Hi all,
I am trying to build a stereo level indicator using EM87 magic eyes I got off ebay.
I have a simple transistor circuit to give me a negative voltage for the grid, which is driven from either the headphone of lineout on the soundcard.
The 6.3V for the heater I am just regulating from the 12V rail on the computer PSU.
Below is a schematic of my circuits for the PSU and the amplifier for the grid.
The tubes light up correctly and I measure the correct voltages on them.
The problem I am having is when I connect the ground from my amplifier for the grid voltage going to PIN1 to the ground of my rectified AC voltage it blows the tracks off the board.
It also tends to destroy the diode bridge causing a short circuit between the AC inputs and the negative DC terminal.
Also it is just mono at the moment and I have been testing it with just a function generator.
I am not sure what the problem is, and as you may have gathered I am new to the tubes...
Thanks in advance for any advise!
I am trying to build a stereo level indicator using EM87 magic eyes I got off ebay.
I have a simple transistor circuit to give me a negative voltage for the grid, which is driven from either the headphone of lineout on the soundcard.
The 6.3V for the heater I am just regulating from the 12V rail on the computer PSU.
Below is a schematic of my circuits for the PSU and the amplifier for the grid.
An externally hosted image should be here but it was not working when we last tested it.
The tubes light up correctly and I measure the correct voltages on them.
The problem I am having is when I connect the ground from my amplifier for the grid voltage going to PIN1 to the ground of my rectified AC voltage it blows the tracks off the board.
It also tends to destroy the diode bridge causing a short circuit between the AC inputs and the negative DC terminal.
Also it is just mono at the moment and I have been testing it with just a function generator.
I am not sure what the problem is, and as you may have gathered I am new to the tubes...
Thanks in advance for any advise!
You need isolation!
With directly rectified mains voltage the 'ground' isn't really ground,it's basically at mains potential.(!)
Buy a valve power supply transformer. It will (A) MAKE THE CIRCUIT SAFE, (B) provide the necessary 250VAC and 6.3VAC for the EM87s
Ahh I thought it could be something like that.
But I think I still sit with a problem. I must make the approx 1.2Vrms from the soundcard into about 0V to about -6V or so.
So I still need 12V for the transistor circuit.
Now if I obtain one of these valve power supply transformers can I connect my grounds from my computer PSU and soundcard one of the 0V outputs on the transformer?
Can the 250VAC be directly coupled to the tubes, obviously through the correct resistors some of the pins?
Steerpike, I see you are from South Africa, I stay in Cape Town. Any idea where I can get one of these transformers?
But I think I still sit with a problem. I must make the approx 1.2Vrms from the soundcard into about 0V to about -6V or so.
So I still need 12V for the transistor circuit.
Now if I obtain one of these valve power supply transformers can I connect my grounds from my computer PSU and soundcard one of the 0V outputs on the transformer?
Can the 250VAC be directly coupled to the tubes, obviously through the correct resistors some of the pins?
Steerpike, I see you are from South Africa, I stay in Cape Town. Any idea where I can get one of these transformers?
Nothing but a transformer should be used if you want to live.
Buy a tube transformer with a 12.6V winding (or two 6.3V windings would be better) so you have all the different voltages you need.
If required, just add a small transformer - but you should be able to find one with all the voltages you need.
As mentioned above, congratulations on being alive, AC mains will usually find a path to ground (unlike a transformer output, which is isolated from ground) and if you're heart is near that path you die.
Buy a tube transformer with a 12.6V winding (or two 6.3V windings would be better) so you have all the different voltages you need.
If required, just add a small transformer - but you should be able to find one with all the voltages you need.
As mentioned above, congratulations on being alive, AC mains will usually find a path to ground (unlike a transformer output, which is isolated from ground) and if you're heart is near that path you die.
Hi,
Sorry, what do you mean by this?
As far as I understand the grid voltage of 0V to -10V must be referenced to the same ground as what is on the tubes pin 3?
Also is there an online store that stocks such transformers or is it easier to get one custom wound?
Thanks again
Sorry, what do you mean by this?
As far as I understand the grid voltage of 0V to -10V must be referenced to the same ground as what is on the tubes pin 3?
Also is there an online store that stocks such transformers or is it easier to get one custom wound?
Thanks again
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The reference is to you directly applying line voltage 230VAC 50Hz to a bridge rectifier with one side to ground, without using an isolation transformer or tube supply transformer.
(1) You have applied line voltage thorugh a diode to ground which is what is causing your traces to blow.
