Why is the fuse blew out?

[xtian]

Odd this may just be my misunderstanding then, but from what I recall someone saying was that they used a 200W bulb to ensure none of the amps they worked on would be impacted. I.e. the wattage of the bulb should be at or over the wattage of the max power of the amp, no? Did I get my wires crossed?

There's already a fuse in the amp, right? We want our light bulb to show fault conditions that occur BEFORE it gets so bad it blows the fuse. A 40watt bulb will show fault conditions with a nice bright glow, without letting so much current through that expensive components get fried while you're going "ooh! pretty lights!"

[pompeiisneaks]

Odd this may just be my misunderstanding then, but from what I recall someone saying was that they used a 200W bulb to ensure none of the amps they worked on would be impacted. I.e. the wattage of the bulb should be at or over the wattage of the max power of the amp, no? Did I get my wires crossed?

There's already a fuse in the amp, right? We want our light bulb to show fault conditions that occur BEFORE it gets so bad it blows the fuse. A 40watt bulb will show fault conditions with a nice bright glow, without letting so much current through that expensive components get fried while you're going "ooh! pretty lights!"

Maybe I misunderstood the purpose of the light bulb limiter, I thought it's design guaranteed no more than a minimum current can go through the amp, protecting it. I.e. it's job is to limit current, not to show you a bright light. The light is a secondary benfit of knowing how much current the bulb is 'eating/limiting' no? OTOH if you have a 40W bulb in a 100W amp you 'can' damage the amp before getting it off even if the bulb runs really bright. Unless you've got lightning fast reflexes?

Again, maybe I've misunderstood something?

~Phil

[Tony Bones]

Maybe this is obvious, but low wattage bulbs limit current more than high wattage bulbs. A 200W bulb might not protect a small amp that has problems.

I use them jut for first power up. If the bulb burns brightly, then I shut everything down and look for the problem. A low wattage bulb (40W) can be used under these conditions to take voltage measurements. Obviously, all voltage measurements will be low with the bulb, but if something looks way off then you know where to start looking closely.

If my initial power up with, say 100W bulb is OK - the bulb glows, but not too brightly - then I remove the bulb limiter and bring the amp up without for further testing.

[martin manning]

A 40W bulb will limit the current into the primary to its full-brightness current even if there is a direct short to ground somewhere (333 mA if the line voltage is 120V). That’d translate to roughly 100 mA HT current, or 6 amps filament current regardless of your local line voltage. The brightness of the bulb is an indicator of the current flowing through it, so it’s a useful indicator. You can observe variation in brightness with time and confirm expected current transients like filter caps charging and filaments warming up, and coming off standby. For a tube amp, using a bulb that is no more than the output power of the amp is not a bad rule of thumb. I like a 60W bulb with a clear envelope, and use that for 50-100W amps.

[xtian]

Tony and Martin explained nicely. I use my bulb limiter only for new builds with no tubes installed, to make sure there are no catastrophic shorts. After I'm satisfied, I start installing tubes and testing operating voltages, but without the bulb limiter.

[pompeiisneaks]

Ok I think that makes sense, i.e. it limits it to 40W total, so you want something at or below the max expected current to actually 'limit' it...

I lerned somtin

~Phil

[psychepool]

Since I haven't yet obtained the parts for the light bulb limiter, so I have taken some steps to do.

I bought UF4007s from the store I used to buy.
And instead of using three pairs, applied two.

When I put the fuse in and connect the plug to the power socket, the fuse still blown.
It also blown even when the power switch is turned off.
It doesn't blown when all wiring connected to the power jack has been removed except the ground.
With the power switch turned off, only one of the two strands of the PT primary is connected to a 220V power supply, is this possible?

I tried putting the same fuse into another well-operated amplifier. Given that everything works fine, it doesn't seem to be a bad fuse.
(It's the same spec that's been in the same package as the fuses that's been blown up so far. I think it was purchased from the same place at the same time.)

The most mysterious part is that the fuse blows without powering it up.
It is hard to guess the cause even though the variables are in extremly controled.
Is there anything I can check out in the current state?

In addition, I'm going to test the main power supply of the PT secondary connected to the rectification diode.
I'll let you know the result after I try.
I want you to let me know what may be the cause, and if there are any further steps you can take, please let me know.

Is it dangerous to try a fuse with a larger currentpect (about 5A)?

[Stevem]

If the fuse is blowing as soon as you plug the amp in, yet the room circuit breaker does not then you must have the amps on / switch wired wrong and that switch is closed now ( shorted ) due to excessive loading from blowing all the fuses that you have.