SAFETY FIRST! 

General advice for old tube radios:

So, you find yourself as the proud new owner of some not-quite-so-new tube radio or amplifier and you have made the choice to try to bring it back to life. NEVER! plug in an old tube radio or amplifier that you found in the garage/attic/flea market/wherever without first performing a careful and thorough electrical inspection (details below). Many people want to plug it in "just to see if it works" or "just to try things out." NO, NO, NO! This is a MONUMENTALLY BAD IDEA!

DO NOT PLUG IT IN! This is the single worst thing in the world to do with antique electronics! If you are lucky (and few of us truly are), all it will do is hummmmmmm. If you are not lucky (yes, this probably means you), a rush of electricity (typically several hundred volts) after decades of sitting around and deteriorating is likely to kill old capacitors, creating cascading failures that can blow a hard to replace rectifier tube, fry irreplaceable coils, destroy other downstream components, or even start a fire... A shorted rectifier can permanently destroy an irreplaceable vintage power transformer or power supply inductors. A typical power transformer for a tube radio produces about 300-400 Volts! Dried, cracked, and missing insulation from old rubber coated wires can come into contact with one another or with the metal chassis and can electrocute you because old radios usually don't have a safety ground!

The BEST practice for dealing with old tube radios is to CUT OFF THE POWER CORD right away! First, it probably needs to be replaced anyhow, and second, this keeps anyone else from plugging it in until you've been able perform a thorough inspection and perform safety upgrades like adding a fuse and replacing damaged wires.

Safety Protocol for YOU:

First, there is a protocol to follow for testing tube radios/amps that needs to be followed BEFORE you plug the power cord into the wall. Failure to follow protocol risks destroying old equipment, starting a fire, or killing yourself. A tube radio power supply harbors LETHAL voltages, often in the range of 200-400v. This voltage can be present EVEN WHEN THE POWER CORD IS UNPLUGGED. This voltage CAN KILL YOU. There are a few basic safety rules that apply here:

1) REMOVE ALL jewelry. ALL of it! NO rings, NO necklaces, NO bracelets, NO danglies, NO nothing that will accidentally conduct electricity!

2) TURN POWER SWIITCH TO "ON" WITH RADIO UNPLUGGED TO DISCHARGE THE CAPACITORS. Doing so will discharge the main power supply capacitors - the ones that hold 400vDC. You can make double sure that you've done this by using a specially made (by you) insulated wire with insulated alligator clips at both ends and a 10k-100k power resistor rated at 1 to 3 watts in the middle. USING ONE HAND ONLY, clip one end of the wire to one side of the capacitor. Using that SAME HAND again, clip the other end of the wire to the other side of the capacitor. Let it sit there for a minute or so and then move on to the next cap. Do this for EVERY big cap in the device. Use ONE HAND ONLY! Keep the other hand behind your back AT ALL TIMES. Exposing yourself to high voltage that spans both hands (and your HEART in the middle) is just asking for an ambulance ride! Tube amp power supplies make excellent defibrillators. A properly beating heart does not respond well to being defibrillated! You've been warned... Also, you'll find many people online just say "short the cap with a screwdriver." NEVER DO THIS! This is potentially damaging to the screwdriver, to the equipment you are testing, AND TO YOU. Depending on voltage and capacitance levels involved, you are potentially dealing with an arc welder. This much power can literally blow off the tip of your screwdriver. If this flying chunk of screwdriver tip hits you or someone else in the eye, now you have a major problem. DON'T BE DUMB!  No need to make yet another entry on the Darwin Award winner list...

Safety Protocol for your RADIO:

DO NOT PLUG IN YOUR RADIO! See above. No matter how tempted you are to "try things out" or "see if it still works" or whatever else, DON'T DO IT! You need to carefully and methodically inspect your tube gear FIRST so that you prevent doing even more damage than time alone has caused. Here is my short protocol for things you want to inspect first. A much more detailed and thorough protocol can be found here: Trouble Shooting Antique Equipment.

