The purpose of this article is to describe different ways in which you can hook an output transformer and plate (anode) circuit together, along with the relative advantages and disadvantages of each. Five different structures are described: standard transformer coupling, parafeed, resistively loaded stage capacitively coupled to the output transformer, tube (valve) based constant current source load capacitively coupled to the output transformer, and solid state (MOS FET) constant current source load capacitively coupled to the output transformer.
These configurations apply to both single ended and to push pull output stages. I will use single ended examples, followed by an illustration of how this is also applied to push-pull amplifier stages.
The 5 Output Structures:
Transformer Coupled Stage
The first illustrated example is standard transformer coupling. In the single ended situation, the major disadvantage of this method is the average DC current that flows in the circuit must be supported by the transformer, making the transformer relatively large so that it does not saturate due to the DC only. This is probably the most common output coupling method, as it is relatively simple. At quiescent conditions, the power supply voltage appears on the plate (minus a little loss due to the IR loss in the transformer. During signal conditions, the output swings above and below the supply voltage.
All the other output structures below will be compared to this output coupling mechanism.
The "parafeed" (parallel feed) uses a large choke to provide the DC voltage to the anode. This choke must be large in value, and provides a relatively high impedance at audio frequencies. The output transformer is capacitively coupled to the anode. Sometimes the capacitor is inserted between the anode and the transformer (as shown in the schematic above), and sometimes the transformer is connected directly to the anode, and the capacitor inserted in the ground lead.
The advantage of this kind of mechanism is the choke and the transformer can be individually optimized: the choke for saturation capability, and capacitance; whereas there is no DC in the transformer, so it can be made relatively smaller, providing both lower capacitance and leakage inductance with respect to its primary inductance. This is a long winded way of saying that it is possible to achieve wider frequency response.
There are four other advantages to this circuit. First, the capacitor inserted allows a low frequency "extension" due to the resonance between the transformers inductance and the capacitance value. The second is tha additional low frequency poles can be more easily controlled (as the capacitor and the inductance of the choke can be used as additional "degrees of freedom" in the design). The third is reduced "hum". In a normal transformer circuit, the hum voltage forms a voltage divider between the plate resistance and the load. For low plate resistances, the power supply ripple is coupled to the output. In the parafeed circuit, the power supply ripple forms a voltage divider across the choke, and is not coupled to the output. Thus, the power supply characteristics are more isolated from the output. The fourth advantage of this circuit is since the transformer has no high voltage DC on it, the transformer can be replaced with an "autoformer" (a single tapped winding) allowing the output autoformer to be further optimized.
Like the transformer coupled circuit, the choke feed in the parafeed circuit allows the output to swing above and below the power supply voltage.
The disadvantage of the parafeed is the size, weight and cost associated with TWO rather large magnetic elements in the system. The second disadvantage of the parafeed is that there is a capacitor in the signal path, and this capacitor is handling a relatively large AC current, potentially adding additional non-linearities to the circuit. It doesn't matter where the capacitor is positioned in that series circuit: the signal current still flows through it.
Resistor Load, Capacitively Coupled to the Output Transformer
In this topology, the output tube (valve) is powered by a large resistor fed from the power supply, and the output is capacitively coupled to the transformer. This is very much like a normal "preamp" stage, except the values are somewhat different.
This type of configuration requires a much higher power supply voltage, since the power supplied to the output stage is supplied through this load resistor.
At this point, it is probably worthwhile to consider an example. Consider a SE output stage consisting of a type 10 (or type 801 if you like). This is a DHT device. I'll consider an operating point of 350V and 11 mA (approximately -30V grid bias). With a normal transformer output configuration, the HT voltage would be about 360 volts (the extra 10 volts to account for the transformer winding resistance). I'll also consider a 12k AC load. This will provide a blazing 700 mW of power.
For a resistor load instead, I'll consider using a 35k resistor to a supply voltage of 735 volts. At quiescent, 11 mA is dropped across the 35k resistor (385 volts), leaving 350 volts to be applied to the anode. With the transformer capacitively coupled, the load on the tube is 35k in parallel with 12k or about 9k. Instead of getting 700 mW, we will get about 500mW. Also the 35k resistor will be dissipating over 4 watts!
Advantages of this configuration: Like parafeed, there is no DC in the transformer, so a smaller unit can be used. Also the big bulky choke is replaced by a relatively "small" power resistor. From a "load" viewpoint, the 35k looks to be in parallel with the anode resistance, so the damping is slightly better. There is no issue of inductor saturation either. This configuration offers the widest frequency response (LF is improved as the transformer is looking at a lower resistance than the anode resistance alone, and there is no parallel inductance to limit the LF. HF is improved as there is no real capacitance due to the choke.). Also, like parafeed, hum is reduced by the effective voltage division between the "pull up" resistor and the anode resistance. The comments relative to an output "autoformer" also apply.
