I have been playing around with tubes for a few years but I have always used published designs. This time around I am trying to design a power supply for various 12A_7 preamps that I plan to build.
Here is the power supply:
The B+ is a little higher than I would like. I thought the voltage doubler was going to double the AC voltage but it appears that it has actually doubled the rectified voltage (I must have missed that when I was reading about voltage multipliers). I bread-boarded a common cathode stage last night to test it out. It seems to work fine but I am trying to think of ways to lower it 70-100 volts. The 12A_7 tube data sheets all seem to have a max plate voltage around 300V. I'm not sure if the max voltage is measured after the plate resistor or not. If my B+ is safe for these tubes then I won't worry about it.
The winding for the heaters is supposed to be 12.6V/ 300mA. What scares me is that it is actually putting out almost 16 volts! The tube worked fine last night but I'm worried about long term stability. I tried rectifying it and then using a regulator but there wasn't enough current left to power the heaters.
I plan on putting trimmers on the heater windings and adjusting down to 12.6V. Is there a better way to do this?
Thanks for reading! Any advise or criticism will be greatly appreciated.
Here is the power supply:
The B+ is a little higher than I would like. I thought the voltage doubler was going to double the AC voltage but it appears that it has actually doubled the rectified voltage (I must have missed that when I was reading about voltage multipliers). I bread-boarded a common cathode stage last night to test it out. It seems to work fine but I am trying to think of ways to lower it 70-100 volts. The 12A_7 tube data sheets all seem to have a max plate voltage around 300V. I'm not sure if the max voltage is measured after the plate resistor or not. If my B+ is safe for these tubes then I won't worry about it.
The winding for the heaters is supposed to be 12.6V/ 300mA. What scares me is that it is actually putting out almost 16 volts! The tube worked fine last night but I'm worried about long term stability. I tried rectifying it and then using a regulator but there wasn't enough current left to power the heaters.
I plan on putting trimmers on the heater windings and adjusting down to 12.6V. Is there a better way to do this?
Thanks for reading! Any advise or criticism will be greatly appreciated.
Remember your meter reads RMS not peak AC. You will charge the caps to peak not RMS.
Just a thought, what is the difference between the voltage after the Anode load with no current draw and with the heaters working IE with current draw. If you exceed the flash over voltage of the tube? I have seen this on EL34's when the cathode resistor goes "pop". With no current draw the volt drop is 0 across the resistor.
Regards
M. Gregg
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So what you're saying is that my voltage is even higher? I understand RMS and Peak in relation to power amp output. Is it the same for DC voltage?
You'll have to forgive me but I don't follow what you're saying. It takes me a while to understand this stuff so I'll need to read it a bunch of times to give you an appropriate response!
I never think of OHM's law. It would solve so many problems!
If I'm not mistaken at 12.6 the current is 150mA so it might actually be:
12.6 = .15 R
R = 84
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When you first switch on, the tubes are not drawing current.
So the voltage drop across any resistors in the anode have no effect on the applied voltage.
If the B+ is higher than the flash over voltage of the tube- it will flash over. ie "short out".
So at the risk of being shunned for asking for the easy answer would you say that I'm safe?
If you would rather me have my own "ah-ha!" moment when this all clicks then please ignore my last question.
I would not exceed max working voltage on any tube. What I am saying is the resistors on the Anode will not have any effect on the applied B+ voltage seen by the anode until current is drawn by the tube -heaters warm up.
Regards
M. Gregg
Ah-ha! That's what I was looking for. I sincerely appreciate your help!
I have heard of putting resistors on the AC before it is rectified. I may try that this evening. This should give the the voltage doubler less voltage to multiply.
bill,
Have a look at this link:
Power Supplies
The section on smoothing may help regards the AC rms and peak charge.
Regards
M. Gregg
Have a look at this link:
Power Supplies
The section on smoothing may help regards the AC rms and peak charge.
Regards
M. Gregg
I deleted it because some time went by and posts were made before I made it. I wanted to review what was said. Voltage doublers are less ideal then full wave rectifiers, I believe they produce a waveform similar to a half wave. Havent looked too closely. But if your aiming for 300v get a 300/sqrt(2) or 212v, closest common value would be 200v or 220
Ha. I deleted mine since yours wasn't there any more. Go figure.
I just looked at a 215V version of my Tx and it is 3x the price! I think I can find a way to lower the voltage for less than that.
Thanks!
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bill,
Have a look at this link:
Power Supplies
The section on smoothing may help regards the AC rms and peak charge.
Regards
M. Gregg
Reading it now.
This is a nice link. Lots of info here.
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I was waiting for someone to post this.
I have been staring at this software for years. For the life of me, I can not figure how to use it. I can draw a supply easy enough but I don't understand how to read the simulation.
I generally try to keep supply voltages to no more than about 85% of maximum for an adequate safety margin, so with a 300v plate limit you probably should be around 255 volts. In theory, with 120 volts ac into a voltage doubler, you'll have about 330 volts dc. You can use a resistive voltage divider to reduce plate voltage to 300 vdc at power up, which would then drop to operating voltage as the tube(s) come into operation. You can start with the equivalent operating resistance of the tube by dividing the nominal operating voltage by the operating current. Then calculate voltage divider from there using Ohms'Law.
Give it a try and see what you come up with.
Mike
Give it a try and see what you come up with.
Mike
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