1) You will need to remove the metal chassis that contains the electronics from its wooden cabinet so you have access to the electronics. Once the chassis is freed, take lots of pictures - up close, in focus, well lit pictures from different angles. Photograph each tube and each connector. Make notes on how things are wired and connected to each other. Note colors of wires, positions of wires. Label all of the tubes with masking tape BEFORE you remove them. Draw a diagram of the chassis and label where each tube goes. Now remove the tubes, get yourself a old bath towel and lay it down on the table. Turn the chassis upside down on the towel and take lots of pictures of the underside before you touch/disturb anything. Use good lighting, make close up images that show details of where wires travel and what they connect to. Only then should you begin to poke further into the chassis. Your notes and images will become an invaluable resource when you start removing and replacing things. If you have an air compressor, this might be an excellent time to use it to clear out some of the old dust/dirt/debris. As always, use a bit of care not to dislodge weak/brittle wires or other structures. It might be best to blow some air across the chassis before your compressor hits maximum pressure to avoid unintended damage. 100 PSI is NOT your friend in this situation... That much air pressure can break brittle wires and dislodge components from where they are supposed to be connected. Go gently and slowly!

2) Inspect the power transformer with your ohm meter. Check the primary for resistance - just measure across the prongs on the AC plug (with the radio UNPLUGGED, make sure the radio is switched 'on'). It should measure somewhere in the 1-10 ohm range and not be open (mega-ohms of resistance). Check each of the secondaries for resistance. They, too, should not be open, nor should they be a dead short. Secondary windings will typically measure tens, hundreds, or thousands of ohms. Check to make sure multiple secondaries are still isolated from one another. You should find mega-ohms of resistance (or infinite resistance) between secondary windings. If these measurements check out, there is a reasonable chance that your transformer "should" be good.

Alternatively, you can use a dim-bulb tester to check the health of your power transformer. It is important to use a regular TUNGSTEN filament (incandescent) bulb - no CFLs or LEDs! Tungsten has a large PTC – positive temperature coefficient of resistance, so the voltage dropped across the light bulb falls off rapidly as the current through it drops. For a small tube radio, start with a 60-100w bulb. A 60w bulb will limit the current to 0.5A (60w divided by 120v). For a radio with more than 6-7 tubes in it, use a 100w bulb. A properly functioning radio/amp will make the bulb glow somewhat brightly at first as the power supply transformer in energized and the capacitors charge and then should diminish to a dull glow after a few seconds. A short circuit in your radio will make the bulb shine continuously at full brilliance -but without burning the insulation or creating high-voltage arcs in your radio!

This protocol comes from Leigh from the AntiqueRadios forum:

One test would be to remove all tubes, then power up with your dim bulb tester. Searching the web for "dim bulb tester" will reveal many examples. They all work the same way - a light bulb is placed in series with your radio to protect the radio's transformer from a shorted condition. The bulb might come up very briefly but should settle down to dark or almost dark. If it shows any significant brightness you have a shorted transformer or circuitry connected to the transformer. At that point you would need to disconnect all secondary wires, then try again. If the bulb still comes on, the transformer is bad. If not, trace the loose secondary wires to find the problem.

If everything looks good at this point, re-install all tubes except the rectifier. Bring it up again on the dim bulb bester. The bulb will light, hopefully not too bright. You should be able to measure less than about 5-6v AC at the heater (larger diameter) pins for the rectifier and several hundred volts AC across the two remaining (smaller diameter) pins. Measure the voltage at the transformer primary. Calculate the percentage (for example: 60v measured / 120v = 50%). Apply that factor to your nominal filament voltages. Then measure those filament voltages. If any are not close to your calculated value, that points to a possible problem.

