The biggest, highest wattage incandescent bulb you can find makes a really good test resistor. Odds are it will have too much resistance to try operating the amplifier at all. If the bulb comes on full brightness and doesn't fade down, you have a short failure somewhere and the bulb limited the fault current to a safe value. If the bulb lights up bright and fades, have a look at the tube filaments. Might need to turn off room lights, but there should be some dim glow. If the amp has a working HV meter, you should see some movement.
If it passes those tests, plugging it in to the outlet direct should be safe to do. And if it flunks the light-bulb test, at least you didn't introduce additional damage on top of what's causing the short. The traditional name for just plugging it in, crossing fingers and flipping the power switch is called a smoke test. And if your luck doesn't hold out using the traditional smoke test, putting the smoke back in is always more expensive than letting it out.
Just be double sure the fuse is the right size.
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