Fuse resistance

[Mick Bailey]

I noticed that after replacing the cathode fuses in an Engl amp the power tube LED was flashing in sequence to playing. The only way is if there's either a circuit fault or the fuses have a high enough resistance for the voltage drop to trigger the LED circuit. I re-checked the amp and checked the fuses - they all read 56 Ohms, which really annoyed me as I'd just bought them and had to make up a minimum order charge as I was out of stock (T63mA).

I did a further check and the resistance holds out even at their rated voltage and close to their capacity. Temporarily swapped them out for some T100mA and no problem.

[Enzo]

Really low current fuses like that do normally show resistance. My HP meter used 32ma fuses.

So other than the LEDs blink, what problem did the smaller fuses cause?

[Mick Bailey]

No other problem that I can see, though the 'scoped output increased just a little with lower resistance fuses. The originals read a little over 10 ohms so I've ordered another lot from a different supplier who lists the resistance. I would think that 56 ohms in each cathode circuit would introduce a little more NFB, as it's unbypassed. The main issue is I don't want to return the amp to the customer with a flashing LED that didn't flash before.

[g1]

I remember some of the slow blows used to have little resistors inside, maybe some still do.
Could you just tell the guy it's the 'back-off!' light?

[Mick Bailey]

I actually prefer the flashing LED. It just needs to be blue to make it a 'now' thing.