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Old 12th July 2009, 05:00 PM   #1
bigwill is offline bigwill  United Kingdom
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Question OPT Voltage spike protection with zeners

Simple premise, zeners facing into each other across the primary of an OPT that will clamp the voltage in the case of accidental load disconnection or amp misbehaviour to something slightly above the maximum anode swing of the amplifier is capable of. Zeners would probably consist of X amount of 200V zeners to make up various ballpark values, 400v, 600v etc

Is this worth doing? I don't think I've seen it implemented before. It sounds like a worthwhile addition to amplifiers considering high voltage 5W zeners are so cheap
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Old 12th July 2009, 06:04 PM   #2
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You may use some modern MOSFET with gate and source connected together. They work as Zeners on high voltages, and most of them have a diode that shunts an opposite polarity. However, if you connect gate to cathode of your output tube through an appropriate thermo regulated voltage divider you can get a "tube on steroids", like I do in my Barracuda amp.
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Old 13th July 2009, 01:28 AM   #3
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Quote:
Is this worth doing? I don't think I've seen it implemented before. It sounds like a worthwhile addition to amplifiers considering high voltage 5W zeners are so cheap
I've read of it as a safeguard in pentodw amps, which are the ones normally associated with OPT primary overvoltage if the load is accidentally disconnected. I can't remember where I saw it though. A simpler safeguard would be to connect a resistor of about 1k permanently across the secondary, so it can never be completely unloaded.
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Old 13th July 2009, 11:03 AM   #4
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by ray_moth

A simpler safeguard would be to connect a resistor of about 1k permanently across the secondary, so it can never be completely unloaded.
I have considered this, but I recall reading somewhere that a small load like that doesn't make much difference. If it would definitely work, I would be all for using that instead...
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Old 13th July 2009, 11:06 PM   #5
tomchr is offline tomchr  United States
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There are some issues - primarily oscillations and RF emissions associated with those kinds of circuits. I suggest looking at some texts related to switchmode power supply design as the same issues are present there. On the primary side of the power transformer of a switchmode supply, you typically find all sorts of voltage limiters, snubbers, etc.

Whether adding such a circuit will prevent amp meltdown if the load is disconnected is another story. But it will limit the primary voltage...

~ Tom
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Old 13th July 2009, 11:29 PM   #6
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Quote:
Originally posted by ray_moth


I've read of it as a safeguard in pentodw amps, which are the ones normally associated with OPT primary overvoltage if the load is accidentally disconnected.
You may see diodes from plates to ground in ADA guitar amps.
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Old 15th July 2009, 10:27 AM   #7
Merlinb is offline Merlinb  United Kingdom
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An alternative is to connect a gas-discharge surge arrestor across the primary. These can be bought with spark-over voltages of 600V and higher, ideal for the task, and they're practically indestructible.
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Old 16th July 2009, 06:27 PM   #8
bigwill is offline bigwill  United Kingdom
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Originally posted by Merlinb
An alternative is to connect a gas-discharge surge arrestor across the primary. These can be bought with spark-over voltages of 600V and higher, ideal for the task, and they're practically indestructible.

Hm, great idea! Wonder how hard these are to come by
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Old 17th July 2009, 04:24 AM   #9
ChrisA is offline ChrisA  United States
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Default Re: OPT Voltage spike protection with zeners

Quote:
Originally posted by bigwill
Simple premise, zeners facing into each other across the primary of an OPT that will clamp the voltage in the case of accidental load disconnection or amp misbehaviour to something slightly above the maximum anode swing of the amplifier is capable of. Zeners would probably consist of X amount of 200V zeners to make up various ballpark values, 400v, 600v etc

Is this worth doing? I don't think I've seen it implemented before. It sounds like a worthwhile addition to amplifiers considering high voltage 5W zeners are so cheap
What is the capacitance of a non-conducting 5W zener? With the zener there you'd have a shunt to ground for some higher frequencies. But likely well above the audio range.

What about placing a fuse in series with the Zener? Then if it ever does conduct the fuse will blow quickly. Then you feed the B+ supply to the xformer through the fuse. When the Zener conducts the fuse blows and disconnects B+ supply and the amp shuts down. Or at least the OPT is no longer powered.

To answer the question about the gas discharge unit. They are very easy to find. They are used in communications antenna to shunt a lightening hit to ground. You can buy them from any place that sell radio gear for a few bucks each. But. I think they need some huge number of amps to trigger. They work when a spark jumps the gap and ionizes the gas which makes the gap more conductive. But they are made to handle 1000+ amps as in a direct lightening strike

Look here for more details
http://www.alphadeltacom.com/pdf/Tra...ech%20data.pdf
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