Why 3/4A Fuse In 5F10 Harvard not 2A Like Every Fender 2X6V6

[rp]

Any reason not to just use a 2A like every other 2X 6V6 Fender, and the 1X 6V6 Harvard? What exactly determines the mains fuse anyway, I always figured it was the rating on the 5V for the rectifier?

[David Root]

2A is for up to 50W output w/ 2x6L6GC. Deluxe Reverb has lots of little tubes, Harvard has only a few tubes 2x6V6, 1A is plenty.

[rp]

2A is for up to 50W output w/ 2x6L6GC. Deluxe Reverb has lots of little tubes, Harvard has only a few tubes 2x6V6, 1A is plenty.

Agreed, but Fender spec'd 2A for all the other 2 6V6 and even 1 6V6 models - tweed, brown or black: 5F1, 5C3-5E3, 6G3, 5B2-6G2 (Princeton) all 2A but the BF-NR Princeton is 1A (w/5AR4). The 1 6V6 Harvard is also 3/4A. Still confused.

Usually I can suss out the logic but they seem to be all over the place on this. I never understood how this was rating was properly derived and never really bothered, just monkey see monkey do, until I ran into this model and got curious.

[David Root]

Not much logic in all of that I think, unless it has something to do with a PT meltdown shorting to the chassis, that might make sense.

[tubeswell]

Mains fuse size is calculated based on the volts-amps of the secondary windings thus:

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.

Now you can work out how they did it for the 5G10. FWIW I make it about .42a primary current draw, which roughly doubled is .75A (in terms of picking the nearest fuse size)

[krash]

FWIW while we are talking fuses, a slo-blo ("time delay") fuse will blow if you draw 2x the rated current for about an hour. It is rated to conduct the rated current indefinitely.

Regardless of what the labeling on the old amps said, I think you may be asking for trouble with a fuse bigger than 2A unless it's a fast-acting fuse and you truly expect to draw 2A continuously. I would put in a fuse rated at the amount of current you expect to draw continuously, plus about 25%. So in an amp that you expect to draw 0.81A, I would put in a 1A fuse MAX, and a fast-acting fuse if you don't have nuisance trips.

That said, I put 2A fuses in all of the amps I build.

[Phil_S]

It's my unscientific impression that fast blow fuses won't handle the in-rush of current on start up. Though it is momentary, it will be problematic. For this reason, a fast blow (usually 500mA) is used from B+ CT to ground, but a slow blow is used on the mains.

[Structo]

I agree with this.
The filter caps act as almost a dead short briefly on power up.

That is why tube devices use the slo blo fuses.

I can see where it might be advantageous to size the fuse for the amp.
I for one don't want a faulty amp running for two hours before the fuse lets go.

[rp]

Tubeswell, terrific. I never really knew what VA really was. This info will go into my files - now I can calculate my own fuses

Still puzzled on Fender's thinking. They are all over the place on this eg: octal Champ 2A, BF Showman 2.5A - WTF? So, yeh, it now dawns on me that the SB mains fuses are really just there to prevent a PT meltdown and a house fire. It's not really gonna save your amp most times. However, I also have had the mains fuses blow right away on f'd-up amps, so...???

I always add a fb HT fuse to my builds but it only works with a tube rec., not a chance with ss rec. I also do my stby BF style with the first cap on the far side of the switch (remember to have the stby on when draining caps!) which seems to help here. Nonetheless the ht fuse can be a nuisance, not sure what I would do if I sold amps. For me if they blow too often I just stick a sb in there - which is just a way of bypassing it.

So, should I be lowering my mains fuses in all my amps? Like, what am I doing with a freakin' 2A fuse in my Champ??? Should we all be doing this?

I've only seen one charbroiled amp in my life. Any techs who've seen alot have an opinion?

[tubeswell]

TLike, what am I doing with a freakin' 2A fuse in my Champ???

If you want more appropriate protection, then the answer has got to be 'affirmative'. I think a lot of the 2A tradition is just that. Here in kiwiland (and in other 'euro' voltage countries) the fuses for the same types of amps should actually about 1/2 what they are in the US (because the mains voltage is 2 x as much, and it all comes back to VA).