To add to what Cap'n said........if you are looking to replace the power tubes, do yourself a favor and make sure the screen voltage (pin 4 on each of the big octal sockets) is no more than 300VDC. This can be checked with the output tubes temporarily removed from their sockets. If it IS more, then look to see if it has a short, fat tube in the dead center, labeled 6GF7A. If it does, then there is also an adjustment pot on the chassis deck so that you can dial in 300 volts. Excessive voltage here is the leading cause of shortened tube life with the 8417.
It's also important to rebias the power tubes anytime you swap them around or get new 8417s. Fortunately, Guild made this very easy to do with a bias test socket on the back panel and two adjustable pots (which are a little difficult to reach). All can be done without removing the amp chassis from the cabinet. If you can't get the bias 'in range', take it to a tech or, if you are solder-friendly, we'll tell you more about what needs to be done.
If you need more help or explanation just ask. There are a half-dozen or so active members here who have become oh-so-familiar with the T-Bass amps. It is worth the effort, too, as they sound quite good when they are topped off and running on all cylinders.