Marshall JCM800 Heater Fuse

[bobloblaws]

Hello everybody,
I have a JCM800 head here that came in with a blown heater fuse. I'm in Canada but I suspect this was an American import rather than Canadian because it doesn't have the typical CSA mandated fuse array that you typically see with these amps. In fact, the mains are protected with a circuit breaker rather than a glass fuse. In any case, the two heater lines have 5A amp slo-blo fuses and one was blown. After replacing it everything seems fine and dandy, heater voltage is correct, but I'm curious what might have caused that fuse to blow. Something I'm wondering about is the power tubes both appear to have some H-K leakage according to my old EICO 667 tester. The meter shows the leakage to be somewhere in the 5M-10M range. Could this possibly be the cause of the blown fuse? Maybe during the start-up surge?

[catalin gramada]

Well..I will check first with heaters resistance meant nominal considering cold resistance of the heaters is way less than hot resistance. Then determine the initial current. Let call it start up current. Then you.ll know how well you 5A fuses suit you circuit. From my point fusing heaters is a trouble maker. Usually the current drawed in heaters secondary when short determine the primary to break out. I think they made it as European safety normative who asked all the circuits to be fused to be conform by EU standards...or something like that...
Late: I just checked the heaters of 5881 I have at hand : 1.45 ohm cold. Imagine You have couple in parallel who show fraction of ohm load at startup. You 5A fuses will be over steressed each time you turn on for a period they show are heating and its hot resistance becomes 4.2 ohm or so...Usually the 'T' type fuses are able to pass an overcharge for short period, but think a heater element will keep it long time enough to determine to blow...Install more robust, bigger fuses then. The slow-blow may resist, say 10 x nominal current for 100ms but not sure about couple of sec or so... This I will check first.

[Mick Bailey]

I'm also suspecting that you may have a case of fuse fatigue. Slow blow fuses can be had with different internal structures and some stand up to initial surges over a longer term better than others. I've had some off-brand ones that glow dull red on a cold start in heater circuits and eventually fail. I think all of this type of fuse have a thermal characteristic, but how this is implemented and the quality of construction has a bearing on reliability. Some appear to be a straight wire, some have a bead, others a spiral, and some a spiral wrapped around an insulating strip.

[g1]

The CSA versions were updated from 5A to 6.3A due to nuisance blowing from turn-on surge. Not sure if this was an issue with US or UK models, and whether 6550 power tubes came into play at all.
Have you found a matching schematic?

[bobloblaws]

The CSA versions were updated from 5A to 6.3A due to nuisance blowing from turn-on surge. Not sure if this was an issue with US or UK models, and whether 6550 power tubes came into play at all.
Have you found a matching schematic?

Upon closer inspection there actually is a CSA sticker on it. It is a 2205 model but I haven't been able to find a schematic that matches it exactly, but there is a sticker beside the fuse holder that says specifies 5A. In any case, bumping up the fuse values to 6.3A might be the way to go.

[g1]

Upon closer inspection there actually is a CSA sticker on it. It is a 2205 model but I haven't been able to find a schematic that matches it exactly, but there is a sticker beside the fuse holder that says specifies 5A. In any case, bumping up the fuse values to 6.3A might be the way to go.

In the late production they updated the stickers as well. (and instructed repair centers to do so)
Where are the heater fuses, not on a separate board?

[bobloblaws]

In the late production they updated the stickers as well. (and instructed repair centers to do so)
Where are the heater fuses, not on a separate board?

They are in a fuse holder near the can caps.

[g1]

That's interesting as I see 2 picofuses on the board, marked H.F.1 and H.F.2, which I would think designates 'heater fuse'. Are they separating the preamp tube heaters from the power tube heaters? Does the sticker under them specify the rating?

[bobloblaws]

That's interesting as I see 2 picofuses on the board, marked H.F.1 and H.F.2, which I would think designates 'heater fuse'. Are they separating the preamp tube heaters from the power tube heaters? Does the sticker under them specify the rating?

Buddy picked up his amp already so I can't check the sticker, sorry. Anyway, the pre-amp and power tube heaters are on the same 6.3VAC circuit. I'm guessing HF1 and HF2 are for the HT supply, I did not see any other glass fuses other than the two in the photo.