What is a widowmaker amplifier?

This is obviously a widowmaker amp because there is no power transformer in sight.

This is obviously a widowmaker amp because there is no power transformer in sight.

A widowmaker amplifier has no power transformer or fuse. Instead, the two-prong, non-polar power cord is wired directly to the rectifier tube. It is very unsafe, because under even minor failure conditions (such as a shorted capacitor or even - in some models - plugging the power cord in backwards), the mains voltage might end up on the guitar strings and the user will be shocked/electrocuted.

On the plus(?) side, power transformers are expensive so widowmaker amps could be manufactured and sold at a lower price. This ensured that every aspiring guitarist in America could own a low-fi death trap if that’s what they really wanted.

How does a widowmaker amp work? The power transformer in a conventional amplifier changes the voltage from 120v mains voltage to some level that is more useful for an amp circuit. A tube amp needs two kinds of voltage: a low voltage (usually 6.3v) for powering the tube heaters, and a high voltage (around 300v or more) for the tube plates.

Without a transformer, a widowmaker amp is limited to 120v. This is really high for tube heaters, and really low for plate voltage. So, it uses strange tubes purpose-built to operate on odd voltages, like the 50c5 power tube (50v heater, 110v plate) and the 35w4 power tube (35v heater). The preamp tube is usually a 12au7 (or similar) using the 12v heater tap and a crazy low plate voltage (for instance, 75v), which works but doesn’t offer the best headroom.

From left to right: a 35w4, 50c5, and a 12au7.

From left to right: a 35w4, 50c5, and a 12au7.

Heaters are usually wired in parallel, but here they are wired in series to consume the full mains voltage (50 + 35 + 12 = 97v, plus a dropping resistance equals 110v). The fact that mains voltage today is up to 15 volts greater than that isn’t a dealbreaker, but high heater voltages promote noise so the amp will sound even worse than it originally did.

Why was it okay to sell these things, if they were so hazardous? A few reasons:

Consumer protection standards used to be extremely lax. Consumer protection was not a thing until the 1970s. Before that, safety features were just another factor in a company’s cost-benefit analysis. If a safety feature made a product too expensive to manufacture, it was removed from the final design and employees were instructed to never mention it again. Thus you had toy laboratory sets that included actual uranium samples, coffee percolators that shattered when picked up by the handle, lead paint everything, and hazardous electronics of all kinds.

The Consumer Protection Safety Act was passed in 1972 and you don’t really see any power transformerless amps after the late 60s. Probably not a coincidence. In fact, late-60s widowmaker amps had a factory-installed isolation transformer, as manufacturers started to realize that consumers didn’t really want to get occasionally shocked by their electronics anymore.

A radio with an identical circuit had been blowing up the market since the 1940s. Widowmaker amps were adapted from a much earlier design: a bestselling tube radio called the All-American Five. (They had five tubes and they were all Made in USA.) These radios were among the first affordable radios on the market and just about everybody had one. Manufacturers kept the price low by eliminating some of the more expensive circuit components, like a power transformer. Instead, the neutral line of the mains was tied directly to the chassis. So, if you reversed the prongs when you plugged it in, the chassis would be live with mains voltage. Note that all plugs were non-polar back then and home outlets were wired randomly, so plugging it in “properly” on the first try would require both testing the polarity of your outlets and opening up the radio’s chassis to check which prong went where.

Radios generally had a capacitor that limited the current to the chassis in the event of power cord reversal, so most of the time the user would only get a shock. Also, the radio would hum loudly if it was plugged in backwards, so most people would immediately reorient the plug for the best sound quality. And besides, tiny radios were so new and exciting and nobody at the time really had any firm expectation that consumer products should be safe.

The mitigating factor for the All-American Five radios is that they were almost entirely encased in a bakelite shell. In typical operation, the user would never have to touch the chassis. (In practice, users were sometimes shocked by touching the chassis mounting screws often located at the bottom of the unit.) However, a guitar amp is meant to be used with a guitar…that has metal strings…that the user continuously touches…obviously, this application involves a lot more risk for the user.

Two-prong cords made everything dangerous anyway. The difference between a widowmaker amplifier and a conventional vintage amp with a two-prong cord is in degree, not kind. Typically, an amp with a two-prong cord has a capacitor (“death cap”) connected from neutral to ground. It’s meant to provide additional shielding, but if the capacitor shorts, it can send mains voltage to the chassis and, by extension, the guitar strings.

Point being, circuit designs that allowed mains voltage to reach the chassis were more or less the industry standard in the 1960s. Power transformerless amps seem egregious now, but actually, when you think about it, just about all electronics back then were pretty horrifying from a safety perspective. So, if you have any guitar amps with a two-prong cord, don’t feel too smug reading this article :) and upgrade your widowmaker-lite to three-prong as soon as possible.

When a radio has no power transformer, it gets a warm-fuzzy patriotic name. But take the transformer out of a guitar amp, and everyone calls it a “widowmaker.” Is that fair or is it just slander?

Google “widowmaker amp” or “power transformerless amp” and every result will be about amps like the Dynavox and how dangerous and awful they are. But google “All-American Five” and you’ll see a lot of nostalgia for these crappy little radios. People write books, articles, and think pieces dedicated to them. They’re restored with loving attention to detail. Their power-transformerless nature is almost always a footnote and never the headline.

It’s true that the stakes are a little higher for the amp, because the chassis is connected to guitar strings and theoretically a radio just sits on a shelf somewhere. But I for one prize the ability to move any piece of your home’s decor from one room to another without being shocked. Also, what if you spill something near the radio? Now your living room is basically booby-trapped.

Anyway, both “widowmaker” and “All-American Five” are totally fair nicknames for the circuit. These products are both an important part of history and also really unsafe. You can be super excited to restore your power-transformerless amp or radio and also a little afraid of it. The past is full of golden memories, beautiful design, handmade quality, corporate greed and electrocution. And that’s okay.

How can I make my widowmaker amp or All-American Five radio safe?

These circuits will be a lot safer with an isolation transformer, a three-prong cord, and a fuse. Note that the isolation transformer must be able to handle to current draw of the tube complement. Probably the most difficult part is finding somewhere to mount the transformer. If you are not experienced modifying circuits, you should find a tech to do it for you.

Of course, “I love that 50C5 output tube sound!” said no-one ever. (Well, no guitarist ever. We’ll withhold judgment on the radio, because we’ve never used one before.) For that reason, we totally stripped the chassis of our Dynavox and installed a different single-ended amp circuit that uses more ubiquitous tubes (a 12ax7 and a 12au7). If the vintage appearance of the cabinet is more important to you than the actual circuit, a totally custom mod is a good alternative to installing the transformer and dealing with whatever other issues the aging circuit has.

One final safety note about All-American Five radios: many of them are housed in a Bakelite case. Bakelite is a plastic made from a blend of formaldehyde and fillers: sometimes wood, sometimes asbestos. Bakelite is safe in its molded consumer form, but all bets are off once you start drilling into it. This is something to think about if you are considering drilling new holes into your Bakelite radio or otherwise modifying its case.


Further Reading

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