Replacing amp power tubes

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My amp is acting up from time to time, everything goes quiet and sometimes stays like that for days, sometimes weeks then it comes back to normal.

Checked the guitars and the leads so I assume its the tubes.

I'm using a Marshall 100W amp, I'm not an expert on this, never replaced tubes but I have a small shitty bass amp, does it come with the same tubes or do they differ from amp to amp (size/watts etc)? I could just swap those and see if the tubes are the issue. I've no local music stores where I could buy tubes so I'll have to order online. I know how to replace tubes, just making sure so I don't end up frying my amp
>pic unrelated