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Old 13th August 2012, 01:46 PM   #271
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and his is exactly why I love this forum!

your input is much appreciated. I will send an email to epcos and ask.

What would you suggest as a more reliable approach? Tube based device maybe?

I need something which is available at a reasonable price.

Thanks!
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Old 13th August 2012, 04:35 PM   #272
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Quote:
Originally Posted by Alexontherocks View Post
Damage from the opt

Click the image to open in full size.

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Maybe the sharp edges of the soldering joint are a starting point for
partial discharge and lead to the flashover.

regards
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Old 13th August 2012, 04:41 PM   #273
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Quote:
Originally Posted by Alexontherocks View Post

The Core is large and the transformer heavy. What would be an inexpensive way of dessicating and potting the OPT?

Thanks
Not inexpensive but the way transformer companys do it:

Drying in the oven, then put into a tank where vacuum is applied and
potted in this tank under vacuum...

regards
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Old 13th August 2012, 05:10 PM   #274
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Isn't there a tube which could somehow replace the gdts?

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Old 16th August 2012, 09:53 AM   #275
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No ideas?


Are there an other, none tube, devices which might work better than gdts?


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Old 2nd September 2012, 07:12 AM   #276
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What about using a high power thyratron operated so that the peak voltage across the opt exceeds maximum inverse voltage?

Something like this perhaps.... if i were to find a cheap lot could it be a solution?

http://www.ebay.com/itm/TG1-2-5-4-TG...item3a68ef134f



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Old 3rd September 2012, 03:23 PM   #277
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Alex,

Unfortunately a thyratron works much like a GDT, in fact it IS a GDT with a control grid. Once the gas inside the thyratron ionizes the only way to shut it off is to remove the plate voltage. There were some speciallized devices that could be shut off, but I don't remember any that could work at this power level.

I have thought about this issue quite a bit and I haven't got an answer.

I experimented with several ways to avoid frying the OPT in the case of operating an amplifier without a load. The worst case is a guitar amp operating at full crank when the speaker fails to an open.

I had best luck with GDT's, MOV devices and even Transorb diodes, but they were all attached to the secondary to clamp the voltage at about twice the expected maximum. These work on mid powered amps, but they just fry in a 300 watt guitar amp.

Your amp should have enough power to melt about any type of clamp device. Any device attached to the primary of the OPT will try to clamp the B+ which has enormous stored energy, and it will discharge that energy through the OPT.
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Old 3rd September 2012, 04:23 PM   #278
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thanks for your post!

Unfortunately the fact that you haven't found a solution makes it almost impossible for me to stumble across one by myself.

I found a good lot of thyratrons (the model I linke to above). a hefty piece of glass!

My concern is that what i witnessed is not a random event (allbeit caused by an solvable issue) but may be related to the interaction between the amp and the huge inductance of the opt. If this is the case I think a solution may be required for the amp to reliably work in the future.

My though is the following:

two tg1 - 2,5/4 are placed across the opt primary section. The insulation for the opt will be upgraded to above 4kv. In case of a spike the reverse voltage barrier is punctured and the thyratron turns on. The problem would be how to turn the device off and remove the bypass.

I am trying to read as much as I can and found this..


http://www2.l-3com.com/edd/pdfs/crow...tionnote.6.pdf

seems an intteresting read.
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Old 4th September 2012, 02:36 PM   #279
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What about a string of thyratron + resistor + ptc thermistor to rapidly quench the current draw and lift the clamp?

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Old 5th September 2012, 09:53 PM   #280
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while waiting for opt repair, thyratron and some other stuff I have worked on the driver board.

I cannot measure beyond 390vpp as the scope clips so I connected the probe to the common cathodes of a single 6bl7 tube. Adjusting persistence and display i managed to get a clear view at the sine wave (while cranking the signal beyond the 390v barrier)

Click the image to open in full size.

if ccs are set differently the peaks are not alligned to the voltage markers.

Unfortunately I cannot tell what vpp is produced but it *should be almost 430/450 vpp.

Sine was ok but square was bad.

Click the image to open in full size.

Click the image to open in full size.

without fb, high voltage swing and possibly a not high enough probe impedance this may be acceptable but it seems to me almost every anomaly one can find in a square wave test is infact there!

Observations?
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