Power-Up Delay for Millenium
Power-Up Delay for Millenium Amp
Requirements
The power supply for the Millenium Amp was designed with the following four requirements.
R1. On power up the heater of the tubes are on.
R2. After T1 seconds, the power to the anode of the tubes are turned on in Low voltage mode.
R3. After T1+T2 seconds, the power to the anode of the tubes are turned on in Full voltage mode.
R4. One the power is turned off, all the timer are reset so that step 1,2,3 needs to be cycled on power-on. All capacitors holding the anode voltage are to be discharged quickly.
Timing Diagram on Power-Up and Power-Down
On power on, the heater of the vacuum tube is turned on. After T1 seconds, low anode voltage is switched on, after another T2 seconds, the full anode voltage is turn on. After the power is turned off, heater and anode power are all turn off with all timing reset, so that the timing cycle will be sequenced on power-on. The capacitors that are holding the voltage when the anode is on is to be discharged quickly when the mains is off.
The final circuit for the implementation is shown below.
Delay element
The main control circuit for the timer are R6, C10 (for T1 which is 10 secs) and R7, C11 for (for T1+T2 which is 26 secs. For T1 delay, when the power is on, capacitor C10 starts charging controlled by R6 and passed to 2 x CMOS 74HC14 inverters to sense the level of the trigger voltage. When the capacitor voltage reach 55% of the supply voltage the CMOS inverter U1A will sense the level and activate the logic inversion. The signal from U1F after a secondary inversion then drive the Q1 to activate the relay K1. R10 is to limit the current to the 12V relay that is applied with 15V. D7 is a free-wheeling diode to allow current to flow in self-induced voltage when the relay coil is switch off. D13 is the LED indicator for T1 activation with R12 as a current limiter for the LED. D11 is there to accelerate the discharge of the capacitor when the power is turn off, so that the timing is reset as per requirement 4. The circuit for T1+T2 timer (set at 26 sec) uses the same identical circuit as T1, so the only difference with T1 is the value of C10 at 47uf and C11 at 100uf, thus creating the difference in timing. The timing is given below;
Next we look at how the anode voltages are controlled on power-on. The controlled is done using two DPDT relays. On power-on the heater is on but the anode voltages (+430V and +380V) are disabled because relay K1 is not active yet.
After T1(10 sec) K1 is activated and the anode voltages are applied. However the applied anode voltages has to pass through R1 (4K7) and R2 (4K7) which limits the anode voltage to the tube. So a low anode voltage is now active to the tubes.
After (T1+T2) (26 sec) K2 is activated and the relay contacts short both resistors R1 (4K7) and R2 (4K7) and apply the full anode voltages to the tube.
This completes the full turn on sequence of event.
Next we look at how the heater and anode voltages are controlled on power-off.
Heater is off when the mains power is turned off.
When the mains power is off, the +15V will be discharged quickly to 0V through the relay K1 and K2. This discharge will also discharge C10 and C11 via D11 and D12 which are now forward biased. The timer T1 and (T1+T2) are effectively reset. At the same time when the capacitor are discharged. the voltage at C10 and C11 will drop to zero and will also pull the logic at the base of Q1 and Q2 to zero, and release the relay K1 and K2. The relays should be released within 5mS of power-off.
Let's examine how the reservoir capacitors C2,C3,C6 and C4,C5 and C7 are discharged.
After the relay are deactivated, the relay contacts in K1 connects the capacitors C2,C3 and C6 to resistors R3 and R4 to ground allowing the capacitor to discharge. At same time K1 is deactivated, K2 is also deactivated which open the relay contacts and allows the capacitors C4, C5 and C7 to discharge via R1 and R2 to R3 and R4 respectively thus completing the discharge for C4,C5 and C7.
Transformer U2
The transformer U2 looks like a strange inclusion. You are right if you are having this thought. Actually a simpler design to have the +380V is to rectify the 310VX2 supply from the transformer directly with 2 rectifier. But U2 was included so as to tap this additional power from 2 extra winding from the heater supply which are able to supply a current of 5A. Since U2 is supply power for the front end tubes, the power demand is low. This frees up the power from the HV windings so that less heat is dissipated from this winding which is expected to be heavily loaded when driven hard.
-15V supply
This is designed to be used as a fixed DC supply bias the grid of U3A and U4A power tubes so that lesser cathode voltage is needed for biasing this tubes. This supply is not regulated because it does not drawn any current and since U3A and U4A already has current control, regulation of this voltage is not needed.
+15V supply
+15V is the unregulated supply voltage for driving the 12V relay and LEDs. Since these are 12V relay series resistors R10 and R11 are added drop the excess voltage. R12 and R13 limits the current to the LED to 10mA.
+9V supply
+9V is digital logic supply which is regulated with a simple 9.1V zener D15 with R5. CMOS 74HC14 was chosen for the digital logic control because it allows for a higher supply voltage of higher than 5V seen in TTL logic.
+430V supply
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