Guitar Amp Screen Resistors
Moderately past the entry point of the learning curve for tube amplification for guitars- My style is primarily R&R.
Here's my question and I don't know if it is off topic as I arrived here during a global search for an answer:
I'm modifying the attached schematic, firstly by converting to 6L6 and using a typical push pull output, (not UL). The output tubes will run in the 430 - 450 VDC range attempting to push 35 watts. Note in regards to the power supply: 'A' is to the right of the choke and consists of 2 series 100uf caps.
I've settled so far on a 1K5 screen resistor but this is just due to schematic interpretations. Is this a good value? I don't have enough knowledge to determine what diff it would make.
I have also used a 1 watt screen resistor on the, resistor is a fuse too, theory but wonder if this is pushing the power rating for the 'R' ?
I've read the screen resistor is responsible for compression of the output? I think I understand that but not sure if someone could expound... Perhaps by allowing more of the pure output signal to pass through the tube?
Thank you for any assistance in this design.
Moderately past the entry point of the learning curve for tube amplification for guitars- My style is primarily R&R.
Here's my question and I don't know if it is off topic as I arrived here during a global search for an answer:
I'm modifying the attached schematic, firstly by converting to 6L6 and using a typical push pull output, (not UL). The output tubes will run in the 430 - 450 VDC range attempting to push 35 watts. Note in regards to the power supply: 'A' is to the right of the choke and consists of 2 series 100uf caps.
I've settled so far on a 1K5 screen resistor but this is just due to schematic interpretations. Is this a good value? I don't have enough knowledge to determine what diff it would make.
I have also used a 1 watt screen resistor on the, resistor is a fuse too, theory but wonder if this is pushing the power rating for the 'R' ?
I've read the screen resistor is responsible for compression of the output? I think I understand that but not sure if someone could expound... Perhaps by allowing more of the pure output signal to pass through the tube?
Thank you for any assistance in this design.
Thanks
Thanks for the reply. I went back and checked those threads again. I did read all the posts last night, however the info in 28 and 29, for me, is inconclusive as I've seen different values for the screen resistors in the past. It does look like 1K5 is too large a value for this application.
I've started an amp build and I'm going to open a new topic at main as I have several other questions.
Thanks again,
Cat.
Thanks for the reply. I went back and checked those threads again. I did read all the posts last night, however the info in 28 and 29, for me, is inconclusive as I've seen different values for the screen resistors in the past. It does look like 1K5 is too large a value for this application.
I've started an amp build and I'm going to open a new topic at main as I have several other questions.
Thanks again,
Cat.
The "EL34" and "6CA7" numbers have been used on both true pentodes, and beam tetrodes over the years. I have seen all 4 combinations although most EL34's seem to be pentodes and the 6CA7's seem to be about 50/50. The "fat bottle" 6CA7's I have seen were mostly beam tetrodes.
I routinely test my guitar and HiFi amps by driving then to extreme clipping with a MIDI guitar preamp that goes past 11. The EL34 and 6CA7's are capable of drawing some serious screen current on peaks that is capable of blowing a 1 K ohm 1 watt resistor. Oscillation was not detected with an RF spectrum analyzer. Most tubes do not exhibit this behavior, but some do. I am assuming that it is an internal alignment issue, since the problem follows the tubes. I have been using 2 watt film resistors for about 10 years with no failures.
The value of the screen resistor will affect the amps "tone" in overdrive conditions, as well as the reliability of the output tubes. A small value resistor (100 ohms) may result in the lowest distortion measurements especially in triode mode. This is good for a HiFi amp that doesn't see heavy clipping.
A 100 ohm resistor in pentode while playing a guitar cranked to 11 into a real speaker load will usually lead to a glowing screen grid. It however produces the highest peak power, and rather sharp clipping, which comes on quick. Metal players tend to like this "tone." Works best with a solid state rectifier, a low B+ (under 400 volts), and a lowish OPT load (3600 to 4800 ohms).
A high value, like 1.5 or 2.2K ohms, will limit the screen current, which will cause a softer clipping that occurs gradually. Tube life is generally better since the peak currents are lower. Blues players like this "tone". Works best with a tube rectifier and, more B+ and a higher load. I use 5000 or 6600 ohms.
The screen resistor value is just another variable that can be tweaked to get the tone you want, within reason. All of this depends on the actual speakers you are using since their impedance isn't constant. Different guitars and playing style will affect peak tube current.
I usually start with a high value like 2 K ohms, then test the sound with a box full of resistors with small alligator clips on the leads that are tried in parallel with the existing resistor. You MUST observe the screen grids inside the tube while (someone else is) playing the amp cranked to 11 (no, well past 11) with the speaker system it will see in use. If any of the grid wires glow, or the tube appears to emit more light when a loud power chord is played, you must increase the resistor value, or be prepared for tube failure. Beware blue glow inside the plate structure too, blue ON the glass is OK.
If you have the equipment, observe the tube parameters during stressed operation. I will put a scope probe on either side of the screen resistor and have the scope subtract the readings and display the result. This will shoe the peak screen current, which can go over 50 mA on a poorly constructed EL34.
I routinely test my guitar and HiFi amps by driving then to extreme clipping with a MIDI guitar preamp that goes past 11. The EL34 and 6CA7's are capable of drawing some serious screen current on peaks that is capable of blowing a 1 K ohm 1 watt resistor. Oscillation was not detected with an RF spectrum analyzer. Most tubes do not exhibit this behavior, but some do. I am assuming that it is an internal alignment issue, since the problem follows the tubes. I have been using 2 watt film resistors for about 10 years with no failures.
The value of the screen resistor will affect the amps "tone" in overdrive conditions, as well as the reliability of the output tubes. A small value resistor (100 ohms) may result in the lowest distortion measurements especially in triode mode. This is good for a HiFi amp that doesn't see heavy clipping.
A 100 ohm resistor in pentode while playing a guitar cranked to 11 into a real speaker load will usually lead to a glowing screen grid. It however produces the highest peak power, and rather sharp clipping, which comes on quick. Metal players tend to like this "tone." Works best with a solid state rectifier, a low B+ (under 400 volts), and a lowish OPT load (3600 to 4800 ohms).
A high value, like 1.5 or 2.2K ohms, will limit the screen current, which will cause a softer clipping that occurs gradually. Tube life is generally better since the peak currents are lower. Blues players like this "tone". Works best with a tube rectifier and, more B+ and a higher load. I use 5000 or 6600 ohms.
The screen resistor value is just another variable that can be tweaked to get the tone you want, within reason. All of this depends on the actual speakers you are using since their impedance isn't constant. Different guitars and playing style will affect peak tube current.
I usually start with a high value like 2 K ohms, then test the sound with a box full of resistors with small alligator clips on the leads that are tried in parallel with the existing resistor. You MUST observe the screen grids inside the tube while (someone else is) playing the amp cranked to 11 (no, well past 11) with the speaker system it will see in use. If any of the grid wires glow, or the tube appears to emit more light when a loud power chord is played, you must increase the resistor value, or be prepared for tube failure. Beware blue glow inside the plate structure too, blue ON the glass is OK.
If you have the equipment, observe the tube parameters during stressed operation. I will put a scope probe on either side of the screen resistor and have the scope subtract the readings and display the result. This will shoe the peak screen current, which can go over 50 mA on a poorly constructed EL34.
This post is quite old but full valid as I have just blind tested by ear using a valiable potentiometer placed instead of screen resistor.
Triode mode= best sound at 270 ohm
UL mode = best sound at 1500 ohm !!
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