Secondary Fuse Value

[Gaz]

Trying to determine a good fuse value for fusing a 100W amp. I am putting the fuse into one leg of the bridge rectifier (per Valve Wizard and others' recommendations). I've seen both 1A slow blow and 2A Fast fuses used in this spot in commercial amps, but not sure what is the best choice for my amp. I took some measurements with my Fluke meter using the Max function. Here are my readings:

From cold at start up: 800mA
With 1kHz sine wave signal at full volume: 1.3A

What do y'all advise?

[The Dude]

It's hard to answer this without seeing a schematic or at least more info. There are lots of different 100W amps. We don't know how many preamp tubes and a host of other info. Please post a schematic.

[pdf64]

Is your Fluke a 'true rms' type? As the current waveforms won't be very sinusoidal, which is what non true rms meters will be 'calibrated' for.

[Helmholtz]

In case you know the PT's rated secondary current, Merlin states that the fuse is typically rated for twice to 3 times the rated transformer current.

[Gaz]

It's hard to answer this without seeing a schematic or at least more info. There are lots of different 100W amps. We don't know how many preamp tubes and a host of other info. Please post a schematic.

I don't have a full schematic, but basically hot rodded Marshall, I just want to move the fuse before the bridge rectifier.

[Gaz]

It is true rms, so readings should be close enough for rock?

[Gaz]

I do, actually it's 550mA. I guess that's why I was kind of stuck between 1A slow blow or 2A fast. I think I could be okay with either one, but not sure...

[g1]

You could try with 1A slow, if you get any nuisance blowing move up to 1.5A slow.

[trobbins]

Assuming its a capacitor input main B+ filter then a slow-blow fuse type is appropriate.

It's preferable to identify what fuse you can purchase, as they typically come in two models - one made for US market (UL-284-14) and one for most other markets (IEC60127-2), and those differ by the max rated current they can handle continuously.

For example if you can only get US model fuses then they would have to be at least 1.3/0.75 = 1.7A rated fuse, whereas an IEC model has to be at least a 1.3A model. Given you have taken a 1.3A reading on the secondary HT winding, then you don't need to size up too much - eg. a 2A fuse from US range or a 1.5A from the IEC range should be ok. If you oversize too much, then the fuse has less chance of protecting the secondary winding.

There is a bit of measurement inaccuracy with a general true-rms meter as the current waveform being measured likely has a very high crest factor that is above the meter spec level (which is typically 3x).

A standard 250VAC fuse is appropriate for the PT secondary even if the winding voltage is substantially more than 250VAC, as protecting that side of the transformer is much more benign that protecting a mains AC supply.

[g1]

Given you have taken a 1.3A reading on the secondary HT winding,

He did not state specifically that the reading was at the HT winding. The HT winding is rated for 550mA.
Not sure exactly what he measured.

[trobbins]

Yeh I had my doubts too which was why I led with the presumption of 1.3A measured on the secondary winding, given the topic is about fusing the secondary. Hopefully the detail will come out :-)

[Gaz]

Yeh I had my doubts too which was why I led with the presumption of 1.3A measured on the secondary winding, given the topic is about fusing the secondary. Hopefully the detail will come out :-)

Yes, I broke the connection to one leg of the bridge rectifier to measure the current on the HV winding. It is rated at 550mA, so my meter must giving an inaccurate reading? It seemed high to me, but I trusted it, not understanding the crest factor was higher than the meter could handle.

I've seen schematics of Peavey using a 2A fast blow in this spot or 1.5A slow on older models. My guess is that the fast blows, well, faster, and is an easier value to find, especially on the road.

Thanks for the help, and thank god for amps during self-isolation. Hope everyone is well out there.

[Helmholtz]

Rated transformer secondary current is the max averaged RMS current the winding should be able to supply 100% of time without overheating.
Amplifier current demand at full power may be higher than rated PT current, as full power won't be delivered all the time.

Crest factor is a meter limit for RMS accuracy. It means the ratio of peak value to RMS value. For a sinewave the crest factor is 1.414 (square root of 2). The crest factor of the transformer current feeding a rectifier with capacitor load is typically much higher as the current consists of short high peaks.

I think your current readings are too high. A 100W amplifier typically draws not more than 500mA DC current at full power. The corresponding transformer RMS current should be less than twice that value or less than 1Arms.

The primary purpose of the secondary fuse is to protect the PT winding from overheating. From the reasoning above it makes sense to double rated transformer current for fuse rating.

A slow blow fuse generally makes sense as it won't be triggered by short turn-on surges and because winding heat-up takes time. With a fast blow fuse a higher current rating will be necessary. Without monitoring of inrush surge (which depend on specific PT and amp parameters) with a DSO that provides RMS conversion, it's hardly possible to predict the optimum rating for a fast fuse.

[Gaz]

Thanks for thanks, explanation, Helmholtz. My readings were confusing me, and now I understand why. It seems like a 1A slow blow would be reasonable on the AC side of the rectifier.

Would it be more accurate to measure voltage across a 1 Ohm current sense resistor?

[Helmholtz]

Would it be more accurate to measure voltage across a 1 Ohm current sense resistor?

Only if your meter allows for a higher crest factor with voltage measurements than with current. Check your meter manual. If I had to guess I would think that a crest factor of 5 should be sufficient. Maybe someone knows more exactly?