Photo 3

If the amp powers up in standby, put it in operate and see what happens. If it blows the fuse, there’s a problem with the remaining filter caps in the high-voltage supply (Photo 3).


Photo 4

If you’ve gotten this far, reinstall the tubes and see what happens. With the amp off, install one pair of output tubes, one in a left socket and one in a right socket (Photo 4). (It doesn’t matter whether it’s the two inside or two outside sockets.) Turn the amp on, let it warm up, and then switch to standby. If the amp stays on, you may get a little hiss from the speakers. Power down, remove those two tubes, and repeat the procedure with the other two power tubes. If all is good again, then the output tubes are probably functioning.

Next, reinstall the two preamp tubes. At this point, either the amp is working, or it’s on but has no sound. If no sound, try replacing each preamp tube to see whether that cures the problem. If not, with the amp on, plug your guitar into the effect loop return jack and turn the master volume up. If you have sound, then the output stage is performing properly, even though the front-panel controls aren’t presently working. That means the problem is in the front end. If there’s no sound, there may be a transformer issue, and you need to start checking both the high voltages and the low-voltage supplies for the IC chips.


Photo 5

If all voltages are good, then the problem is probably one of the 8-pin audio op-amps. I suggest replacing IC 1 thru IC 5 first (Photo 5), as they are in the primary signal path. It might be fun to install sockets and experiment with other compatible dual op-amps (such as the 1458, 4558, TL072, or TL082). They all sound a bit different, especially overdriven. You can also try different preamp tubes, such as the 12AX7, 12AT7, and 12AU7.

I haven’t covered every possible failure, but hopefully these tips will help you sharpen that Tusc!