Turning an old tube radio into a guitar amplifier can be a tricky process if the radio you are using requires total rewiring, and it should not be attempted by anyone without a sufficient background in analogue circuit building. However, a tube radio with an audio input on the back is fairly easy to turn into a guitar amplifier that can provide you with a unique sound for recording or practice situations where high volume is not a concern.
Connect a ¼" mono cable from the output of your guitar to the input of a direct box with a ground lift switch. The ground lift switch is essential for connecting to an old tube radio to prevent unnecessary humming sound.
Plug a ¼" mono-to-stereo RCA cable from the audio output of the direct box to the input of the tube radio. The ¼" mono side of the cable should be plugged into the guitar's direct box and the RCA cable should be plugged directly into the tube radio. In most cases, ¼" mono-to-stereo RCA will be the right type of connection. However, this will vary based on the tube radio that you use. A wide variety of cabling to connect a direct box via ¼" is available from pro audio retailers and electronics supply stores, who can often make a cable that is not readily available.
Turn the volume on the guitar and tube radio all the way to their lowest settings. Move the guitar a few feet away to prevent any feedback or excessively loud humming when you turn the volume up.
Select the audio input on the control panel of the radio. Increase the output volume on the radio to a very low level.
Increase the volume on the guitar and play something to test whether or not you are getting signal or if there is a lot of noise.
Use the ground lift switch if you hear any humming or buzzing noises along with your guitar's signal. If you are using single-coil pickups, move around a little bit to try and reduce the hum. Proximity to the guitar amp, or tube radio in this case, is often the cause of humming and noise with single-coil pickups.
You may find that using an old tube radio as a guitar amplifier gives you a fairly brittle sound. Try a humbucker-style guitar or switch to your neck pickup. Alternately, use an equaliser pedal to reduce some of the high end and add a bit more bass if necessary.