![[-]](https://theultimatetone.com/images/collapse.png "[-]")
06-22-2023, 03:03 PM
Hi Guys
In TUT3 The Ultimate Tone vol.3 we introduced the notion of "transconductance multiplication" output stages. Simply put, this is just the use of paralleled output tubes. The sonic difference between what most guitar amp hobbyists and players might refer to as a "50W" output stage and a "100W" output stage are explained using the tube characteristics. Most players encounter this sonic difference in the 50W and 100W models within the same brand, say as Marshall plexi or 800 amps, or as a Fender Pro versus a Twin.
"Transconductance" (gm) is a characteristic relating the input stimulus of an active gain element (BJT, jfet, triode) to the resulting output, where the input and output quantities are different "domains". For example, transconductance for a tube is the relationship between the voltage at the control grid and the current at the cathode (equals plate current). Voltage and current are two different domains. A BJT used in a gain stage often has the opposite condition, of a current at the base and an output voltage at the collector.
With a standard tube output stage, we can view the voltages on a scope or measure them with a meter, but we can see a signal voltage at the grid causing a much larger signal voltage at the plate. This voltage gain is the result of the transconductance of the tube and the resistance or impedance of the load. If we double the transconductance by paralleling similar tubes, the voltage gain doubles. We can add more tubes, and use dissimilar tubes, in which case we add all the individual gm values and multiply by the load to find the new voltage gain.
If we place a feedback resistor from the plate to grid, we lose voltage gain and alter the circuit performance. For one thing, we must usually insert a DC blocking cap in series with the feedback resistor in order to keep the idle condition the same. The direct resistive link would otherwise cause the bias to shift dramatically positive and melt down the tube and OT. In many examples of this circuit form, the feedback resistor is taken to the input side of the grid coupling cap. which is often the plate output of the driver or splitter. In any case, the local feedback around the output tube decreases its output impedance and distortion contribution. The reduced drive impedance into the OT helps improve the OT frequency response and the OT parasitics (leakage inductance, coil capacitances) become less problematic.
The DC blocking cap introduces a problem of its own in that another RC constant is added to the circuit, which potentially decreases stability at low frequencies. The next post will show a very simple SE PA that overcomes the problems and illustrates the basic principles of the "trans-tube" amplifier.
In TUT3 The Ultimate Tone vol.3 we introduced the notion of "transconductance multiplication" output stages. Simply put, this is just the use of paralleled output tubes. The sonic difference between what most guitar amp hobbyists and players might refer to as a "50W" output stage and a "100W" output stage are explained using the tube characteristics. Most players encounter this sonic difference in the 50W and 100W models within the same brand, say as Marshall plexi or 800 amps, or as a Fender Pro versus a Twin.
"Transconductance" (gm) is a characteristic relating the input stimulus of an active gain element (BJT, jfet, triode) to the resulting output, where the input and output quantities are different "domains". For example, transconductance for a tube is the relationship between the voltage at the control grid and the current at the cathode (equals plate current). Voltage and current are two different domains. A BJT used in a gain stage often has the opposite condition, of a current at the base and an output voltage at the collector.
With a standard tube output stage, we can view the voltages on a scope or measure them with a meter, but we can see a signal voltage at the grid causing a much larger signal voltage at the plate. This voltage gain is the result of the transconductance of the tube and the resistance or impedance of the load. If we double the transconductance by paralleling similar tubes, the voltage gain doubles. We can add more tubes, and use dissimilar tubes, in which case we add all the individual gm values and multiply by the load to find the new voltage gain.
If we place a feedback resistor from the plate to grid, we lose voltage gain and alter the circuit performance. For one thing, we must usually insert a DC blocking cap in series with the feedback resistor in order to keep the idle condition the same. The direct resistive link would otherwise cause the bias to shift dramatically positive and melt down the tube and OT. In many examples of this circuit form, the feedback resistor is taken to the input side of the grid coupling cap. which is often the plate output of the driver or splitter. In any case, the local feedback around the output tube decreases its output impedance and distortion contribution. The reduced drive impedance into the OT helps improve the OT frequency response and the OT parasitics (leakage inductance, coil capacitances) become less problematic.
The DC blocking cap introduces a problem of its own in that another RC constant is added to the circuit, which potentially decreases stability at low frequencies. The next post will show a very simple SE PA that overcomes the problems and illustrates the basic principles of the "trans-tube" amplifier.
