Safety words
As I will be documenting processes and methods of working with lethal voltages and hazardous equipment here, I believe that I have some modicum of responsibility for the safety of anyone who may stumble upon these pages and attempt to do these things for themselves. In that interest, I am going to make this post a single point of reference in all of my posts.
Background
I have been a mechanic of some variety for the last 17 years. Most of that time, I have been an industrial mechanic, however I have also been an aircraft electrician and a car mechanic. Suffice it to say that I have ample experience working with voltages up to 500 volts AC/750 volts DC, the risks involved, and safety measures to be taken to mitigate those risks as much as is practicable.
I realize that the majority of people who are going to be playing with the circuitry on this site are synthesizer DIY nerds, pedal builders, tinkers, and electronics hobbyists - none of which are going to have much experience (if any) working with voltages any higher than around 18 VDC... MAYBE 24VDC, or, if you're spicy, 48VDC. Which is totally fine - in our modern high-efficiency world, there is rare occasion for any consumer-level, entertainment electronics which would use voltages much higher than that.
But vacuum tubes are from the old days. And in the old days, everything used high voltages. The Philco model 40-180 radio that sat in many American living rooms (and with which many Americans received updates on the then-ongoing war) in the 1940s was putting out 270 volts AC from its power transformer. What's worse is that many, if not all, of these radios were equipped with non-polarized power plugs with the neutral soldered directly to the steel chassis inside the cabinet, with a small-value capacitor going from neutral to the chassis - in some cases, overly cautious designers would specify a capacitor from hot to chassis ground and neutral to chassis ground. These capacitors have become known as 'death caps' over time. Rob Robinette has a fantastic and thorough writeup (which I consider to be required reading for anyone getting into antique electronics) on the perils of these capacitors and what to do with them on his website - link is here. Another fantastic website by John Fuhring details the specifics of these hot chassis radios and is also HIGHLY recommended.
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| The capacitor highlighted above is an example of the 'death cap' that is endemic in antique electronic circuitry. This example is from a Webcor reel-to-reel tape machine, manufactured and sold in the late 1950s. |
To be an electronics hobbyist at this time meant that, unless you were strictly tinkering with battery-powered radio sets, dealing with dangerous AC voltages was part of the gig. The only time you would have comically low voltages of 12 volts was if you were dealing with getting power to the heater filaments of the tubes. (As an aside - if you went back in time and told anyone in the 1940's that you could create a 14 watt power amplifier, with no transformers or coils, that ran on 12 volts DC, they would think you were full of horseshit.)
As for the specific details of tube circuit safety - a lot of far more educated and eloquent people than I have written about safety over the years. I am going to list what I consider to be critically important pieces of literature and media regarding safety below.
(if you are wondering why almost all of the information given is referring to guitar or stereo amplifiers, and whether or not that distinction applies, you are not educated enough to be messing with this stuff. You have gaps in your knowledge that you must seek to fill first.)
REQUIRED READING
(Uncle Doug is a fucking treasure. If you are learning about tubes, you REALLY should watch all of the videos on his channel. The way the man explains how these things work is easy to understand and he is an incredible teacher)
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