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i have been troubleshooting a guitar amp that blows the 125 ma slo blow fuse upon startup. The fuse does not blow when the amp is brought up to 115 VAC with the Variac. However when starting the amp without the gradual Variac increase in voltage the fuse blows. I removed the 100 uf 350 volt filter cap and installed a 33 uf cap. The amp now starts up without blowing the fuse. However I see a millisecond filament glow when turning the amp on. My question is: Is the filament glow stressing the fuse, or just me?

update:

Thank You for your response. After a half dozen slow start up observations on a pair of these amps exhibiting the 125 ma fuse glow or blow, I thought to replace the first 100 uf filter cap with a 20 ufd cap. The problem of fuse blow at normal start up and fuse glow at slow start up was eliminated.

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  • \$\begingroup\$ Both. I'd be looking for a proper slow-blow fuse from a different manufacturer. \$\endgroup\$ Apr 15 '15 at 22:03
  • \$\begingroup\$ Replacing filter caps with lower capacitance values may speed up the "charging time" thus eliminating the fuse glow, but the original values were chosen for a reason. The ripple from the power supply will increase significantly when jumping from 100uF to 20uF, which could lead to a 50/60 Hz hum or buzz coming through your amp (the filter cap is used to filter out the mains hum). \$\endgroup\$
    – DerStrom8
    May 22 '15 at 16:00
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Oh, tube amplifiers and fuses. One of my favorite topics:

So, your 125mA fuse blows, on the secondary side of the power transformer I guess.

On the secondary side of the transformer, you'll deal with voltages around 400V. A fuse rated at 125mA will blow at around 50 Watt power-usage. So you don't need "just a small tube amp" to exceed this current.

As you've told your tube glows during turn-on and then goes back to normal operation. That's because you'll likely have a bunch of filter-capacitors in your amp that - on turn-on - draw more than 125mA while they charge up. Drawing 125mA and more is quite common even if the tubes of the power-amp are still on standby.

  • Back then in the 70th high voltage electrolytic capacitors had a very high "ESR" or "equivalent series resitance". In other words each capacitor always have a resistor in series. This limits the current running into the capacitor. 125mA for the fuse may have been a good value back then because of the ESR.

  • Nowadays capacitors have a much lower ESR, so during powering up the amp a much higher current will run through the fuse.

During normal operation (aka rocking out) you will on average never draw more current than what happens during turn-on time.

That may be the reason why your fuse lights up. The main culpit is, that the inrush, or turn-on, current is much higher than the average current you'll see while the amp is running.

There are two sane ways to solve this problem:

  • If you want to retrofit an old amp, add some kind of inrush current limiting device between the power-supply and the power-amp section. A beefy NTC resistor will do this okay, but there are more elegant solutions than that.

  • If you're designed the amp or build something based on a circuit from the 70th to 80th, consider ramping up the B+ voltage slowly. Your custumers, fuses and filter-caps will thank you.

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Unless it is a really small amplifier, less than a few watts. I do not think a 125ma fuse would hold up. Check to see if it is the right value. Perhaps if you replaced it before, did you read the voltage instead of amps.

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