I thought it would be most important to write an article on safety first, considering that we are dealing with lethal voltages. When we are dealing with a tube amp the voltages are high, often around 450 volts. The current through the wires, however, is normally very low. It is in the milliamp range. The problem is that is only takes 0.1 amps through your heart to kill you, and far less can cause permanent damage.
In our high voltage tube amps, there are many places that can have a potential difference with enough voltage to drop you into a pine box. Potentially the most lethal components are the large filter capacitors. They are designed to have a large charge at your amp's highest voltages. When the amp is shut off, these caps will retain much of their charge for days unless drained. They are then capable of dropping their entire charge into a load, or you, in less than a second if you're not careful.
In my amps I place a high value resistor across the filter capacitors to allow them to discharge when the amp is turned off. Before you work on your vintage rig you need to make sure the amp doesn't contain these static voltages. You can build a little tool for draining this charge with nothing but a 2-5 watt resistor in the 75k ohm range (can vary from 40k - 100k ohms). Connect this between ground and the plate of one of your preamp tube's plate. Should I mention that the map should be unplugged when you do this? I hope not! Now I'm a little cautious after being shocked before, and I touch my tool to each plate on the preamp and then the power amp also. As a precaution you can leave the tool connected between ground and a preamp tube's plate.
My tool consists of a 100K ohm resistor soldered to about 4" of wire on both ends and then soldered to alligator clips on to the wire. This allows me to attach it to the amp while working on it and also easily remove it when I am done. Don't forget to remove it when you are done!
It is often the case that you need to poke around in your amp while it is on and live. This is very dangerous, but is necessary for certain bias setting and for finding bad solder joints or wires that are too close to each other. All that needs to be kept in mind is that you CAN NOT do this with your hand, and that it must be done with a nonconductive tool. I have a tool made of a hard plastic, but many people use chopsticks or something similar. To check a tool for conductivity, place your multimeter on resistance and put both leads about a 1/2" apart along the length of the tool. You should not get a resistance reading. If you have a low resistance reading the material is too conductive for your purpose.
I hope you found this article informative and useful.
November 2, 2004