With three power tubes out of four working, the amp will produce an asymetrical waveform, but at lower volumes you won't notice the difference - only if you crank it, when it will seem a little low on power and the distortion will be a bit odd-sounding.
If you switch to 50W and the bad tube is in the 'on' pair, the result will be worse - a more or less half-wave waveform, with very noticeable loss of power and bad-sounding distortion at anything above very low volume.
If the tube isn't glowing, its filament has failed - not the 'active' elements which will be fine, so it won't blow the fuse. Quite often, this not actually a dead filament but is caused by a bad solder connection between the tube pin and the wire that runs down inside it from the glass bottle; you can often fix them by re-soldering the tip of the filament pins (2 and 7) with new solder - but be careful not to make the pin bigger or it will stretch the socket contacts. If you add too much solder, wipe it off when hot (with a cloth) or scrape/file it off when cold. I've repaired about a couple of dozen "dead" tubes this way, mostly JJs which seem to be prone to it for some reason.