(2) you have created a potentially lethal circuit which could electrocute a person who touches it.
You should use a transformer with a 230V and 6.3 v outputs to isolate your circuit from mains AC for safety purposes.
(1) You have applied line voltage thorugh a diode to ground which is what is causing your traces to blow.
(2) you have created a potentially lethal circuit which could electrocute a person who touches it.
You should use a transformer with a 230V and 6.3 v outputs to isolate your circuit from mains AC for safety purposes.
Ok, I understand that I should now use a transformer, but with using one how will I make sure my other circuits share the same ground?
Transformer floats the entire circuit it supplies. Any solid connection (such as ground to ground at the input, but could just as well be any other voltage potential to any other voltage potential on the other component) equalizes potentials connected together and all voltages within certain component are referenced to this common potential as far as you're concerned. If your supply isn't floating (= the way as shown in your initital schematic), something will go bang if there is low enough an impedance path between two different potentials once another common reference is established.
Note that there is really no such thing as "ground" in your circuit. It would mostlikely measure ungodly high number of ohms to real ground (the one you stand on).This is just a reference point that is commonly used as it connects together both input source, amplifier and output (speaker).
Note that there is really no such thing as "ground" in your circuit. It would mostlikely measure ungodly high number of ohms to real ground (the one you stand on).This is just a reference point that is commonly used as it connects together both input source, amplifier and output (speaker).
First thing, do I still need to rectify the AC?
From what I understand the heater can be run off the 6.3V straight from the transformer.
As for the 250V, I assume it must be rectified, however if rectified then you get 350VDC.
I have attached an image below of a simple schematic.
I assume that the GND on my transistor circuit can then be connected to the ground of the rectified 250VAC from the transformer.
From what I understand the heater can be run off the 6.3V straight from the transformer.
As for the 250V, I assume it must be rectified, however if rectified then you get 350VDC.
I have attached an image below of a simple schematic.
I assume that the GND on my transistor circuit can then be connected to the ground of the rectified 250VAC from the transformer.
An externally hosted image should be here but it was not working when we last tested it.
You don't have to rectify heater supply, just leave it floating (unconnected to anything else).
You have to rectify B+. You can drop whatever excessuive voltage in supply filter. Level indicator circuit doesn't need very sophisticated filtering, all you want is to avoid potential flickeriing so few tens of microfarads in value (say 47 uF for example) in combination with sufficintly high wattage rating resistor to drop excess voltage (use Ohm's law to determine its value) should be more than enough.
You have to rectify B+. You can drop whatever excessuive voltage in supply filter. Level indicator circuit doesn't need very sophisticated filtering, all you want is to avoid potential flickeriing so few tens of microfarads in value (say 47 uF for example) in combination with sufficintly high wattage rating resistor to drop excess voltage (use Ohm's law to determine its value) should be more than enough.
Ok, thanks for that.
One last thing, can I connect the ground of my transistor circuit to the ground of the rectified B+ supply?
One last thing, can I connect the ground of my transistor circuit to the ground of the rectified B+ supply?
I think that is a great plan, before I blow more things up/and/or kill myself...
Thanks for the help so far
Thanks for the help so far
Fort777 USED to sell valve PSU tansformers, but it seems they've gone to you-know-what. Such transformers are quite rare, and often expensive since they aen't much in demand. Best plan here I think is to buy TWO 220:6:6 transformers (around 15 to 20 VA) e.g., Fort777 transformer
Wire the two 6V windings on one transformer to the two 6V winding on the other transformer. Those 6V windigs will give you the required 6v for the valve heater, and the 12V for the transistor circuits. The 220V-side of the second transformer will give you almost 220V, isoltated and 'safe', for the valve anode and target.
Wire the two 6V windings on one transformer to the two 6V winding on the other transformer. Those 6V windigs will give you the required 6v for the valve heater, and the 12V for the transistor circuits. The 220V-side of the second transformer will give you almost 220V, isoltated and 'safe', for the valve anode and target.
Thanks for the help Steerpike. Its a bit of a battle finding these things locally.
I will definately try out your idea with the 2 transformers.
And as I mentioned above I will post my schematic here.
Just one more question for now. Are the tube sockets available anywhere locally?
I will definately try out your idea with the 2 transformers.
And as I mentioned above I will post my schematic here.
Just one more question for now. Are the tube sockets available anywhere locally?
Yes, RS has them
RS Noval sockets
*
RS Components SA (Cape Town)
Unit 4
Woodbridge Business Park
Koeberg Road
Montague Gardens
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