What rating fuse to use?

[paggerman]

Hi, I have recently built yet another amp from bits saved from the junk yard. In this case it was an old 16mm projector. I have used a couple of Gibson designs (GA-20 and GA-30) to create my design (schematic available soon if anyone wants). The valve complement is 5V4, 6SJ7, 6SN7, 6V6, 6V6. I have retained the original 5V4, 6SJ7 and 6V6s but have replaced the 6SN7 as it was a bit weak in my tester. When I fire it up, it works. A bit of hum and buzz (which can be worked on) and a nice, warm sound. However, after a few minutes the fuse blows. I have now blown 3 of these and each time I open the guts to find if anything is damaged or obviously shorted. After the first blown fuse, I found a poor solder connection on the grid of one of the triodes. the second time I found that a tone cap leg was perilously close to the unused terminal of the tone pot (but I don't think it ever touched). As it works for a while, I am now thinking that I don't have the right rating of fuse and that it is living right on the edge until it can hold on no more. I am using 0.5A 250V - the actual mains voltage is 110V from a step down. From other threads I understand that voltage doesn't make a difference and have seen Fender Bandmaster and Bassman schematics have 3A fuses. Should I be using a 250v 3A fuse? (250V fuses being much easier to obatin here). Many thanks.

[Enzo]

The voltage rating of a fuse means the highest voltage it can safely break. Most fuses are either 250v or higher current fuses at 32v. I have seen 125v fuses, but they are rare, and I don't know where I could buy them. They tend to come in imported amps from China.

A half amp fuse is extremely small for such an amp. I don't know that you need a 3A fuse, but at least a 1A or 2A.

[tubeswell]

Calculating Mains fuse size



multiply volts with amperes (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
PT:
300v x 200ma = 60va
6.3v x 4a = 25.2va
50v x 80ma = 4va
89.2 va altogether

89.2va / 110v = 0.81a

Match to 1A and multiply by two = 2A mains fuse.

[Jazz P Bass]

You could also hook up an ammeter to one of the mains legs and actually "measure" the current that the amp is drawing.
On a "typical" tube amp, I would think 1/2 amp is way too small.
It is a good size for trouble shooting purposes though

[g1]

Just thought I'd check to make sure we are talking slow blow fuses?

[Rhodesplyr]

You could also hook up an ammeter to one of the mains legs and actually "measure" the current that the amp is drawing.
On a "typical" tube amp, I would think 1/2 amp is way too small.

The main thing an amplifier's fuse needs to survive on a regular basis is the turn-on surge.

So, if you switch on your amp, and the fuse blows, it's rated too low (assuming your PT isn't shorted). You can then experiment with larger values until you have one that survives normal use. There's no real need I know of to go higher.

A 5A fuse is specified for my Leslie 32H amp, but when I had finished rebuilding it, I only had 3A fuses handy, so I popped one in, hoping it would get me through a test run. It worked and continued to work, and I've been using 3A fuses in these amps ever since.

I'll leave the slow-blow vs. fast-blow issue to others.

[paggerman]

Thanks very much for everybody's informed advice. I'm glad you have all supported my thinking - it means that some of what you guys have taught me in the last year or so has sunk in!

Just an update - I got some fuses this morning and have been having a play this afternoon. no outages, no problems. Not even any buzzing. I was very conservative with the anode resistor on the 6SJ7 to avoid microphonics. That has backfired a little as the sound is very clean. I still can't play guitar, though.