Guitar amp fuses

[Goathanger]

Hi,

We've recently replaced a F3.13A 240V fast blow fuse in a Peavey Valveking amp, with a slow blow S3.15A 240V, due to incorrect advice.
The original fuse blew.
Is this going to negatively effect the amp at all?
After using the slow blow fuse for an hour or so, the amp volume faded out then produced no sound at all. This is when we noticed we had been told to use the incorrect fuse.
The fuse has not blown and the amp still has power.
Basically I'm wanting to know whether the installation of the slow instead of the fast blow could have damaged the amps internals, or whether there was an existing issue that caused the original fuse to blow in the first place?

Thanks

 

[Nigel Goodwin]

There was an existing issue that caused the fuse to blow in the first place - which is obvious because the amp is still faulty.

 

[unclejed613]

there are many things that could have caused the original fuse to blow, and later when you replaced the fuse, to kill the output of the amp. without more info, that's kind of difficult to pin down. one place to start is by measuring the plate voltage for the outputs, which could be anywhere from 350-600V (be careful!!!!!). second, measure the voltage across the cathode resistors of the outputs (these will usually be large ceramic resistors of a few ohms, and the voltage will be anywhere from about 20mV to a volt or two depending on the resistance) a reading of zero millivolts means no current. the tubes are generaly run at 15 to 40mA depending on the tube type (calculate the expected voltage across the cathode resistor using ohm's law). also, NEVER operate a tube amp without a load.

 

[dr pepper]

A mains fuse I'd expect to see a 'T' slow blow(maybe what your bad advice was considering), however the fuse may be for the ht which would explain a fast blow.
Its possible you have done more harm, you'd be reasonably unlucky however.
The fault as implied could be anything, a bad rectifier cap, bad rectifier tube, bad o/p triode or pentode, does that amp use el34's?
Do you know if any of the tubes were lighting up blue before the fuse blew (had to be carefull spelling that).

 

[unclejed613]

quite often the B+ (HT) fuse is internal inside the amp, and as dr pepper said is an F (fast) type fuse, while the external fuse (the primary line (mains) fuse) is a slow-blow (T for time delay) type to take the inrush current and the cold filament current. if you popped the B+ fuse after replacing the primary fuse, the audio might fade out as the B+ caps discharge. blue (gassy) tubes could be partially functional, but at too high a current. or you've lost the negative bias on the output tubes and they may be amplifying, but are running abnormally high current.

 

[dr pepper]

Thats pretty much what I was thinking too.
The only thing as well as soft tubes is maybe a short somewhere causing excessive anode current, the most famous being what is called 'that cap', the capacitor that connects from the phase splitter or driver stage to the o/p tubes.
I think the reason why the ht fuse is inside the case is that most fuseholders are meant for 250v ac, some amps run at 600vdc, outside the rating of a standard panel fuse holder.

 

[unclejed613]

that's also the "expensive" fuse..... bypass that one with a gum wrapper and the things that burn up as a result are the most expensive, output transformer especially... which is another reason to have that one inside the amp, relatively safe from "bubba roadie" (i'm not insulting all roadies here, just the ones that like to play technician without knowing what makes things tick) and his magic gum wrappers.