3) This is probably a good time to make your first modification to your old piece of gear: add a polarized AC plug and add a safety fuse. Old AC power cords have plugs with two blades that are the same size so you can plug it into the AC wall outlet either way. This is a bit dangerous with old gear, so replacing the plug with a new polarized plug is a good first step. Then, follow the power cord back into the chassis and add a fuse to the "hot" wire. Do this somewhere AFTER the cord strain relief (where the cord physically enters the chassis) and BEFORE the power line hits the power switch and transformer. The power flow sequence should be: AC power cord, fuse, switch, transformer. You can use any type of fuse holder here, depending on how "original" you want to keep things looking under the hood. The image below shows a grey fuse holder that I attached to the chassis in the top right hand corner of the power supply for my Radiola-17. The power cord is new and enters from the bottom right of the image. I made a knot in the cord just inside the hole it travels through to keep the cord from getting pulled and coming lose. Then, I cut the "hot" wire and added a safety fuse that is visible in the top right corner of the image below. Typically, a 1A slow-blow fuse will be fine. When in doubt, start with a smaller fuse first, say 0.5A. I would not recommend going above a 1A fuse unless you have a rather large radio with more than 5 or 6 tubes. Refer to the metal plate that is usually riveted to the chassis for the actual power consumption of your radio and choose your fuse accordingly. NEVER purposefully install a fuse with too high of a rating - this defeats both the function and purpose of the safety fuse.  For example, NEVER use a 5A fuse because a 1A fuse blows. At most, try a 1.5A fuse next.  If the 1A fuse blows, it's telling you there is a problem that you need to solve.  Find the problem and fix it...

When attaching a new power cord to your new outlet plug, the ends of the wire need to be tied with and "Underwriter's Knot" to prevent strain on the cord from pulling the wires loose from the screw terminals on the blades of the plug.  Period appropriate cloth covered wire and properly polarized AC plugs can be found on ebay or a number of antique radio restoration web sites.

4) Check each of the tube pin pair combinations for shorts. Identify what type of tube you have, the numbers are usually stamped in silver on the glass envelope, either on the top or the side. You might need to wipe the tube with a damp paper towel to remove the dust and grime before you can read anything stamped on the outside. Sometimes you need to hold the tube at an angle to reflect the light from a lamp. Look here at Frank's Electron Tube Database or Google the tube number and find a data sheet for it. This will help you identify the pins and their function. The image below is the pinout from a random tube that I found on the internet. Start with the heater/filament pins - these are typically the pins that are connected to one another in the tube diagram. With a four-pin tube, the heater pins are typically the fatter two pins. In the tube pinout diagram below, pins 2 and 7 are the heater. They are labelled as such with an "H" and you can see in the diagram that they are the only two pins that have a physical connection to one another.

Test the heater pins with your ohm meter for resistance. A conductivity test between the heater pins should reveal a small resistance - typically under 10 ohms. Make sure there are no other shorts across any of the remaining pins. Test the remaining pins in pairs, all combinations, one pair at a time with your meter. If any other pair of pins besides the heater pins show connectivity (anything under 1,000 ohms or so), you probably have a shorted tube that needs to be replaced. Using your ohm meter for now is probably sufficient to make sure you have a tube without gross problems. If you want to do more thorough testing, you'll need to find a tube tester.  

5) Inspect the power supply capacitors. You can perform one or two tests with your typical inexpensive Digital Multi Meter, and you can perform a few more if you have an expensive DMM. but NO digital multi meter (unless you've dropped a few grand on a digital capacitance bridge tester/meter) is equipped to help you test what really needs to be tested in antique tube radios and amps: do any of your capacitors "leak"? This is a simple test: caps are supposed to allow AC to pass and are supposed to block DC from passing. If DC passes through your cap, you have a leaky cap. The ONLY way to test for this is to remove the cap from the circuit, apply a working voltage (200+ volts DC) to your cap and see if any of the voltage leaks through. Here is where we get to the heart of the story. See my page for details about how to build a capacitor leakage meter.

In case you missed it the first time through, here is an EXCELLENT detailed writeup for how to thoroughly inspect a tube-based power supply BEFORE applying AC power. Be sure to read Max Robinson's write up about Trouble Shooting Antique Equipment. The first one third of this page is very detailed and definitely worth a read to help keep you from screwing things up even worse than they are already likely to be from the simple passage of time.

Once you've verified the proper functionality of each capacitor (and replaced them accordingly), you are probably most of the way toward making your radio work...