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Old 14th June 2013, 03:06 AM   #281
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Hi guys,
I begun working on electronics on my own shop back in 1989; it was a tv set repair; many tube units were still being repaired at that time. They were the worst thing to put your hands on; live chassis, +B of around 300V, and crt HT of 25KV. One day I remember I flipped one tv set on one of its sides and separated a few centimeters the chassis from the tv case to make some meassurements on the backplane, I was sliding my forearm in carefully with one multimeter probe in my hand when accidentally I touched some +B spot; went catatonic for a few minutes until I realized I was still alive. That was the first and only time I had such a huge voltage shot. Learned the leason the hard way.
Later on worked for siemens in the electromedical division, on X-ray and CT scanners. They were very emphatic at the training courses in germany about the hot spots where if you touch you go.
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Old 17th June 2013, 07:04 PM   #282
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On valve amplifiers I always put an LED in series with a resistor across B+.
This not only shows when high volts are present but slowly discharges the B+.
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Old 17th June 2013, 10:44 PM   #283
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Quote:
Originally Posted by nigelwright7557 View Post
On valve amplifiers I always put an LED in series with a resistor across B+.
This not only shows when high volts are present but slowly discharges the B+.
Do you try and cope with the wide range of DCV that could be deemed hazardous (eg. 50 to 500V) ?
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Old 17th June 2013, 10:48 PM   #284
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Originally Posted by trobbins View Post
Do you try and cope with the wide range of DCV that could be deemed hazardous (eg. 50 to 500V) ?
The LED does stay on (but dimmer) as the voltage dies away.
By the time it I has gone out it is pretty safe to touch.
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Old 9th July 2013, 07:21 AM   #285
Rewind is offline Rewind  Sweden
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I got new Valvo EL84 tubes from Germany yesterday. When I put them in the amp it started to pop like crazy in the Radian 2" compression driver! I remember seing rust on the staples on the tube boxes, and then I noticed champagne coloured oxidation on the EL84 pins. They must have been stored in a moist environment over in Germany.

So I took out my trusty Dremel tool and started grinding down the pins until I saw bare, shiny metal. That solved it. Sound improved as well. The connection must have improved a lot.

I will do this a safety precaution on my other amps as well, since it may also improve sound quality.

One time a tube preamp lost connection to the tube pins and it burned through a resistor on my $640 Lynx soundcard. Had to send the Lynx across the Atlantic for service. Never dared to use the preamp again. I use a passive preamp with a step attenuator now - very clean sound.

Last edited by Rewind; 9th July 2013 at 07:30 AM.
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Old 16th August 2013, 08:01 PM   #286
Matt BH is offline Matt BH  United Kingdom
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Hey Rewind, don't really know about using a Dremel for this. Seems a bit harsh but I use a fibre glass pencil. Works a treat for all sorts of contacts etc.

Cheers
Matt.
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Old 17th August 2013, 02:41 PM   #287
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Quote:
Originally Posted by Rewind View Post
I got new Valvo EL84 tubes from Germany yesterday. They must have been stored in a moist environment over in Germany.

So I took out my trusty Dremel tool and started grinding down the pins until I saw bare, shiny metal. That solved it. .
Be careful that you aren't creating minute cracks in the glass base. Annealed tube glass cracks very easily under stress. My favourite is a small brass brush on a model drill, no more.

richy
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Old 17th August 2013, 02:57 PM   #288
Rewind is offline Rewind  Sweden
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Originally Posted by richwalters View Post
Be careful that you aren't creating minute cracks in the glass base. Annealed tube glass cracks very easily under stress. My favourite is a small brass brush on a model drill, no more.

richy
Yeees, I should maybe have been more careful. Everything sounds good here tho.
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Old 29th August 2013, 08:54 AM   #289
M Gregg is offline M Gregg  United Kingdom
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Just something might be of interest,

Thoughts from another thread.
Another reason to use discharge resistors, if the B+ is up for weeks after power off. That's when little fingers go to work pushing hair clips into the vent holes. Doing what daddy or mommy does with their hobby!

Regards
M. Gregg
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Old 29th August 2013, 06:24 PM   #290
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"Salute, Genius!"

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Water conducts electricity!
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