Orange Dual Terror blows HT fuses

[xtian]

Customer reports amp blows HT fuses. Replaced power tubes, no change. I open it to find 220uF reservoir cap obviously bulging. Replaced e-caps.

Unloaded (tubes out) voltages look OK. Cathode biased. With all tubes in, survives the bulb limiter test. But without bulb limiter, blows HT fuse.

Next step?

[JazzGuitarGimp]

Uhm.... Rip out that pcb and PTP that sucker?

[Phil_S]

What you already know, something is drawing too much current. Divide and conquer is the time tested technique.

First, though...does the fuse blow immediately when you flip the switch or after it heats up (any reasonable delay)? If at the time of switching, I'd look at the power supply and rectifier.

After that, divide the amp into sections, add one tube at a time in the amp starting at V1 until you find that adding a particular tube blows the fuse. Then you know which section is the culprit. (Or go backwards by pulling the power tubes first...)

Yucky PCB. Maybe an unintended bridge has developed? Maybe there was arcing at a tube socket or on the board that is not easily seen but is clearly conductive, but not with the bulb limiter in place.

Just some ideas....good luck.

[Leo_Gnardo]

Charging a big hi voltage cap can open an HT fuse. Some manufacturers call for slo-blo HT fuses. I'd expact that to be the case if the fuse is between rectifier and HV filters. Can usually get away with a normal fuse if the switch is between filter caps and output transformer center tap.

FWIW I was having fuse-a-poppin' problems in a big home built tube bass amp. Left the fuse in line with the OT CT , BUT put the standby switch on the screen grid supply as in recent SVT-CL and similar. Worked a charm.

[xtian]

Going to have to award this one to Leo. The fuses that the client gave me were Fast blo. I bought some Slow blo fuses, and all is well.

BTW, Orange does spec the fuse as Type T (slow-acting) on the rear panel. This spec might not be obvious to a casual user.

[vibratoking]

Good call Leo. BTW, I've experienced the slo-blo problem quite a few times. Fairly common occurrence for the wrong fuse type to be inserted. Whenever I open the fuse holder I am looking for the wrong one.

No offense meant to anyone, but interesting that the PCB became an instant suspect. Good thing you didn't jerk your knee and replace the whole damn thing...just kidding.

[Phil_S]

Occam's razor. http://en.wikipedia.org/wiki/Occam%27s_razor
I should know better

[Leo_Gnardo]

BTW, Orange does spec the fuse as Type T (slow-acting) on the rear panel.

Thanx guys! I've learnt to look for the T now. For Time delay, I guess. Sometimes it's printed on PC board if amp has that kind of fuse holder. I gotta squint real hard under good light to read it if it's on the fuse. With a magnifying glass. Microscope will be needed next.

I stock a good collection of regular 1/4 x 1 & 1/4 fuses but it's getting popular now with the mini 5 x 20 mm. Grr, more dang inventory. GMC series along with the GMA. Sometimes they look much alike. Sometimes there's a little doo-hickey in the middle of the fuse wire. And some are packed with sand so ya can't see what's in there. And some values cost a lot. Go figure.

[xtian]

1.25x reading glasses are my new friends.

"What a drag it is getting--" yeah, whatevs.

[Leo_Gnardo]

"What a drag it is getting--" yeah, whatevs.

Beats the alternative.

ShowPower used to have shirts printed:

Every day above ground is a good day.