Series String Amps: Demystifying the Widowmaker

NOTE: Some folks have suggested that this post may not state the facts accurately. In particular one correspondent suggests that an isolation transformer only insulates the circuit from the line, and then you become the ground path. Another suggests that the same result could be achieved with a GFCI plug. I'm going to get to the bottom of this, so stay tuned and be careful with these damned things.

Here's a circuit for a series string amp and in all respects it is typical of the breed although you sometimes see these with four, fiver, and sometimes more tubes. The idea's the same.

In a never ending quest for ever less expensive radios, RCA released model schematics for what are known as All American Five radios.

By putting the filaments in series and adding a suitable resistance, a power supply of a sort could be cobbled up without buying a power transformer. The typical offering in a guitar amp works on the same principle. Get as close as you can to 120v with the filaments in series, add in enough resistance to make up the difference, a couple of good stout electrolytic capacitors and use the chassis as the return path and Bob's yer uncle as the Brits say.

Well. In a radio it's usually sitting on a shelf inside a plastic or wood cabinet and you only touch a plastic knob so what's the worry? The Arvin folks, known then, as now for their metal stamping expertise released a line of metal case radios using this technology but did a baked enamel finish that is a pretty good insulator. Sometimes I cringe when I see people who have stripped and chrome plated these sets but nevermind. The subject's guitar amps.

The evil here consists of two things-the chassis as a return path for line voltage, and the guitarist or harp player's intimate connection to that return path through the guitar or mic cable. Standing barefoot on a cement floor or wearing sweaty leather shoes only makes it more dangerous. You can get the whole 120v of AC through your hands or mouth and it CAN kill you.

There is one, and only one, way of making this safe to use and that is through the use of an isolation transformer. That is a device which ISOLATES the circuit thus described from line voltage and thus saves you from being the ground return path. There is NO other way. A three prong cord will not make you safe.

You can obtain isolation transformers pretty easily from a number of sources or, you can make your own using a pair of Radio Shack door bell transformers back-to-back and accomplish the same goal.

Having said all that, these can be really fun amps to play with, but be advised: They're not really safe in their unmodified state.

UPDATE: One of my colleagues has posed the question of using a GFCI self contained cord with these amps. I myself have not used them, and the idea seems to be OK, but I think the difference is that with a GFCI setup you're interrupting a potentially dangerous problem, and depending on a relay to do it. I think the difference is that relays do not always do what you want when you want, and the problem is always there-a hot chassis connected to one side of the line. With a suitable isolation transformer, you eliminate the hot chassis problem completely and totally forever.

Thanks to Clark Huckaby for the use of his schematic. Just sharin' the wealth and keepin' it real, folks.The isolation transformer setup is the work of Dennis Poirier. A great fellow.

Building a Solid Workbench Addendum

One of the things everyone should have in their shop is a good solid work bench. I was getting by with a $40 Chinese made plastic topped banquet table that I got at Menard's, but soon after I put it together the overhang end started to gradually fold and collapse. Last week I knew something had given way because the end of my bookshelf was sagging badly.  The corner was folded under and fractured, and total collapse was imminent.

I built a workbench for the garage a month ago using the plans I found here and decided that it could be adapted to my particular space considerations. Although the plans call for rather tall legs and a long top, I made mine with 30 inch high legs and a 60 inch surface. You can adjust the dimensions any way you like to fit your particular needs and whether you plan to work standing up or sitting down.

For this kind of work it is better to attach the top with flush surface wood screws which I did. Then, if you like, you can attach a sheet of tempered masonite for hardness.

The biggest cost items are the Simpson Strong Tie RTC24 corner braces, which run nearly six dollars each. That puts you at about $55 with braces, hardware and a tube of construction adhesive. The Simpson people have been making structural connectors for wood structures since the 1950s and their products are good stuff. When you're done, you will have a very strong and rigid workbench or work table.

It takes a couple of hours to cut your lumber and put the bench together.

Here's a picture of the bench in use.

Fender Excelsior, You Fathead

These are pics of my recently acquired Fender Pawnshop Excelsior. Like others I have undertaken a few minor modifications to make the amp more to my liking.

The first was adding a tone control to replace the two position bright-dark switch. A 500k pot is installed and wired just like the switch with the existing wiring. You can widen the switch hole a little with a small file or two, and then use a fender washer to dress up the area and cover the screw holes. Be careful not to scratch the surface of the panel with your file-I did, and I'll probably strip this down and have it electroless nickel plated before long. A trio of vintage chicken head knobs dressed things up a little.

I don't know about you but I don;t like orange pilot lights. I snipped the wires to this eyesore and installed a typical Fender type lamp socket with a red jewel and a #47 bulb. I piggybacked off the wires for the second 12AX7 preamp tube socket which read a nice 6.7 volts. Plate voltage runs about 370 on these amps by the way. At the same time I added a pair of JJ 6V6 power tubes and a Sylvania JAN 12AX7 in the first position.

How's it sound? The tone control gives a little more flexibility and the tube upgrades are useful. Plus Leo would have approved of the red jewel.

Plus you get to see my newly reconstructed shop complete with killer work bench I built Sunday.

My apologies to the ghost of Jean Shepherd-if you spent any time connected to a radio in Jersey youy get the joke.