How to Install a Power Tube with a Broken Guide Key
Have you ever had an output tube that was missing it's guide key? Maybe you broke it off when attempting to install or remove the tube from your amp but wiggled and rocked it side to side a little too much?
Well there is a very good possibility that your tube will still function fine. This is because the guide key has nothing to do with the tube's ability to keep a vacuum. Its only purpose is to ensure the tube aligns correctly with the socket that you are plugging it into. Of course without the guide key it can be very difficult to align and if you get it wrong you may damage the tube when you power up the amp.
But thankfully we can show you an old trick that will help you get it right. Below you will see a pictorial showing how to do it. All you need is some masking tape, a felt-tipped pen, and a room with good lighting.
Step 1: Take a piece of masking tape and wrap it around the tube's base.
Step 2: Examine the bottom of the tube base to determine how the guide key was originally attached. Usually this is easy to do and you'll notice the ridge side always faces between pins 1 and 8. Now sometimes this can be a little tricky because some tubes won't have pin 1 on the tube base. For example the common 6L6 and 6V6 tubes are often without pin 1 (and pin 6 too, but don’t worry about that right now).
What can you do about this? Well, sometimes the tube base will be numbered so in a well lit room you will be able to see the numbers. If not, we suggest you examine another tube from your set and look at how it's aligned on it.
Step 3: Next, get a black felt tipped marker pen. Carefully make a line on the tape that is in line with where you determined pins 1 and 8 are located. This line should be in between these two pins just like the original guide key's ridge side was facing.
Step 4: Now you need to get a good view of the socket to see how to align the tube. The socket will have a large keyhole in the center. You will notice a groove on one side and this is where you will align the mark you made on the tape. Once you have aligned the mark on your tube with the socket's groove, simply plug in the tube. If it's difficult to see the socket while plugging in the tube, you can tape the amp chassis and draw a mark pointed at the keyhole's groove. This will make it easier to see, especially if the chassis has not been removed from its cab.
Now the tube and amp should function normally, saving you the time and trouble of finding a replacement tube.