I have come up with another amplifier circuit which is capable of delivering 15 Watts output power. I have built this using my favorite Pentode just like my previous circuits. This one is a big project with several sub-circuits, so I`m gonna go through each sub-circuit separately
First let`s go through the amplifier circuit itself. I`ve decided to go with a completely different topology this time, suitable for Hi-Fi. The first stage is a Pentode voltage amplifier with a local negative feedback to decrease the gain to around 3-4 times. It`s your choice how to distribute the gain between the two negative feedbacks. The second stage is a phase splitter, with a few advantages over a typical cathodyne phase splitter.
The use of a constant current source instead of a tail resistor gives absolute symmetry between the two outputs. For Hi-Fi symmetrical driving of the power tubes is essential. Another apparent advantage is the well balanced and equal output impedance. As long as you don`t overdrive the power tubes vigorously, the output impedance will be in perfect balance. There is also one more advantage to this type of phase splitter. Unlike cathodyne, where the voltage gain is little less than unity, this phase splitter has a gain of around 25. So if the power tubes requires 10V and your signal source is only 200mV, you can set the first stage to have a gain of 2 (which will make it extremely linear). With the gain of phase splitter, the power tubes will be driven accordingly.
I recommend using a gain of 3 for the first stage. This will force the global feedback to have deep effect on the signal as well. Anyway, the power amplifier works in a Maximum Power Distribution Ultra Linear Mode. That means that the transformer is tapped at 20% of the primary and fed to the screen grid via a small resistor. This will result in a 15W of Power at only 1.1% distortion from the power stage. Combined with the global and local feedback the overall distortion won`t pass above 2% at 15W.
Even then, the distortion is mostly second order, so it should sweeten the sound. For Ultra Linear operation, it`s essential to filter the power supply extremely well. Also it should not sag a lot under a low frequency attack. The solution is using a Pi-Filter (C-L-C), with a sufficiently large capacitance to supply the current demand during an attack. This is good enough for the power stage. The preamp and the phase splitter need more stable supply, so an additional RC-RC network is added to compensate for the noise, and the momentary sags of the power supply.
Engulfing the amplifier in global negative feedback from output to input will flatten the frequency response. This will minimize the speaker`s reactive component, influencing the amp and to further reduce the overall distortion.
SUPPLY CONTROL CIRCUIT:
The second part of this amplifier circuit, revolves around maximizing the lifespan of the tubes. It also employs a mechanism, through which an idle state of the amp for more than 6 minutes which will result in a sort of a sleep mode. When the amp is first turned on, the heater voltage will be at 3.3V, where the typical voltage for these tubes is 6.3V. This will minimize the surge current at cold start. It will also keep the tubes at a ready-on state, with just enough current through the filaments, to keep them partially on. At this point the High Voltage supply is not applied.
Applying the HV supply with lowered heater voltage will wear out the tubes extremely fast, due to cathode poisoning. After an input signal is detected, the comparator U2 starts producing a square wave signal at the output, which is passed through an envelope detector. The capacitor C3 starts charging, reaching the maximum voltage of 11V. When the capacitor charges above 2V the comparator U3 enables and forces the Q4 transistor to switch the relay, increasing the heater voltage from 3.3V, to 6.3V. Up to this point the amplifier hasn`t been supplied with the high voltage.
The output of U3 rises to 12V, which through R13 disables the transistor Q2. This allows the capacitor C1 to be charge and after it charges to 4.7V the comparator U1 enables Q3, which enables the relay switching the HV supply for the tubes. There is a 16s delay for the high voltage supply, after the tubes receive the full 6.3V filament voltage controlled by R4/C1.
This cycling of the heater/HT voltage increases lifespan of the tubes and lower power consumption in absence of a signal. When the input signal is disabled, the comparator U2 no longer charges the capacitor C3 and it starts discharging through the resistor R11. After 6 minutes the voltage falls below the 2V threshold and the output of the comparator U3 disables the relay driving transistor, and the heater voltage falls down to 3.3V again (in standby). This action also enables the transistor Q2, which discharges the capacitor C1 and disables the HV supply as well.
The amplifier enters standby mode with a quiescent current less than 20mA for control circuitry & around 1A for the heaters. When you apply an input signal, the whole process starts again and the amp quickly (15-16s) comes back to life. You can alter the turn on time of HV supply by decreasing the value of C1. But going below 22uf is not advisable since it will result in a 3.6 seconds delay.
Take the supply voltage of the control circuitry from the 6.3V AC heater power supply. A voltage doubler rectifies and doubles the voltage at around 17-18V. Then an LM7812 steps down and regulates the voltage at 12V needed for the control circuitry. Potentiometer R15 compensates the dis balance created by the voltage doubler. Alternatively you can have a separate winding for the supply voltage of the control circuitry. This is the most advisable method.
Hope you all liked this tube amplifier circuit. Do try this and post your outcome. In case if you come across any queries do mention it in the below comment box, i will help you out.