Disadvantages of this configuration: More "heat" is generated, power output is somewhat lower, and the power supply must be much higher! The output cannot swing "above the supply" so the supply must be great enough to accomodate the expected swing. This is probably a long way around of saying you need to plot 2 separate load lines; the AC load line and the DC load line. (You, of course KNOW how to do that from reading the "Of Loadlines..." series on these pages).
You will almost *never* see this configuration used, because of the mentioned disadvantages. [I would like to add that I find something magical about the sound of the resulting amplifier, in spite of its disadvantages].
Constant Current Loading
The idea behind the constant current load is that with a constant current load, the valve is almost perfectly linear (near ZERO distortion). However that discounts that the AC load is still the transformer reflected impedance. However, in principle, it provides essentially a parafeed connection topology with a more "perfect" choke... it has a high AC impedance and still powers up the circuit. The more "perfect" nature is that the high impedance does not change with frequency as it does with a choke. I will describe 2 implementations; one using a tube constant current source, and one using a solid state implementation.
A Tube Constant Current Load
The circuit shown above provides about 11 mA with 385 volts across the constant current element (6SN7). The effective impedance is about 50k or so. Since the 6SN7GTB can handle 450 volts and 11 mA within its ratings, it would operate OK in our example above. (In fact, for 350 anode quiescent voltage, the HT supply could be 800 volts).
Advantages: Like the previous 2 topologies, there is no DC in the transformer. The frequency response will be good as well, since there is no loss in effective impedance at low frequencies. The power will be slightly less than with parafeed (as the resistance at midband will be lower than with the choke), but greater than the resistor pull up configuration. Less size and weight than the parafeed. Also, unlike the resistor example, as the output tube swings towards cutoff, (higher than quiescent voltages) the current tends to remain constant, so the current can be delivered to the load. With the resistor load above, this is not the case. This means that in principle, to deliver the same power as with the resistor load example, a lower HT supply would work successfully. (The example shown works to about 80 volts drop across the 6SN7. For the "blue example" above, rather than 735 volts, "only" 600 volts would be needed for the HT supply). Note that the comments relative to using an output "autoformer" also apply to this circuit as well.
Disadvantages: Like the resistor load example, much higher voltage is required. An extra tube is needed. Lower efficiency than the parafeed or transformer example.
There is a nasty disadvantage to any constant current source load. This type of topology is really only applicable to triodes. Since a pentode presents a fairly constant current sink, trying to power it from a constant current source will be fairly unsuccessful, sort of like the unstoppable object hitting the immovable object. Also, cathode bias circuits (with bypass capacitors) are not very successful, since for DC bias purposes, the cathode resistor presents a high DC resistance equivalent at the anode, and the quiescent point is not very stable.
A Solid State Constant Current Load
The circuit shown above will provide about 11 mA for about 385V drop. With the parts shown, the effective impedance is about 400k. (So it is a much more "constant" constant current source). It is effective for voltage drops of 20 volts to about 1000(!) volts across the constant current circuit, so it allows for 500 volts "quiescent" conditions across the current source. (In the example above, the HT supply could approach 1kV). The transistors would need to have heat sinks.
Advantages: Essentially the same as above, except with the higher impedance of the circuit, the power available approaches that of the transformer coupled circuit alone. In the "blue example" above, the HT supply would need to be only 550 volts. Notice that there is a substantial range between the minimum required supply voltage (550 volts) and the maximum possible voltage before blowing the transistors (nearly 1kV).
Disadvantages: Similar to above. There might be considered to be 2 additional disadvantages: a heat sink is required (and the cases of the transistors are at high voltage), and the "solution" is solid state, which might not be satisfactory to some potential users.
Push Pull Equivalent Structure
Push Pull Example Discussion
Push pull offers an advantage that SE amplifiers don't offer relative to the output transformer. Since the DC current on each side is the same, the effective DC in the transformer core is cancelled, so there is no need to provide for a large DC component. That usually means push pull amplifiers are almost always "transformer only" coupled.
However, some mismatch (and thus unbalanced DC) can occur in the circuit, leading to (perhaps) a change in characteristics of the amplifier as a function of loudness or over time. This can be avoided by using any of the same techniques discussed in the SE section above to provide the DC paths to the output tubes (valves). In the schematic above, I used the transistor constant current source example. Notice that the same "disadvantage" as the above applies... the HT voltage must be much higher to allow the tube to swing "above" its quiescent point, just as noted above.
There is also an equivalent additional advantage to this topology. In addition to being immune to tube unbalance, an autoformer approach can be used. In this case, a single winding is again used, but it is center tapped (and this is the ground connection), and tapped at 4 ot 8 or 16 ohms away from the center tap. This looks "odd" but works just as effectively. In ASCII art, this winding is.....
Plate G Out Plate
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