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1. ## Fuse ratings

I needed a fuse for my SB-230 rebuild, to go in the HV line in series with a Glitch resistor. It is a 620mA fuse, so if EVER I exceed that plate current, the fuse goes BYE BYE!

But it is rated at 125V. I am putting 2500V through it! Is it going to go PUFF!!!!??
How can a fuse be rated for voltage when it is essentially ZERO Ohms when working. Once open it is a ZILLION OHMS. So how can you have a voltage across ZERO ohms???

2. Ham Member
Join Date
May 2001
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This may help you. http://en.wikipedia.org/wiki/Fuse_(electrical) I also tried to use "a ZILLION" in a formula, but research showed http://en.wikipedia.org/wiki/Indefin...itious_numbers so used infinite instead that formula worked. Current was zero, voltage was = to the applied voltage, just as predicted, so physics has not changed.

I would not put a 125V fuse in a 2500 volt circuit, it might not work at the current you think it will. While it might open around the current you think it will, it definitely will not open in the time you think it will.

You will never have zero ohms across anything it will always have some minute (small not time). It may be .00001 ohms but it will be there. I don't remember every putting voltage through anything, current however does pass through something, as long as voltage is applied. But seriously the Wikipedia on fuses is good.. Regards.. Fred

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Mar 2006
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I would not put a glass type fuse in a HV line EVER. If you "REALLY" want to protect and shut down the amp "in time" you should simply make a simple transistor circuit that monitors the Vdrop across the plate shunt resistor calibrated to trigger and lock a small relay latched in series with the antenna relay circuit. Adjust the trigger point of the transistor to trigger at whatever plate current you'd like. This is often done with grid trip circuits and can easily be carried over to protect the the plate current as well. When a glass fuse blows in a HV line it explodes usually and makes an awful mess.

4. The fuse if you should use one at all....
Should go in the cathode circuit of the tube. The DC return for the cathode.
This will protect the tube and make it possiable to use a regular fuse.

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The 125 V fuse will likely arc and remain in a low impedance state when the fuse element melts.

For a glitch resistor the Pulseater series of resistors should survive.

6. Originally Posted by AF6LJ
The fuse if you should use one at all....
Should go in the cathode circuit of the tube. The DC return for the cathode.
This will protect the tube and make it possiable to use a regular fuse.
Yep, this is the solution. Henry amps all have a cathode line fuse, it's only a volt or two above ground so a "regular" fuse works fine. A normal 125V (most are really 250V) fuse, once "blown," may still arc internally at 2500V, rendering it rather useless.

7. Originally Posted by WB2WIK
Yep, this is the solution. Henry amps all have a cathode line fuse, it's only a volt or two above ground so a "regular" fuse works fine. A normal 125V (most are really 250V) fuse, once "blown," may still arc internally at 2500V, rendering it rather useless.
My Ex put a 250V 1A fuse in the B+ line to an amplifier. The B+ was 800V there was a fault one day and the fuse exploded, peppering everything nearby with glass shards.

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Aug 2001
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Originally Posted by WA6MHZ
I needed a fuse for my SB-230 rebuild, to go in the HV line in series with a Glitch resistor. It is a 620mA fuse, so if EVER I exceed that plate current, the fuse goes BYE BYE!

But it is rated at 125V. I am putting 2500V through it! Is it going to go PUFF!!!!??
How can a fuse be rated for voltage when it is essentially ZERO Ohms when working. Once open it is a ZILLION OHMS. So how can you have a voltage across ZERO ohms???

 The continuous anode-I rating of the 8873/4/5 is 350mA. A 620mA fuse is of no value. Also, a fuses' V-rating needs to be observed to assure that it will open quickly, unexplosively and protect the device it is supposed to be protecting.
To protect the SB-230's 8875 from harm:
1. Put a 30Ω to 50Ω glitch-R in series with the +HV to guard against gold sputtering from the grid.
2. Put a 0.375A, 3000V fuse in the +HV to protect the oxide cathode from flaking.
3. Use a low VHF-Q parasitic oscillation suppressor to reduce VHF/UHF amplification,
4. Add MOF R to the cathode to protect the cathode from more than c. 30W drive.

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Minor correction. The Heath SB-230 uses an 8873 tube, not the 8875.

Glen, K9STH

10. Has this question of voltage rating of fuses come up at work. The voltage rating is to ensure the fuse has a clean clear of the fuse when an overcurrent happens.

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