Don't know if this should be posted here or the Tubelab forum. But here goes.
I have simulated characteristic curves in LTSpice for an 807 used in an UNSET topolgy. This is shown below along with a possible load line. My question is what is the advantage of UNSET 807 compared to triode strapped 807? Is it primarily output power possible?
I have simulated characteristic curves in LTSpice for an 807 used in an UNSET topolgy. This is shown below along with a possible load line. My question is what is the advantage of UNSET 807 compared to triode strapped 807? Is it primarily output power possible?
Max G2 voltage for 807 is 300V, although at least one datasheet states 400V in triode connection.
6W seems optimistic though, 807 / 6L6 are known for producing very low power in SE triode, like 1,5W or so.
6W seems optimistic though, 807 / 6L6 are known for producing very low power in SE triode, like 1,5W or so.
The UNSET circuit's main advantage is getting pentode power levels with triode curves, which offer lower plate resistance and output impedance. The separate screen supply also allows for running sweep tubes in "triode" mode. The ratio of R1 to R2 controls the shape and tilt of the triode curves.
At idle there is some voltage dropped across the mosfet similar to cathode bias. The voltage dropped across the cathode resistor in a cathode bias circuit is "wasted" since it gets turned to heat in the cathode resistor. In the UNSET circuit the mosfet is driven to near saturation (very low voltage drop) as the vacuum tube is reaching peak current. This gets the full power supply voltage across the OPT (minus the drop in the output tube) at peak current, reclaiming what would have been dissipated in the cathode resistor.
At idle there is some voltage dropped across the mosfet similar to cathode bias. The voltage dropped across the cathode resistor in a cathode bias circuit is "wasted" since it gets turned to heat in the cathode resistor. In the UNSET circuit the mosfet is driven to near saturation (very low voltage drop) as the vacuum tube is reaching peak current. This gets the full power supply voltage across the OPT (minus the drop in the output tube) at peak current, reclaiming what would have been dissipated in the cathode resistor.
George, thanks for the refresher. In your opinion is the so called "Pentode sound" whatever that is a result of the curves that goes away when using UNSET on a pentode? BTW my SE Pentode 1625 amp sounds really good, but I have only lived with 2 amps.
A pentode tube operating without any feedback has a very high output impedance. This makes the amplifier's behavior highly dependent on the characteristics of the speaker system. Most amplifiers HiFi that employ pentode output tubes use local or GNFB to lower the output impedance. Most guitar amplifiers do not, which is part of the reason that there are so many different sounding guitar speakers.
As we have seen it is possible to design a "current drive" amplifier with a speaker system designed to function with current drive work quite well.
Most of today's speaker systems are designed for use with a solid state amp having a near zero output impedance so their behavior with a highish impedance driving source is uncertain. Some may sound good, but many will have booming flabby bass, especially those with large woofers.
When RCA introduced the 6L6 pentode tube in the late 1930's, Schade proposed a feedback method to make it sound more like a triode. This employed feedback from the plate of the tube to it's grid, but that was not practical at the time, so a blocking capacitor had to be in the loop somewhere. This causes phase shift at low frequencies, so it wasn't too common in commercial amps, though circuits with this approach appeared in several RCA tube manuals that employed a feedback resistor from the plate of the output tube to the plate of the driver tube.
P-channel mosfets weren't even a dream at the time, but it is possible to drive the cathode with a transformer, but I have never seen it done in audio amps. The technique was employed in ham radio RF amps, but more for skirting the FCC's "DC power input to the final RF stage" than anything else.
As we have seen it is possible to design a "current drive" amplifier with a speaker system designed to function with current drive work quite well.
Most of today's speaker systems are designed for use with a solid state amp having a near zero output impedance so their behavior with a highish impedance driving source is uncertain. Some may sound good, but many will have booming flabby bass, especially those with large woofers.
When RCA introduced the 6L6 pentode tube in the late 1930's, Schade proposed a feedback method to make it sound more like a triode. This employed feedback from the plate of the tube to it's grid, but that was not practical at the time, so a blocking capacitor had to be in the loop somewhere. This causes phase shift at low frequencies, so it wasn't too common in commercial amps, though circuits with this approach appeared in several RCA tube manuals that employed a feedback resistor from the plate of the output tube to the plate of the driver tube.
P-channel mosfets weren't even a dream at the time, but it is possible to drive the cathode with a transformer, but I have never seen it done in audio amps. The technique was employed in ham radio RF amps, but more for skirting the FCC's "DC power input to the final RF stage" than anything else.