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Old 10th October 2013, 01:57 AM   #11
smbrown is offline smbrown  United States
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Kevin, care to share a schematic?
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Old 10th October 2013, 02:56 AM   #12
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As a ham radio operator for decades, I've worked on a lot of RF amplifiers. Once worked on one with 7500v on the plate.
I got shocked once on a 1800v amp. I was shorting the caps to ground before servicing it and the voltage jumped through the insulation on the screwdriver, ran through my arm and popped open a 1/4" opening on my elbow...
Be careful
Mark
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Old 10th October 2013, 08:30 PM   #13
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Originally Posted by SY View Post
The wrist strap will not help you- that's useful for static, not anything with current.
A wrist strap for working with HT supplies should be maid of braided wire and have a good connection to ground – (I have one that is about a " wide and looks like the ground strap of a car battery) – the rubber ones have more resistance than your skin and aren't even "useful for static" (IMHO).

The point I was making earlier is that one shouldn't be afraid of 1000V and unafraid of 500V - both can kill you. Always work with one hand in your pocket; always discharge caps†; never work "live" unless you absolutely have to (and even then, make sure you clip your probes on with power off first, then switch power on, take your reading, switch power off, and wait for the voltage to drop back to a safe level before you remove your test clips).

†Include bleeder resistors on all caps in the HT supply. If working on a device that doesn't have them, build yourself a "discharge rig." My high-voltage one is a 100K 25W resistor attached across a spare set of 2500V-rated DVM probes with an alligator clip on the black lead. I clip that to ground, carefully touch the other to the cap to be discharged, and wait until the meter drops below 20V. Then I wait a few minutes and repeat to allow for dielectric absorption – (it's not that the DA "rebound" will reach lethal volts, but you don't want a "surprise spark" causing you to flinch and wreck something).
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Old 10th October 2013, 08:47 PM   #14
SY is offline SY  United States
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Quote:
Originally Posted by digitrax View Post

The point I was making earlier is that one shouldn't be afraid of 1000V and unafraid of 500V - both can kill you.
Yes, but the 1000V one can kill you in more ways, some of which are surprising (for the millisecond before you die).
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Old 10th October 2013, 08:50 PM   #15
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Originally Posted by kevinkr View Post
All wiring in my amps carrying a kV or more is double insulated...
Never a bad idea in any amp, but for those that don't know about it:

Pomona 6733 hookup wire:
18ga w/10KV silicone insulation < $25 a (50') roll and worth every penny for this sort of work
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Old 11th October 2013, 03:01 AM   #16
nhuwar is offline nhuwar  United States
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Why would you wear a anti static wrist strap when working with high voltage. It would be more accurately called a death strap in this use.


The gm70 is a cool looking tube. One day I hope to build with it.
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Old 11th October 2013, 04:37 AM   #17
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Originally Posted by SY View Post
Yes, but the 1000V one can kill you in more ways, some of which are surprising (for the millisecond before you die).
True, but one should take the same precautions with 500V (or even 300).
In other words, treat any high voltage as if you were working with flyback voltages don't assume that, if it's under a KV, you're safer.
Insulate everything and don't touch anything you don't have to.
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Old 11th October 2013, 10:35 AM   #18
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My GM70 SE runs on 1100v and I've never experienced the pops or cracks that Kevin describes.
I'm using Pomona wire and everything is carefully insulated, I've even epoxy potted as many exposed parts as I could. Others parts are varnished. I've also rounded any sharp metal part that carries HV.
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Old 11th October 2013, 03:22 PM   #19
kevinkr is offline kevinkr  United States
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I had the biggest problem with the copper version of the GM70, the graphite plate ones on hand seem to settle down quickly and permanently.

I have some upgrades planned for the driver stage, and when I do this I will take a file to the corners of the plate pin solder tab as you did. Everything else has large quantities of air around it.

No plans at this time to make the design publicly available.
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Old 12th October 2013, 09:26 AM   #20
M Gregg is offline M Gregg  United Kingdom
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NB, Wrist strap.

If you use a wrist strap connected to earth while working on HV equipment. (don't do it!)

You are guaranteeing a good contact shock and will probably not survive.
You must try and isolate yourself from all contact with earth. You are Trying to create no connection with your body and any form of earth.
Chassis earth is one example. The other is any return path to the equipment supply. That means if the amp is disconnected from earth the chassis will be connected (internally)to the 0V side of supply and is just as dangerous.

Think like this, how do birds sit on power lines? The answer is the bird is at the same potential as the power line..ie there is no flow through the bird except the resistance between its feet. What would happen if the bird was connected to ground? BANG NO BIRD!

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M. Gregg
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Last edited by M Gregg; 12th October 2013 at 09:32 AM.
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