phil likes to use a single 1K resistor, followed by two 100Ω ohm resistors, 1 for each tube.

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

Post by geoff 1965 »

i've just seen something related to this lately and that is the mullard tubes built at that time and being used by marshall & vox when the amps were designed i.e. 1960's were very robust valves and the screens could handle 100R.
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Re: Values of the screen grid resistors influence sound

Post by musicheals »

Thanks a lot.
Yes, my main resistor right after the OT voltage is 1.8k.
I thought, that the screen resistors limiting the current 8O
Anyway, it seems to me, that higher screen resistors make the sound less aggressive and somehow a bit lifeless.
I tried 100, 220, 470 and 1k and stayed with 100 ohms to have a bit protection. But I would like to remove them. Is there no danger with a rail resistor of 1.8 k ?
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

I definitely don't recommend removing them. But keep in mind other things are happening too. Take voltage readings on the power tubes with the different screen resistor values. Look at the relationships of the voltages with the different screen resistor values.

The correct thing to do would be to find a similar relationship with the correct screen resistor value (always making sure the plate voltage is higher than the screen voltage). The screen resistor value itself is not magic.
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Re: Values of the screen grid resistors influence sound

Post by TriodeLuvr »

geoff 1965 wrote:
Thu 05/13/21 10:08 am
i've just seen something related to this lately and that is the mullard tubes built at that time and being used by marshall & vox when the amps were designed i.e. 1960's were very robust valves and the screens could handle 100R.
Mullard's own EL34 amplifier was an ultralinear design with essentially equal voltages on anodes and screens. They used 1K screen resistors, so the actual screen voltage would have been about 5V less than the anodes (similar to the Dynaco ST-70 hi-fi amp). Mullard's documentation states that the 1K resistors were used because they increased linearity when the amp was driven above 15W.

It's worth noting that Dynaco's ST35 ultralinear used no screen resistors on the 6BQ5s. Again, anodes and screen voltages are virtually the same. Mullard's EL84 pentode amplifier used a 1.2K ohm dropping resistor off the anode supply and a 47 ohm resistor in series with each screen. Their documentation states that the 47 ohm screen resistors are included "as a normal measure to prevent parasitic oscillations." Mullard's ultralinear EL84 design operates the screens at the same voltage as the anodes and also includes the 47 ohm resistors for suppression.

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Re: Values of the screen grid resistors influence sound

Post by musicheals »

I took readings on the anode and screen grid without the screen grid resistors. The screens are 5 V less than the anodes.
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Re: Values of the screen grid resistors influence sound

Post by JMPGuitars »

musicheals wrote:
Thu 05/13/21 12:28 pm
I took readings on the anode and screen grid without the screen grid resistors. The screens are 5 V less than the anodes.
Share the readings of all the power tube pins, and your cathode resistor value (I'm assuming the amp in question is cathode biased).

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

Post by zaphod_phil »

musicheals wrote:
Thu 05/13/21 12:28 pm
I took readings on the anode and screen grid without the screen grid resistors. The screens are 5 V less than the anodes.
Make sure to measure all voltages with respect to the cathode
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Re: Values of the screen grid resistors influence sound

Post by zaphod_phil »

TriodeLuvr wrote:
Thu 05/13/21 11:07 am
geoff 1965 wrote:
Thu 05/13/21 10:08 am
i've just seen something related to this lately and that is the mullard tubes built at that time and being used by marshall & vox when the amps were designed i.e. 1960's were very robust valves and the screens could handle 100R.
Mullard's own EL34 amplifier was an ultralinear design with essentially equal voltages on anodes and screens. They used 1K screen resistors, so the actual screen voltage would have been about 5V less than the anodes (similar to the Dynaco ST-70 hi-fi amp). Mullard's documentation states that the 1K resistors were used because they increased linearity when the amp was driven above 15W.

It's worth noting that Dynaco's ST35 ultralinear used no screen resistors on the 6BQ5s.As you know, screen voltages in UL amps can swing above anode voltage and there's no resistor to limit screen current.Those screen grids wouldn't have survived long 8O

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Re: Values of the screen grid resistors influence sound

Post by musicheals »

I measured....
11V on the kathode

308V on the screen grids incl. 100 ohm resistors

314V on anodes

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 zaphod_phil »

zaphod_phil wrote:
Thu 05/13/21 4:09 pm
TriodeLuvr wrote:
Thu 05/13/21 11:07 am
geoff 1965 wrote:
Thu 05/13/21 10:08 am
i've just seen something related to this lately and that is the mullard tubes built at that time and being used by marshall & vox when the amps were designed i.e. 1960's were very robust valves and the screens could handle 100R.
Mullard's own EL34 amplifier was an ultralinear design with essentially equal voltages on anodes and screens. They used 1K screen resistors, so the actual screen voltage would have been about 5V less than the anodes (similar to the Dynaco ST-70 hi-fi amp). Mullard's documentation states that the 1K resistors were used because they increased linearity when the amp was driven above 15W.

It's worth noting that Dynaco's ST35 ultralinear used no screen resistors on the 6BQ5s.

Jack
As you know, screen voltages in UL amps can swing above anode voltage and as there's no resistor to limit screen current,Those screen grids wouldn't have survived long 8O
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Re: Values of the screen grid resistors influence sound

Post by zaphod_phil »

geoff 1965 wrote:
Thu 05/13/21 10:08 am
i've just seen something related to this lately and that is the mullard tubes built at that time and being used by marshall & vox when the amps were designed i.e. 1960's were very robust valves and the screens could handle 100R.
I believe that's very true
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Re: Values of the screen grid resistors influence sound

Post by TriodeLuvr »

zaphod_phil wrote:
Thu 05/13/21 4:09 pm
As you know, screen voltages in UL amps can swing above anode voltage and there's no resistor to limit screen current.Those screen grids wouldn't have survived long 8O
The EL84 and 6BQ5 (and EL34) were designed for ultralinear service. Tube life was not a problem, and I know this because I performed warranty service on Dynaco products back in the day. Most of the historical issues with these tubes occurred in guitar amplifiers like the VOX AC-30, which ran the EL84s well above their dissipation rating.

I agree that a 1K or higher value screen resistor can contribute a lot to the tone of these amps. I just don't think it's reasonable to be so concerned about screen dissipation and failures in these particular tube types. Use the resistors, but do it for the right reason. :D

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Re: Values of the screen grid resistors influence sound

Post by musicheals »

..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
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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.

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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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