16 - 18V tap on Stereomour Transformer
- Thread starter drewh1
- Start date
rlyach
New member
Drew,
I don't want to speak for those who have been working with this amp for years but I believe that the answer is no. At least not easily. You could tap into the 12.3 AC filament and build a rectifier, but the transformer may not be able to supply the current to the stepper motor and the 12AT7 at the same time. The 12AT7 takes ~300mA. Stepper motors can take a lot of current, depending on the amount of torque they supply. Also, you may not have enough voltage. the 12.3AC has a 17V p-p that you can rectify but as soon as you pull some current from the regulator you just built that 17V will probably drop below 16V. You could also use a voltage doubler from the 12.3VAC output and drop the voltage down with a regulator or resistor, but again you may not have the power available. You could also draw current from the 400VDC supply using a large high power resistor, but this may affect the power to your output tubes. I don't know the current rating of the HV supply. I think your best bet is to add a small switching power supply inside the box specifically for the stepper motor. You can just tap into the 120VAC main. This is just food for thought and I am sure PJ may have a more complete answer.
I don't want to speak for those who have been working with this amp for years but I believe that the answer is no. At least not easily. You could tap into the 12.3 AC filament and build a rectifier, but the transformer may not be able to supply the current to the stepper motor and the 12AT7 at the same time. The 12AT7 takes ~300mA. Stepper motors can take a lot of current, depending on the amount of torque they supply. Also, you may not have enough voltage. the 12.3AC has a 17V p-p that you can rectify but as soon as you pull some current from the regulator you just built that 17V will probably drop below 16V. You could also use a voltage doubler from the 12.3VAC output and drop the voltage down with a regulator or resistor, but again you may not have the power available. You could also draw current from the 400VDC supply using a large high power resistor, but this may affect the power to your output tubes. I don't know the current rating of the HV supply. I think your best bet is to add a small switching power supply inside the box specifically for the stepper motor. You can just tap into the 120VAC main. This is just food for thought and I am sure PJ may have a more complete answer.
You could add a small 12V transformer and power supply to the Steremour, just screw it to the wood on the back side of the chassis.
I think there's a little headroom on the 12V winding, but I agree with Randy, those motors can draw surprising amounts of current.
I think there's a little headroom on the 12V winding, but I agree with Randy, those motors can draw surprising amounts of current.
drewh1
New member
I can use an external power supply for the remote system, it just seems not very elegant. I suppose I could take 120 V off after the switch so at least it would only be powered when the amp was powered. The motor only uses current when turning (400 ma) and 4ma standby current. So maybe that is a better solution.
Hmm, 400mA might be doable, considering the motorized pot is not always running, hopefully PJ can chime in!
If this is the case, you'll have to do some head scratching about whether pin 9 on the 9 pin socket should be lifted.
If this is the case, you'll have to do some head scratching about whether pin 9 on the 9 pin socket should be lifted.
drewh1
New member
In fact, the motor will not be used very much, occasional adjustment. I am finding I only move the volume control 2 notches or so to adjust for the various source levels. Since I have my music collection on a server, I tend to skip around a lot and don't want to get up every time! OK, so I am a little lazy 
I like the idea of tapping the 12 volt since it simplifies the circuit and I don't really want a switching power supply lurking underneath polluting my everything with EMI!
I like the idea of tapping the 12 volt since it simplifies the circuit and I don't really want a switching power supply lurking underneath polluting my everything with EMI!
Paul Joppa
Moderator
The 12.6v winding can handle 600mA rms easily, and close to 1 amp for short periods (say, less than 10 seconds?). You could rectify it and get 14-15 volts unregulated. I suggest the best would be to leave the 12AT7 with AC heater supply, and I'll strongly encourage you to retain the grounded heater tap (pin 9 on the socket) - this tap serves other purposes as well.
drewh1
New member
So I want to make sure I have this right:
1. I tap off of the 12.6 v AC on the transformer (where the red and black go to 9 pin socket, pins 4 and 5). i assume it is better to tap off the transformer than the socket?
2. Use a bridge rectifier to convert 12.6 v AC to variable 12 to16 v DC (unregulated, low current so voltage will drift up?)
can I use any reasonably rated bridge rectifier (100v 4amp?) ? or should I use a low vF Shottky 2A 40V?
1. I tap off of the 12.6 v AC on the transformer (where the red and black go to 9 pin socket, pins 4 and 5). i assume it is better to tap off the transformer than the socket?
2. Use a bridge rectifier to convert 12.6 v AC to variable 12 to16 v DC (unregulated, low current so voltage will drift up?)
can I use any reasonably rated bridge rectifier (100v 4amp?) ? or should I use a low vF Shottky 2A 40V?
