Determine Mains Fuse Required

[Littlewyan]

This may seem really easy to some, but how can you work out what Mains Fuse is required for your Amp? Obviously its easy to work out the HT Fuse but the Mains Fuse stumps me.

[xtian]

It has been said more than once that this is not an exact science. I rely on other proven designs to suggest ratings.

[Littlewyan]

I've gone by Marshall's on this occasion which is 250mA (1Watt Amp). I guess Amp Manufacturers will build a prototype first and determine the size fuse from measuring the current consumption. I needed to know simply because I'm having faceplates made up for my amp and wanted to specify the fuse size on the rear faceplate.

[stiltamp]

Our transformer maker recommend a time lag fuse which has the double
current of the nominal primary winding value.
I follow the recommendation quite often, it works fine.

[tubeswell]

Its not hard - multiply volts with amperes draw (for each winding) and you get VA. Sum up the VA values and you get the whole VA of the secondaries. Divide that by the mains voltage and you get the primary amperage. Take the nearest value and multiply by two.
For instance for a fender bassman 5F6A PT (2 x 5881 and 3 x 12AX7 and a GZ34):

HT = 300v x 150ma = 45va (note the spec'd rating of the HT winding in 5F6A PTs is typically 200mA, which makes the power supply a bit stiffer)
Heater = 6.3v x 3a = 18.9va (note the spec'd rating of the heater winding in 5F6A PTs is typically 4A)
Rectifier = 5v x 1.9a = 9.5va
Bias = 50v x 1ma = 0.05va

Total secondaries = 73.4 va altogether (98.7va if you go by the 5F6A stock secondary winding current ratings)

73.4va / 125v = 0.59a

Match to 1A and multiply by two = 2A mains fuse. (Note that the Fender schematic shows 3A, but that is overkill).

[Littlewyan]

Thanks for that. So basically its best to go with the circuit measurement and not the PT limit, as the PT could be way over spec and so the circuit would fry itself before the fuse blew. Is that right?

[tubeswell]

works for me.

[davent]

Could a Kilowatt meter be used to measure the current draw of the working amp then the fuse value be extrapolated from that.

http://www.degreedays.net/kill-a-watt-meter

[martin manning]

Sure, that'd be better than a crude calculation, but either way you have to make a wild guess at the peak inrush current and its duration to choose a fuse rating that will survive it. Then, finding the lowest rating that won't suffer nuisance failures will probably require some experimentation.

[Merlinb]

Thanks for that. So basically its best to go with the circuit measurement and not the PT limit, as the PT could be way over spec and so the circuit would fry itself before the fuse blew. Is that right?

The fuse is mainly there to prevent fire and to some extent to protect the power transformer, but not the circuit itself. If a fault is bad enough to blow the main fuse the circuit is probably fried anyway (or rather, it is the frying that blows the fuse!). So using the PT limit is acceptable, although if you can get away with a smaller fuse, all the better.

[Littlewyan]

Well this time I got lucky as Marshall have already done the testing for me. I don't however have a B+ Fuse due to space, I'm going to try and install one but if not is it really a big issue? Trainwrecks go without them just fine. As I'm using a Pre Amp valve for my output stage will that short out like power valves do or will it just go microphonic or die like a normal pre amp valve? As the biggest issue I think you get in amps is a power valve shorting out internally.

[tubeswell]

3 possible ways a triode could short:

1) heater to cathode short (more likely if h-k voltage rating exceeded)

2) grid to cathode short (would imply loss of bias)

3) grid to plate short (implies a melting and deformed grid and most likely extreme red plating and thermal runaway)

[Littlewyan]

So basically as long as the amp is built correctly it should be fine. I'll still try and include a B+ fuse, might be able to use a chassis mounted fuse folder. Can't have too much protection.

[Merlinb]

the biggest issue I think you get in amps is a power valve shorting out internally.

Biggest issue is the rectifier shorting, which will do its best to destroy the power transformer and reservoir capacitor.

[Littlewyan]

And the only protection from that is a mains fuse and maybe a quick reaction from the user switching off the amp?