This is my first post here and I am not an electronics engineer (just a simple guitar player) who would appreciate some advice.

I have a relatively old (no longer produced) valve guitar amp which has two fuses - one which is located under the mains cable inlet (which has a 1.0 A slow blow fuse which is easily obtainable) and one which says it's protecting the tubes and that's a T0.375AL slow blow. I've figured that is 0.375 mA slow blow. My problem is having scoured the internet, it's nigh on impossible to get that rating easily (you can in the US but I'm in the UK and the postage costs are going to be almost 8 times more than the cost of the fuses).

I can get 0.315mA slow blue fuses easily in the UK - my question is am I likely to damage my amp in any way if I were to replace a blown .375 mA fuse with one of those? Equally, is the reduced amperage of the .315 mA fuse likely to cause a higher risk of the fuse blowing? Thankfully, I have not had a fuse blow but thought I should carry spares in case one ever did.

Thank you for the responses I have already received and I will do my best to answer:

The fuses are in a valve guitar amp and are protecting the tubes (American terminology for valves). I had always assumed (simply) that putting in something of a lower value (.315 vs .375) would have provided better protection since it would have blown sooner and thus protected the circuit(s). If that's not the case and it should be that I replace with a higher rated fuse, what is acceptable? 400ma slow blow? And to be clear, my amp isn't blowing any fuses at all - I just want to have some spares in the kit bag should it ever do so to enable me to swap for a new one and continue performing. Thanks again.

  • \$\begingroup\$ Do you have to do anything special to turn off the amp? A cool off period or some such thing? \$\endgroup\$
    – pgvoorhees
    Commented Feb 14, 2022 at 19:11
  • \$\begingroup\$ yes, it will work safely, at risk of un-needed blows under borderline operation. \$\endgroup\$
    – dandavis
    Commented Feb 14, 2022 at 19:58
  • \$\begingroup\$ It might be ok. I suggest buying one or two and trying it (at your least-favourite engagement.) \$\endgroup\$
    – rdtsc
    Commented Feb 14, 2022 at 20:00

2 Answers 2


Generally, your replacement fuse should have same or greater voltage rating and maximum interrupting current rating.

Lower rated current could cause false blowing.

The other thing which could be important is melting integral.

Without knowing what the fuse protects, nobody can assure that replacement will work.


Using a smaller amperage fuse is likely to only cause the fuse to blow in normal usage.

But for a normally sized fuse to blow would indicate something else is a problem first. Fuses normally don't blow for no reason.


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