Power Supply Tube Preamplifier.

P

Pim Driessen

Member
Dear Forum Members,
I have a question about a current limitation in a power supply for a Tube preamplifier output 215 Volt.
The question is Does this work if you accidentally create a short circuit?.
Here is the diagram of 2 different power supplies, the first one broke due to a measurement error, of which it is not entirely clear what went wrong.
The second diagram is of a tube preamplifier from Audio and Technology from a long time ago, I personally think the current limitation of Audio and Technology looks better.
When I simulate in Tina Software, the current limitations of both seem to work, but as I wrote before, the power supply board of the first diagram broke after a short circuit.
What is the opinion of this Forum about the two power supply diagrams?.
Greetings,
Pim

 
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Need to know if the voltage rating of the pass transistors is greater than the supply voltage.
Also what is there power rating, and are they on a heat-sink?
 
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The most important thing about using bipolar transistors in high voltage applications, is to double check its SOA curves, and stay well away from its secondary breakdown region.
 
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I suspect the secondary breakdown value of the transistor was exceeded under short circuit conditions, but the BUS11 data sheet I found does not show that value.

Below is the safe area graph for a similar transistor.
At 200V, the maximum allowed current is about 80mA.
The first circuit appears to have a current limit due to T4's base-emitter resistor of about 0.7V/10Ω = 70mA, which is close to that DC limit.
It will also be dissipating about 15W under short-circuit conditions, which may be more than the heatsink can dissipate with exceeding the maximum transistor junction temperature.

Best type of current-limit circuit for this application, would be a fold-back or electronic-fuse type which limits the current to a very low or zero value under shorted conditions.

 
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The BUS 11 Transistor was still intact, the other transistors were defective, I replaced them all including the BUS11 transistor. Below is a graph that was calculated by a friend.

An electronic fuse would be a solution, I'll look it up, in the meantime the tube preamplifier plays, but not with the power supply where a photo was placed by me, I got the power supply with electrolytic capacitors and resistors completely silent. The second electrical diagram has a slightly different current limitation, especially the 475 K resistor gives me the feeling that this is better designed than in the first diagram.
 
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Pim Driessen said:
An electronic fuse would be a solution,
Click to expand...
Below is the LTspice sim of an example electronic fuse circuit (Q4 and Q5) that could be added to your circuit:
Q3 turns off when the output is close to 0V due to a short, which then pulls the bias voltage at Q1's base to ground, stopping the output current (yellow trace).
The power would then need to be removed to reset the fuse.

Some of the resistor values may need to be tweaked for your particular circuit.

 
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Thanks a lot.
That looks good, I need to look into that, and also whether it can be built into the existing power supply.
But since there are a number of transistors in the existing power supply that can only handle 200 volts, I am considering building the diagram below, and then on a perforated board and point to point.
 
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Pim Driessen said:
Changed my mind and found a more simple design, is it possible to insert the electronic fuse into this diagram?
Click to expand...
It's more simple because that design has no output regulation, as it just sets the output to be some fraction of the input voltage with no feedback, so the output voltage will vary with the input voltage.
Is that okay for your purpose?

If so, than you can connect Q5 from my circuit to the junction of your R3-R4, my R4 to your VF1 output. and my R5 to the your input VS1.
 
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Good that you noticed, it is not convenient that there is no feedback.I have found another diagram, did not know that it was still present in my files,Can the electronic fuse be used here?
 
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Pim Driessen said:
Can the electronic fuse be used here?
Click to expand...
Yes.
Connect my Q5's collector to your U1's gate, my R4 to the output, and my R5 to the input voltage.

Also change C1 to 10nF.
If the fuse trips on startup (no output) than increase its value but to no more than just allows it to startup.

Of course my two transistors must be rated for at least 300V.
 
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I'm going to add it to Tina Software, and then it will be placed here.
I'm going to add it to Tina Software, and then it will be placed here.
By the way, I didn't realize that the entire circuit as described by you, has to be built in, that will be an extra printed circuit board.
 
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This is confusing, all transistors in your circuit are in the electronic fuse, so they should also be able to handle a high voltage, e.g. BF459, can the circuit be built with all five transistors BF459?
At the output I put a switch with a 1 ohm resistor, that's the second picture.
It seems to work.
The voltage and current drop out.
 
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