Bogner/Line6 heater fuse popped

[DonMoose]

Gentlemen -

A Bogner/Line6 head landed on my bench this evening with a report that 'it just sort of quit' in the middle of a jam. No smoke or smell, no drama, just went silent. The digital stuff and footswitch all work fine, but no sound.

HT fuse seems fine (ohm meter sez 0.2), but the heater went open.

What would kill the 10A heater fuse but leave the HT intact?

Oh - and anyone got a schematic?

[JazzGuitarGimp]

It's possible the blown fuse was a nuisance failure. I'd try replacing the fuse (with the correct value) and then bring the amp up slowly on a variac.

[DonMoose]

Sounds like a good bet - just have to find a 20mm 10A glass fuse. (looks like a slow-blow).

Thx!

[JazzGuitarGimp]

Hi Don,

I meant to ask this in my last post: what is the tube compliment in this amp? 10A is a lot of current. I assume it has a quad of large bottles?

[alvarezh]

HT (B+) and filament windings are separate, you could blow the filament fuse and still have HT voltage.

I am not familiar with the amp but if it helps: I have had blown fuses on amps where the tube sockets are PC mounted. Replacing tubes causes hair cracks on the soldered legs of the tube socket (I inspect them with a magnifying glass) which cause high current consumption. As a matter of fact, every amp I get with PC mounted tube sockets I re-solder the legs by default even if they look good.

Slow blow fuse for sure on the heaters.

All the best.

[Leo_Gnardo]

Some amps have a fuse on EVERYTHING, even the bias supply. Why? I guess to satisfy the safety agencies in each & every country around the world and maybe beyond. Some companies, Peavey first, now Bogner too & certainly others, comply by fusing every dam' thing so they don't have to make slightly different models (differing only in fusing) that will be accepted in various countries.

What I do in the case of an open filament fuse, is clip in the appropriate fuse, make sure the amp is running OK for a couple hours, then replace that fuse with a short jumper of 16 ga. solid wire. How many amps have run for how many years with NO filament fuse? Just about all of 'em.

Consider - at switch-on, cold filaments have an extremely low resistance, and draw up to 6 times their rating but only for a half second or so. (I've measured it.) Usually if a fuse is going to blow, that's when it goes. You've seen the same thing happen in light bulbs, that big flash when switched on, as the filament "fuses itself." I'll admit it's a bit unusual for a filament fuse to pop mid-session.

[DonMoose]

Mr Gimp (and any curious lurkers) - 2x 12AX7 and 4 big bottles (100W) - switches and settings for American (6L6/5881) or British (EL34).

I assume it's a pretty nice amp. Had no idea the thing was basically a tube amp with permanent FX in always-on loop. The other Line6 things I've fiddled with have been disappointing.

alvarezh - a couple of the socket areas look like there were issues cleaning flux - will retouch, along with a resistor with only half-fill in one of its thru-holes. Thanks for the tip!

[Structo]

Couldn't a heater/ cathode short cause the heater fuse to blow?

Can you test the power tubes?

[DonMoose]

Couldn't a heater/ cathode short cause the heater fuse to blow?

Into a 1-ohm Rk, an F-K short would add 4.5A to the peak output of the heater, so yeah, it would. But shouldn't that also take out the HT fuse? The heater winding looks like 4.454Vpk/10A = 315mOhm and brings the cathode 4.454V (nearly 10% on a -50V bias) closer to the grid. I'd expect to (briefly) redplate at least one of the outputs, unless biased pretty cold.

Can you test the power tubes?

I have a tube tester, but we know how effective those are

[JazzGuitarGimp]

An easier, and probably the most certain way to see if it's related to the tubes is to pull them and see if the fuse blows. If it does, you know it probably isn't a tube. If it doesn't, then you likely have a shorted tube. Does the amp have a standby switch? If so, does it power up okay in the standby position?

[DonMoose]

Once the stupid fuses arrive (20mm 10A SB - not found local/convenient), I'll be able to get on it.

With fuse out, DCohms says 100 to GND on both sides of heater and 200 side to side. The heater secondary resistance looked reasonable- don't recall number, but will double-check to gnd.

[JerryFJA]

Is it the Alchemist? Check the large cap in the DC heater circuit. Should be C7
Jerry

[DonMoose]

It's a Spider Valve HD100 - no evidence of DC heaters.