Your presumptions are all sound, I'd recommend 1N5820 diodes for your bridge.
rlyach
New member
Drew,
I put the circuit in circuit lab. I was not exactly sure what the RS values for the filament winding was so I made an educated guess. I have also included two resistors to model the filament of the 12AT7. There are two problems you will face. First, because the the filaments have their center point connected to ground (pin 9) your stepper motor will not have the same ground reference. If the motor has its chassis shorted to ground this may be a problem. Second, you will not reach 16VDC, especially when the stepper motor is running. The specs for the motor you gave say that it should work down to 12VDC. If we can get someone to give us the detailed specifications for the 12.6 Vac winding we can get a better idea of the performance of the rectifier under load. Also, I used 1N4007 diodes but you can update the parameters for any diode. Here is the link to the circuit. You can open it, make changes, simulate it, and save it to your own workbench.
https://www.circuitlab.com/circuit/92f74h/stepper-supply/
I put the circuit in circuit lab. I was not exactly sure what the RS values for the filament winding was so I made an educated guess. I have also included two resistors to model the filament of the 12AT7. There are two problems you will face. First, because the the filaments have their center point connected to ground (pin 9) your stepper motor will not have the same ground reference. If the motor has its chassis shorted to ground this may be a problem. Second, you will not reach 16VDC, especially when the stepper motor is running. The specs for the motor you gave say that it should work down to 12VDC. If we can get someone to give us the detailed specifications for the 12.6 Vac winding we can get a better idea of the performance of the rectifier under load. Also, I used 1N4007 diodes but you can update the parameters for any diode. Here is the link to the circuit. You can open it, make changes, simulate it, and save it to your own workbench.
https://www.circuitlab.com/circuit/92f74h/stepper-supply/
The UF4007 will drop a little more voltage than a 1N5820, and the 10uF cap will be more responsive to the load at 10,000uF.
Let's hop that the motor supply is allowed to float with respect to the metal on the motor itself!
Let's hop that the motor supply is allowed to float with respect to the metal on the motor itself!
rlyach
New member
I have updated the circuit to use the 1N5820 instead of the 4007. This reduces the forward voltage by about 200mV but increases the Rs by about 25 miliohms. I also increased the cap to 10000uf. I did a quick check and found several suppliers for a 12,000uF cap. The only question is what is the output series resistance of the winding? I gave a guess but it will make a difference. Keep in mind that the 400mA load is constant in this simulation, making this very much a worse case scenario. I would have liked to filter the output a little more but we don't have the headroom. All this is still dependent on being able to float the stepper motor below chassis.
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Paul Joppa
Moderator
The 12-v winding is about 0.65 ohms (including the reflected primary) with an unloaded voltage of 12.94 volts (assuming the power line is providing 120.0 volts, of course).
Use Schottky diodes! Silicon diodes will have a reverse recovery spike, which radiates energy from the wiring and couples to the other windings inside the transformer capacitively. If the voltage is marginal, consider 80SQ045N 8-amp diodes; they have an even lower voltage drop.
Use Schottky diodes! Silicon diodes will have a reverse recovery spike, which radiates energy from the wiring and couples to the other windings inside the transformer capacitively. If the voltage is marginal, consider 80SQ045N 8-amp diodes; they have an even lower voltage drop.
rlyach
New member
I updated the circuit with the 80SQ045N (Vf=.26V typical at 25C). I also used a 12,000uF cap and updated the transformer data. The results look too close to call. The DC output of this rectifier on the filament winding with the filament of the 12AT7 drawing 300mA and the stepper motor drawing 400mA is ~13.2V. If I run the simulation unloaded with 4mA standby I get ~15V. The large capacitor will handle the initial surge to get the motor moving. You will have 54ms of surge current before the voltage drops to 13.2V. I also looked through the documentation and could not determine if the stepper motor has a grounded chassis. If it is, this wont work. You could order the optional $10 120Vac to 15VDC power supply, put a metal shield/box around it, and mount it far from the tubes and transformers.
Paul Joppa
Moderator
Three points:
1) the 12AT7 draws 150mA at 12.6 volts
2) All these calculations were made assuming a 120v power line; for reliable operation you would want 16.5v under this condition so that +/-10% on the power line will still give 15-18 volts. Assuming the spec means the voltage must never ever under any circumstances for any length of time fall below 15.000 volts. Which I doubt - see #3 below.
3) There is no spec for ripple on the power supply - you'd have to ask John at Bent Audio about that.
1) the 12AT7 draws 150mA at 12.6 volts
2) All these calculations were made assuming a 120v power line; for reliable operation you would want 16.5v under this condition so that +/-10% on the power line will still give 15-18 volts. Assuming the spec means the voltage must never ever under any circumstances for any length of time fall below 15.000 volts. Which I doubt - see #3 below.
3) There is no spec for ripple on the power supply - you'd have to ask John at Bent Audio about that.
