Fulton-Webb Imperial reverb blowing fuses.

[jvm]

I have a Fulton-Webb Imperial Reverb that blows B+ and mains fuses randomly. Customer states that amp has to be on for 3-4 hours before this will happen. Tubes have been subbed with known good tubes with the same results. 45 watt, 2-6L6 amp. Nothing really strange in the design. Any ideas where to start?

[Jazz P Bass]

What is the fuse rating?
Is it slo-blo?

[jvm]

Mains fuse is 3A, B+ fuse is 1A. Both slo-blo. No marking on chassis so I don't know if these are original fuse ratings or not.

[R.G.]

Anything that takes that long is very likely to be a thermal problem. It may be a thermal that opens/closes an intermittent contact, or something heating up and failing. For a 4-hour time constant, the power tranny and OT are suspect, just because that's the approximate range of their thermal time constant, not because they are prone to be bad. Another possibility is heat in general is overheating a filter cap or the cap that filters the bias voltage (if it has one, don't know the schemo).

Get the light bulb limiter on it to catch it when it fails. Also possibly go after it with a hair dryer or heat gun (CAREFUL with that heat gun!).

[jvm]

Do you think he may just need to run a cooling fan on the amp? 4 hours is pretty much maximum "on" time for my amps. I notice there are four filter caps crammed in very close to the PT. In fact, two are touching.

[Gtr_tech]

Sounds like old filter caps getting hot from leakage. I've seen it happen with bias adjusted too far to the hot side too.

[jvm]

The amp is about seven years old as far as I can tell. I'm thinking a cap job may be in order. Smaller caps might make for better air flow. Things are pretty tight in this chassis. Two of the filter caps are gooped to the PT itself.

[R.G.]

I notice there are four filter caps crammed in very close to the PT. In fact, two are touching.

That is very bad juju indeed. Filter caps need to be cool. Touching the PT is a problem in general. PT heating the filter caps would account for the time lag. When electros get too hot, they get leaky, current skyrockets, internal power dissipation shoots up, and they go into thermal runaway.

You might fire it up and use your harbor freight $30 infrared remote scanning thermometer (!!! Great invention!!!) to sense temps on those things over time. If they get over 70C on the surface, they're probably dying the thermal death.

There is no certain information here, just speculation.

[Gtr_tech]

Oohh....I didn't know it was that new. Caps could be a problem, but not real likely unless they're super cheap-o's. Check over the bias circuit and make sure one/both PI coupling caps are not leaking.