[AMPS] Fuses

[Richard W. Ehrhorn]

I never (knowingly, at least) experienced a "Rocky Point bang" until I 
began working external-anode, ceramic-metal tubes, Larry. First the 
3CX1000A7/ 3CV1500A7, then 8877, 8874, 3CX800A7, 3CPX5000A7/3CX15000B7, and 
4CX800A/Gu-74b. Never, so far as I know, with 807(!), 6146, 813, 4-400A, 
4-1000A. Suppose 807 & 6146 are red herrings - 600-750V probably couldn't 
break down the internal spacing short of mercury or neon fill. But with 
4-1000As @ 4-5kV the only Bang!s I could identify came from violent VHF 
parasitics - usually blowing away everything connected to the screen.

But for the 3CX1000/3CV1500, which have thoriated tungsten filaments and 
banged at least as often and as loud as any, one might have wondered if the 
oxide-coated cathodes in the newer-generation tubes had been somehow 
involved in creating "barnacles" and bangs! I guess not.

73,  Dick  W0ID



----Original Message-----
From:	Larry Molitor [SMTP:w7iuv@axtek.com]
Sent:	Monday, March 22, 1999 7:06 PM
To:	amps@contesting.com
Subject:	RE: [AMPS] Fuses


At 17:08 22-03-99 +0000, Peter G3RZP wrote:
>40swg  (0.0048 ins dia) wire fuses at about 2 amps. Suspended between
>suitable supports, it should go in well under 2millisecs, thus reducing 
the
>energy dissipated in the tube. Then it has to be replaced, and if the tube
>is a tetrode, some sort of protection is needed for the screen.

Peter,

I have been using 40 ga. (or thereabouts) wire as a HV fuse on my tetrode
amps for many years. In every case when the fuse was installed, there was
no damage when the bang occurred. Replace the wire and try again.

For test purposes, I have a board with a 40 ga. wire fuse and a 25 ohm
glitch resistor mounted on it with HV pigtails. When bringing up a new amp,
I always put this in the HV line. Saves lots of parts.

By the way, The only "rocky point" type bangs I have experienced have been
in external anode ceramic metal tubes. Most of them have been in 4CX1000's
operated at 2500 volts or more. It's not a question of if it will go bang,
but when! In the 25 plus years I have been running 4-400's and 813's in
TCGG, I have never experienced a flashover. 572's or 811A's either for that
mater. Is it luck of the draw or a characteristic of the external anode 
tubes?

73,

Larry - W7IUV
w7iuv@arrl.net
http://www.axtek.com/w7iuv


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