Express blows fuse when powering it on

[tob]

My latest Express build behaves a little weird when turning it on:

With bulb limiter (60W bulb) it powers up fine (Power OFF, Standby OFF -> Power ON, Standby OFF -> Power ON, Standby ON).

When plugged into the wall socket directly after this procedure the power up process above works fine, too, and the amp sounds fine, just like my other builds so far.

But when it's being fired up without the bulb limiter exercise beforehand the fuse blows at step one (Power OFF, Standby OFF -> Power ON, Standby OFF). Since the Standby switch is open at that point, something has to go haywire from the socket up to C16 right before the standby switch or in the heater circuit (lifted above ground by 2 100R resistors at the first tube).

Before I'll dig into it...does anybody have any ideas as to where I should look first? Has anyone experienced anything similar?

Thanks for your input!

[sst4270]

Good gut shot pictures would help.
Voltage chart as well.

Is this a brand new build? firing up for the first time?

Or has this amp been working fine and now all of a sudden it's blowing the fuse?

[fishy]

Do you have a slow blow fuse in there?
Is the fuse rated correctly?

Pics

[passfan]

Do you have a slow blow fuse in there?
Is the fuse rated correctly?

Pics

Ditto

[tob]

Good gut shot pictures would help.
Voltage chart as well.

Is this a brand new build? firing up for the first time?

Or has this amp been working fine and now all of a sudden it's blowing the fuse?

It's a brand new build and it has behaved that way from the start.

Unfortunately the amp and I are in different places at the moment but I'll post gut shots asap.

I'm puzzled by the fact that it works the one way and it doesn't the other way...my gut feeling is that it has to do with the first two 40µF capacitors (C15, C16). They could have some residue voltage from the bulb limiter run that's not there when the amp is cold...

[tob]

Do you have a slow blow fuse in there?
Is the fuse rated correctly?

Pics

I use a 1A (230V!) slow blow fuse. Same as with any other amp I've built so far.

Will post pics as soon as I have the opportunity to take some.

The voltages all look coherent. B+ is a bit high at 410V but the voltage drops as expected down to B+5. The heater voltage is at 6.3VAC.

[gearhead]

Am not a cap expert, but sounds like cap(s) before the standby switch trying to charge too fast on startup. The light bulb slows that down, so the fuse doesn't blow. Assume you do the successful startup shortly after? If so, assuming I'm correct, the cap(s) would still be mostly charged and not trying to draw a high chargeup current.

[paulster]

At 240V I found I needed to go up to T1.6A to avoid occasional fuse blows at power-up for all my Wreck builds. I run DC heaters so my heater supply has an inrush current on top of the HT supply.

Any less than that and they'd blow at seemingly random and almost always inconvenient times.

Using the current limiter it will be fine because the bulb limits the inrush current and the standing current draw will be less than an amp once the caps are charged.

[Structo]

I'm not sure which schematic is the most accurate but a couple I have show two 47uF caps before the standby switch while others show them after the standby. (Trudy?)

I think most agree that it is usually best to have some filtering before the standby switch so that the in rush of current is not as bad when the standby switch is thrown.

Another trick you can do is to install a 3 watt 100K resistor across the standby switch, so that while the tubes are heating, there is a trickle of voltage charging the caps, that way when you do throw the standby to play, the in rush is not as bad.

Curious if you have the two 100K series resistors after the first filters acting as bleeder resistors?


Finally the schematics show a minimum 2A fuse while the other shows a 3A fuse.
Depending how close the total current draw is at standby, especially if the caps are after the switch, 2A may not be enough.

[paulster]

Finally the schematics show a minimum 2A fuse while the other shows a 3A fuse.
Depending how close the total current draw is at standby, especially if the caps are after the switch, 2A may not be enough.

Of course these are for 120V though, whereas tob is in Germany on 230V mains, so 1A would equate to the 2A at 120V.

It isn't enough though, as the massive iron of the PT itself has some serious inductance which causes a current inrush, plus the first two filter caps should be before the standby switch as well.

Since I went up to 1.6A I've never had a fuse blow at startup, so I'm happy with that value for running on European voltages.

[tob]

Moving to a 1.6A slow blow fuse solved the problem. Thanks guys!