Yes, there are some vintage amplifiers out there that want to kill you.
This is an old (1950s?) “Saturn” three-tube amp, known as a “Widowmaker” due to its lack of an isolation transformer and the possible shock hazard. The amp utilizes a 35W4, a 12AU6, and a 50C5 tube. It has a small, 4” car radio type speaker. It has a single combination power switch and volume control. Lots of these 3-tube amps were made; they were cheap to produce and sold as a “Student” amp. They were sold at Sears and Montgomery Ward and various department stores.
The amp was particularly hazardous in that the incoming (110 VAC) power was grounded directly to the chassis. There was no fuse in the circuit. There was a 50/50 chance of having 110 VAC delivered to your guitar (and you).
I found this fragment of the schematic – you can see that one leg of the incoming 110 VAC goes directly to ground, and no fuse anywhere. Yikes!
I researched the amp. but couldn’t find the “Saturn” brand anywhere, and it didn’t conform to any of the Valco-Supro type amps out there.
The circuit in its entirety was traced and drawn. All electrolytic and coupling capacitors were replaced. Several mistakes were fixed. Here is the amp circuit as found
The “Isolation Module” was inspired by this article by gmoon: https://www.instructables.com/Isolation-transformer-upgrade-for-old-guitar-amps/
This is the best article I have found on how to make a widowmaker amp safe. You will need Ohm’s Law and some math to calculate the power handling of the transformer and the total circuit current, and to size the dropping (current limiting) resistor, but it is pretty straightforward, and gmoon describes the calculations very well.
The module consists of a three-conductor power cord, a fuse, an isolation transformer, a full wave bridge, and a filter capacitor. The full wave bridge augments the 35W4, which is a half-wave rectifier, and after filtering it makes the amp much more quiet. The voltage is higher with the FWB, so the 135R dropping resistor was increased to 470R to compensate. The switch on the volume pot of the amp was wired to the incoming hot leg, after the fuse. With the isolation module installed, the former “Widowmaker” is safe to operate – the power is truly isolated, and properly grounded. I mounted the components on a length of 1″ x 3″ wood, and secured it inside the bottom of the cabinet. Here is the layout – note that the isolation transformer is grounded:
The amp sounds very good, probably about 3 Watts, and there is very little, if any background hum.
I think this method is the safest and best way to modify any widowmaker amplifier and bring it into the 21st century.
Total parts cost was about $50 – $60. Kind of expensive for an amp that is worth maybe $150, but when you consider that it might save your life, it’s probably worth it.
