[Amps] HV fuses

[Vic K2VCO]

Also I would add that if the tube is a tetrode or pentode, you must have a screen current 
trip circuit as well. Otherwise, if the fuse blows for any reason you will destroy the screen.

On 10/25/2013 1:51 AM, John G3UUT wrote:
> In a microwave, I believe that these fuses are before the rectifier in the AC part of the
> circuit.  I'm sure everybody knows this but I'll mention it again.  As AC drops to zero
> volts during a cycle, the chance of getting long lasting destructive arcs (plasma arcs) is
> much lower, therefore using them on DC in an amplifier is potentially problematic.
>
> Having said this I've used a microwave fuse in an amp, putting it before the glitch
> resistor on about 2.4 kV and it seemed to blow ok.  They are very inexpensive and
> certainly attractive for use in amps and so a bit of research on their usability would be
> very useful.   The fuse I used was in a white plastic insulating enclosure so that if it
> blew up removing the bits of glass wouldn't be difficult and I had a glitch resistor to
> limit the potential arc current.   I suspect the voltage and the value of the glitch
> resistor are going to be the biggest factors determining whether there will be an arc.
> Take care doing the experiments.
>
> 73 John G3UUT
>
> On 25/10/2013 07:49, Fuqua, Bill L wrote:
>> I would wait until New Years or July 4th to give one a try.
>> Hi Hi
>> 73
>> Bill wa4lav
>>
>> ________________________________________
>> From: MU 4CX250B [4cx250b at miamioh.edu]
>> Sent: Friday, October 25, 2013 1:48 AM
>> To: Fuqua, Bill L
>> Cc: Amps at contesting.com
>> Subject: Re: [Amps] HV fuses
>>
>> It didn't sound right to me either, Bill. The seller provided no data
>> sheets other then claiming a 900 mA/5kV rating for the fuses. I think
>> they were intended for microwave ovens, but am not sure. As I said, I
>> bought them out of curiosity because they were so cheap.
>>
>> Sent from my iPad
>>
>>> On Oct 24, 2013, at 23:38, "Fuqua, Bill L" <wlfuqu00 at uky.edu> wrote:
>>>
>>>    This does not sound right. I assume you are saying that they are glass fuses.
>>> I suspect if you blow one in a HV powe rsupply it will shatter or explode into pieces if
>>> you have a reasonable value of filter capacitor.  Do you have a data sheet on these
>>> fuses?
>>> 73
>>> Bill wa4lav
>>>
>>> ________________________________________
>>> From: Amps [amps-bounces at contesting.com] on behalf of Jim Garland [4cx250b at miamioh.edu]
>>> Sent: Thursday, October 24, 2013 7:58 PM
>>> To: Amps at contesting.com
>>> Subject: [Amps] HV fuses
>>>
>>> Out of curiousity, I bought several 900 mA/5 KV fuses from an eBay Chinese
>>> supplier. They arrived today. They fit fine into 3AG fuse clips, but are
>>> about 50% longer. Internally they have a spring connected to the fuse link
>>> which pulls the connection apart when the fuse opens. If they work, they're
>>> pretty cool. and VERY inexpenisve.  I'm wondering why one couldn't parallel
>>> two or three of them to multiply the current rating?  Assuming they're
>>> reasonably well matched, so that the current divides equally among the
>>> fuses, it would be a very compact and inexpensive HV protection circuit. I
>>> would mount fuse clips on a piece of G10 circuit board and mount the
>>> assembly on ceramic standoffs. When one fuse opened up, all would open up,
>>> but replacing three fuses would cost  only a dollar or so. I'll test some of
>>> the fuses ( I bought twenty) to see how accurate their current rating is.  I
>>> have no convenient way to test their HV rating, but I'd use them only at
>>> 2500V in my dhomebrew amplifier.
>>>
>>> 73,
>>>
>>> Jim W8ZR

-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/