Making Widow Makers Safe

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Making Widow Makers Safe

Postby jviss on Fri Nov 04, 2016 7:02 pm


New user, first post.

I have recently become interested in low-end '60's amps, like Harmony, Kay, Silvertone, an others. So far I have a Harmony H303-A and a Kay K503A. I got both of them working without electrocuting myself. What I want to do now is make them safe, and do so according to correct safety and engineering practice.

The root problem is that these typically use unpolarized, two prong electrical plugs, and use the metal chassis as the system ground. Both of mine have power transformers, which some folk refer to as isolation transformers, but unfortunately the chassis can still become hot in a fault condition. My Harmony has a 50V tap on the secondary connected to the chassis; the tap is supply the heaters with 50V. It floats above ground potential, but when shorted to ground no measurable current flows. Reverse the plug to see different voltage float levels. The Kay has the tube heaters across the primary, and one side of the secondary connected to chassis. Both use the chassis as the system and signal ground.

Many people install a three conductor power cord, connect the green wire to the chassis, fuse the 'hot,' or line conductor, and be done with it.

What I would like to know, is there a correct way to do this other than that? Can the Kay's heaters remain across the primary, with a three conductor power cable and fuse installed?

Thanks very much!


p.s. very interested in a good book on tube guitar amp design

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Joined: Fri Nov 04, 2016 2:57 pm

Re: Making Widow Makers Safe

Postby JJman on Tue Nov 08, 2016 1:35 am

Harmony H303A: The heaters are interesting but there is no reason to change them. Change to 3-prong power cord with ground wire to chassis. I don't see the fuse but it should be directly connected to the incoming hot wire. The other fuse end goes to the switch. The other end of the switch to the primary. Neutral from wall goes straight to the other primary.

Kay 503A: Having the heaters on the primary is pretty bogus but they are shown floating off the chassis. I would add the 3 prong cord and wire in the same manner as above. But I would also add a fuse between the hot wire coming off the switch and (only) the "top" side of the 150R (R3) dropper. This way a short (to chassis) in the heater string would blow that fuse instead of the filament on one/some tubes. The string uses ~150ma on paper but more in today's world. Problem is the initial current is higher when they are cold. May not be possible to find a value that makes sense. They’re similar to having the old series-type Christmas bulbs. And the filaments are fuses too, but slow. The grounded power cord ensures against shock regardless of the heaters.

Make sure the main fuses are small in value. I would not use more than 1amp in either. Many old amps have high value fuses in them from ignorant prior users.
-If it says "Vintage" on it, it isn't.
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