Fender Hot Rod Deluxe - Fuse type fast or slow

[DrGonz78]

So I am replacing the fuse on this Fender deluxe and it is a 3A type. I can tell by the old fuse that it is a fast acting type. However, I am not sure if it was originally slow blow or fast acting. Just not sure who might have stuck this one in there and if it was correct. Anyone know? Thanks.

[Jazz P Bass]

Funny how that is not indicated anywhere.

I use Slo Blo fuses on all of my repairs.

The amp specifications call for a 180W draw from the mains.

So a 2 amp fuse would do in that case.

Then you have turn on surge.

At any rate, if the amp goes tits up, a fast acting or a slo blo 3 amp fuse is still going to open.

[g1]

I think in this case it's a fast blow. They don't specify on the schematic, but they do show the export 240V versions as F1.6A.
Also, the HR Deluxe III shows a "T" for the heater fuse but no T for the mains, so that implies fast blow to me.

Also, they have a thermistor which should help soak up the turn on surge.

[Jazz P Bass]

So if it is not specified, then the fuse should be a fast acting.

Makes sense.

[DrGonz78]

Cool thanks guys. I was thinking it must be fast blow too and all I had on hand was slow blow. Just wanted to make sure though as you never know who replaced what last. So off to get some fast blow fuses.

[nevetslab]

Adding my 2 cents......while their schematic draftsman failed to specify the fuse type for 120V, they did state F1.6A for 220-240V. F1.6 is Fast. It wasn't listed in their owner's manual either, for some reason. I checked a number of other Fender amp schematics. Slow Blow (3AG 1/4" x 1-1/4" size) or Time Delay (GMC 5 x 20mm size) are traditionally on the AC Mains side. Yet 3A FAST is what's called out in the Hot Rod DeVille, same current draw.

Now, there is a small NTC Thermistor on the primary side of the power transformer, which will limit the inrush current a bit. I didn't got thru all the schematics, but when I looked at my list of amp models & their specified fuses that goes out with our spare parts trunk for major festivals backline gear, I found both Fast And Slo-Blo listed on all the various Fender Amps.

With the Fender amps, I suspect if there's NOT a NTC Thermistor in line with the primary, then they're calling out SLO-BLO. If that isn't the criteria (with Fender), then it's back to common sense. I'm usually installing Slo Blo 3AG's or Time Delay GMC's on the primary. HT fuses are nearly always Fast Blow.

In general, the fuse value is selected to allow full power at high line (8% above nominal) & lowest load impedance, allows turn-on at high line without the in-rush current blowing the fuse, and with around 30% margin above that. If there is a major fault in the amp, it will generally blow a Slo-Blo fuse in nearly the same mSec time period as it would a FAST Blow fuse of the same rating. The value needs to be high enough to prevent nuisance blowing under high transients.

On SS Amps, whose mains current draw is dependent on load impedance, the value selected can be exceeded on the bench, using steady state sine wave. Using instrument, music, Pink Noise showing periodic peak clipping or 1/8 power sine signal source at lowest load impedance, the factory fuse value usually will not blow, though certainly would at steady state.