Replacing a Two-Fuse Plug

[Todd, KA1KAQ]

I've got an old Kaar radio here that I want to cobble up a power cord for. Been searching with zero luck for the original Elco Varicon 20 pin connector, but since it was apparently a special part for this series of radios, chances of finding one are slim. So I got some of the small female disconnects at Napa as used on old speaker tabs, and will build a plug to at least make it functional.

By the looks of the schematic, it originally used one of the Killah dual-fuse plugs ala Johnson, Heathkit, etc. Though I have a spare around here, I'm going to forgo the thought of using it. Instead I'll used a 3 wire grounding plug.

My question relates to fusing. It's been a few (many) years, but I don't recall a specific need to have a fuse in each side of the line, only one in the hot side going to the on/off switch. Since I don't want to drill and blast, I can just stash one under the chassis. Originally there were two 1.5amp fuses.

Just want to be sure I'm remembering right and not missing something obvious here. One 3amp fuse, good to go?

[K9DXL]

I might be wrong here, but I'd go with one 1.5 amp fuse.  It's the weak link idea, and I don't think the mfr. would put a pair of 1.5 amp fuses feeding a three amp load.  What's the power consumption @115 VAC (might be on a plate or decal on the unit somewhere.)  That should give you an idea of the size fuse to install.

[KA2DZT]

The same 1.5 amps flows through both fuses.  So, one 1.5 amp fuse is what the answer is.  Fuse the hot lead only.

Fred

[Todd, KA1KAQ]

Yep, you guys are right on the money. I got confused looking at the plug wiring diagrams with the 6 & 12 volt plugs next to the AC. 6 volts = x2 amps from 12 volts. Interesting in that they wired the DC plug so you could just flip it over 180 degrees from 12v to 6v and change the fuse.

Had chores to do outside today and ended up doing other things out in the station tonight so I didn't get to it anyhow. Next thing I need to do is dig out a 3 wire plug & cord.

[WQ9E]

Hi Todd,

My response got lost during server issues it appears but the others have covered it.

The only thing I would add is that while you are setting it up with the 3 wire plug it is preferable to put the hot/load side line bypass after the fuse.

I have replaced most of the original 2 fuse plugs on my Johnson and Heathkit gear, the exception is the Ranger I to drive the Desk KW which has to plug into the outlet in the Desk KW.  For those who want to keep the original style plug and be a bit safer you can carefully mark the side of the plug which is the hot side which will go to the switch and put the proper fuse in that side.  Then put a very large slow blow fuse (or copper slug) in the other side so that the neutral fuse won't open.  Of course you then have to be very careful to always connect ground first and also orient the plug properly to take advantage of this change.

Johnson was creative in some cases, for example the Ranger uses a 3.2 amp "slow blow" fuse in one side and a 5 amp regular fuse in the other side.  I am a prolific user of inrush limiters and with those in place a 3 amp regular fuse works fine for the Ranger.

For those wanting to stay authentic the last time I was in Farm and Fleet they still had the traditional 2 fuse plugs in the electric fence supply area.

[N0WEK]

A little further afield...

I've got one of those old slate and knife switch fuse panels here that I picked up somewhere. It's early enough that both sides of the line are fused and both sides switched by double pole knife switches. About the time the industry figured fusing the neutrals was a very bad idea Buss Fuse came out with the official "penny in the fuse box" "fuse". All the neutral lines in that box have the official shorting fuses installed.

I've thought of using that box for my lights and shack power by switching nice noisy "clunk" relays (out of sight) with the slate panel. It'd add to the ambiance of the Tesla/Frankenstein look I'm going for.

[WQ9E]

I remember reading an article written by the engineer of a 1920s era radio station and they had a slate knife switch panel like you describe.  One knife switch was in series with the HV feed to the final and the announcers would open the switch enough to create an arc to light a cigarette.  The federal radio inspector saw that little trick and wrote them up for it

I do like the knife switch look.

[KA2DZT]

The two fuse double pole knife switch was probably made for 230 volts.  They did make single pole knife switch/fuse boxes.

Fred

[Todd, KA1KAQ]

If I were going for originality I'd just open the plug, jumper the contacts, and install a fuse under the deck. It would look stock but be inherently more safe. I stopped worrying years ago that the Johnson police, Collins cops, Hallicrafters or Heathkit Rangers would show up some night for a random snap inspection.  

As far as line bypass caps Rodger, I don't recall seeing any in this rig and have no plans to add any. It's not something I'll be using very often, but do want to be able to fire it up now and then for a demo.

I've got one of those old slate and knife switch fuse panels here that I picked up somewhere.

I've thought of using that box for my lights and shack power by switching nice noisy "clunk" relays (out of sight) with the slate panel. It'd add to the ambiance of the Tesla/Frankenstein look I'm going for.

Rumor has it that Fred mined the copper for these panels and built them by hand in his spare time.  

I picked up a few panels maybe 8 years back with the same intention. A simple cleaning with Windex and a hose made them look new. Unfortunately I had to leave several behind during the final phase of the move due to space & weight issues. Still have 3-4 of them, though. Someday, when the Dream Station gets built.......mhmmm....