Blowing Fuse - What's the Problem?

[Lenny1716]

While playing my 1967 Fender Bandmaster (AB763), the 2 amp slo-blo fuse blew. I removed the two power tubes (6L6's), to eliminate them as a possible cause, and put in another 2 amp slo-blo, which also blew immediately upon turning on the power. Is there perhaps something specific to zero in on to determine the cause?

[xtian]

Pull all the tubes. Still blows fuses? (Actually, a lightbulb limiter is your friend, so you don't burn thru a bunch of fuses.)

Possible causes:
bad electrolytic caps
bad diodes in rectifier
short in OT and/or PT
short in death cap
plain old short somewhere

[tubeswell]

What xtian said.

Fuses blow because there is too much current. So you have to find where the short is

[Stevem]

I have never seen the diodes give up in these amps unless it was hit by Lighting, or run off a generator will very poor regulation !

In all likely hood you have a bad filter, in fact on a amp that I recapped for a I guy only 1.5 years ago I just had a cap go bad and the amp had the same issues as yours, and this cap when I pulled it out of the amp checked good on both my ESR meter and my cap meter, it was only the sign of it's expanded positive end that gave it away!

[Leo_Gnardo]

Lets not overlook the often overlooked bias filter cap. Yeh that one over in the corner. Way too often I've found it to be the culprit in a case like this. When installing fresh filters, include the bias cap(s) without fail, and your amp won't fail you.

[Structo]

Just a heads up for newer members.

The safety fuse on the primary side of the power transformer is mainly to prevent fires due to a short circuit on the primary side.

We use slo blo fuses for this since the current frequently is higher when the amp is first turned on due to the power supply demand due to the first filter capacitors charging. As they charge, the current demand slows.

So a Slo Blo fuse handles this surge without blowing.

This is why many builders prefer to have the standby switch after the first large reservoir capacitor.
That way the first cap charges before loading the rest of the capacitors.

To truly protect the amp you should install an HT fuse that will stop the B+ current when there is a fault on the B+ side of the transformer.

[Davidg]

I had 2 bad diodes in my 66 Bassman so it is rare but still a possibility. Not to discount anything Stevem said but the bottom line is u must look at all possibilities and u have 0 chance of finding anything without a current limiter. Or u canjust start throwing parts on it til something sticks, that is what most "techs" seem to do. Caps are due if they haven't been changed and would be a good place to start. Or check for continuity to ground where there shouldn'be any.