If the original fuse was 2A and the replacement 3A then you havent fixed the problem you'll just make some other component fail further up the line. I had similair issues with my Classic 30, it ate up some EL84s and then popped the fuse and kept popping it. Took it in and Graeme had a fiddle, resoldered a few things and with a new set of tubes it was good to go.

I assume the Classic 50 is like the 30 and uses the weird bent PCB board which is prone to issues.

Toast wrote: I replaced the 2A, 250V fuse with a 3A, 250V and everything seems fine; it played beautifully for 40 min. Is this OK? Is there any chance of damage to the amp with the 3A fuse in?

Fuses are rated to protect circuitry. They should blow before anything else does - as a protection. They are a deliberate weakest link. If you put in a higher current fuse then something else in the circuit is being allowed to draw more power (potentially up to 50% more power in this case) than it should.

Think of the circuit breakers in your house. If the circuit is capable of handling 30a you don't want a 40a circuit breaker! That would allow the cable to be overloaded. It probably won't go up in smoke instantly, but it can be overloaded now because the circuit breaker will allow more current than the circuit is designed for. If you upgraded the circuit breaker because it kept on tripping then you did the wrong thing - you should have reduced the load (or put in heavier duty wiring).

So I'd get that amp checked out.

IIRC, the classic 50 has four fuses. It should give the values on the board. As others have said, stick with the rated fuse for safety. As to the fuse blowing in the first place - it is fairly common (they get old and blow), and a fresh set of valves can stress the fuse a little more. I've heard of amp techs who change the fuse as a matter of course when they change valves.

So it might not be a problem. Replace the fuse with one of the correct value and see how it goes. If it keeps on blowing fuses, then take it in to a tech to look at.

Thanks for the great advice, guys!

I've gone back to the specified 2A fuse, so I will see how it goes.

This is the first fuse it's blown in 5 years of intense use, so it's probably nothing - just the new tubes. 8)

hope it works out.

someone correct me if i'm wrong, but as the C50 is fixed bias, it might be that the new tubes are causing it to run a little differently, hotter perhaps, drawing more current. if the fuse blows again it might be worth it to take it to a tech to get the bias appropriated.

out of interest, what was in it before?

Yep, fixed bias.

The valves that were in before were:
EL84s x 4: all Sovteks
12AX7s: 2 x Electro Harmonics and 1 x Sovtek

I would rebias. But then, I would not have to pay a tech to do it.