[Amps] SB 220 Exploding Resistor

[Carl]

> Date: Mon, 18 Jan 2016 14:13:10 -0700
> From: John Lyles <jtml at losalamos.com>
> To: amps at contesting.com
> Subject: [Amps] SB 220 Exploding Resistor
> Message-ID: <569D5566.3090503 at losalamos.com>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> If you get a strong enough wirewound resistor, this can suffice as the
> surge current limiting resistor for a spark over in a tube. With 250
> amps at the beginning of the overcurrent (2500 V/10 ohms) spike, there
> is enough magnetic field to break a wirewound R as the windings have
> mechanical force between turns. I have always used bulk carbon for
> series R, like Kanthal Globar makes. They are exactly the right part for
> this.
>
> http://www.globar.com/ec/bulk-ceramic-resistors/globar-bulk-ceramic-non-inductive-resistor-applications.html
>
> http://www.globar.com/ec/bulk-ceramic-resistors/axial-leaded-resistors.html
>
> Finding them through distribution might be a problem, though. Has anyone
> else used these?
>
> John
> K5PRO
>
> ##  The energy absorbing types are the AS  series.   The SP series is what 
> is used in dummy loads, etc.
> I have used these real small  globars back in the 70s... for the parasitic 
> suppressor in several amps,  but
> I don’t remember if they were  type  AS  or  SP.   Dunno if  Globar will 
> sell you just 1-2 of them ?
> SP types are rated for  350 deg C.
> AS types are good for  250 deg C
>
> ##  There is no issue using wire wounds for a glitch R.... provided they 
> are sized correctly.
> 2500 /10 ohms  =  250A.
> 3000 / 10  ohms =  300A
>
> ##  2500 / 50 ohms = 50A
>      3000 / 50 ohms =  60A
>
> ##  A  single 50 ohm  @  50 watt  WW works quite well..and does not 
> implode.  However, I use a HV
> fuse in series with the 50 ohm glitch Resistor..... to interupt the peak 
> fault current...asap.
>
> ##  I have also tried paralleled  100 ohm  @  50/100 watt  wirewounds.
>
> ##  Ameritron sells the globar type  AS  glitch resistor.... forget the 
> value they use.   If its real low,  like
> 10-15-20-25 ohms,    you could always series  2-5 of them.  But I believe 
> they are not cheap.   I believe
> they only sell one value.
>
> ##  I have also tried  the bigger, tubular type  SP resistors for a glitch 
> R, and they work good.   But these were obtained
> surplus.  I have also tried large tubular type  AS, and they work very 
> well.
>
> ##  But the wire wounds are cheap to buy new, and in any value and wattage 
> you want.   Readily available.
> 50 ohms will limit fault current to a low value.... and is not high enough 
> to cause heat issues, when used on
> high duty cycle modes.  Also, the V drop across  50 ohms, with  500-1000 
> ma of plate current is not an issue either.
>
> ##  You will not implode a 50 ohm  @  50 /100 watt WW any time soon.
> ##  surplus  globars show up on the market from time to time..and also 
> work good.
> I bought several  50 ohm, 90 watt CCS  type  SP  globars, same as used in 
> a heath dummy load, these also work
> excellent for glitch use.  I believe ameritron also sellls em.
>
> Jim   VE7RF


That 250A "assumes" the 2500V is a constant. In an amp with a wimpy 
transformer the B+ tanks.

Ive used a 12 Ohm 15 W for decades since I picked up a few hundred new ones 
at an auction.

RCA recommended 15 Ohms minimum for the 8122 at 2500V and 10uF output C. In 
the NCL-2000 the resistor is 15 @ 10W and in over 50 years Ive never seen 
one blow. It is listed as a special surge current rated in the parts list 
and Im pretty sure it is a WW.

Carl
KM1H