HT fuse blow

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HT fuse blow

Post by calling card »

A Hiwatt HGS100H, these being the later breed pcb based relative of the DR103, my son's monster. It blew an HT fuse, standard spec is 1 amp fast blow. Put a new one in, runs for a time then pops, metal splatter inside glass, now emitting hum when another fuse goes in.

My initial thoughts are visual check of power tubes and see if one looks different, wiggle & tap check for anything untoward. Pull outside pair of valves, check, pull inside valves, check.
Next up, get chassis out and inspect for things that look to have been hot, rectifier, resistors etc.

Apart from admitting defeat and sending it off for repair/assessment, any ideas on what most probably culprit is?

Oh I should add it was running set at 8ohm into a Rockit quad with oldish 10" Fanes, cab is rated 8ohm but reads about 12ohm with meter. (first thing my son checked on failure).
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Re: HT fuse blow

Post by jeremyb »

Does it still blow with the tubes out?
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Re: HT fuse blow

Post by calling card »

Not pulled anything out yet.
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Re: HT fuse blow

Post by jeremyb »

calling card wrote: Wed Jun 29, 2022 11:56 am Not pulled anything out yet.
If it blows with the tubes out then its probably a shorted filter cap, rectifier or power tranny, hopefully its just the power tubes :)
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Re: HT fuse blow

Post by RectifiedAmps »

Sounds like a bad power tube, hence the hum. But like jb says, check it without power tubes installed. An impedance mismatch won’t cause a fuse to blow, but you’re probably measuring 12ohms DC resistance on an 8ohm impedance cab, so it’s probably not mismatched at all.

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Re: HT fuse blow

Post by calling card »

Thanks guys for the strategy suggestions, yeah best you can hope for is a tube a bit off. I did say to son that meter test on cab would vary a bit from labeled impedance, couldn't remember if it's a higher or lower though.
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Re: HT fuse blow

Post by RectifiedAmps »

If you were measuring a single speaker the resistance will usually be lower than the impedance, but for a quad the wire runs can be long enough that they might add a few ohms. Most meters aren’t very accurate when measuring low ohmage either.

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Re: HT fuse blow

Post by calling card »

Check over last night, powered up & watched power tubes first. One of the centre pair making pretty blue lights in the centre area of bottle and 180 degrees out the other side, it got really hot compared to the others. Pulled that pair and in a little while HT fuse blew. Pulled all valves, switched on with standby in play mode, fuse held.
Marked the blue glow tube & set aside, plugged two others into centre pair sockets and the same socket as before made blue lights and elevated heat with the different tube.
Around about this time a red indicator light appeared on one of two fuse blocks labeled as 'output valves' one is V1 & V3 the other V2 & V4. The V1 & V3 failed. Has 500mA fuse.
Last night I glanced at it thinking V1 - V3 normally denotes preamp but looking again this morning and see Output Valve labeled above the fuses.
It only has the HT marked fuse as far as any other fuses go. Usually I'd expect to see a fuse on primary power side too. Although if you look at the back panel this fuse is very close to jug cord socket.
The odd tube out is 2nd in from the left.
Going to take it in for professional assessment.

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Re: HT fuse blow

Post by sizzlingbadger »

Blue glow is not normally a sign of an issue. If the the issue is related to the socket rather than any particular power tube then I expect there is component that has been damaged. Probably best to get it to a Tech and have it checked it out. There is a mains AC fuse in that IEC socket.
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Re: HT fuse blow

Post by calling card »

The morning reading matter has been 'blue efflorescence' and how not usually bad, even good. This thing reminded me of a tig welding torch the way it looked.
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Re: HT fuse blow

Post by calling card »

Can't let go... these things are quite the orphan child of Hiwatt, lack of info except gearpage like insults to MIC.

I found a schematic for the 50w version. seems those LED light fuses marked as Output valve go to pins 1-8 cathode side which find ground through the fuses thus lighting up LED when fuse breaks.

Still going to tech, so curious.
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Re: HT fuse blow

Post by calling card »

So it had a visit to Mr McPherson, great turnaround at one week!
I asked my son what the fault was, he said a resistor was cooked, which one I asked - screen resistor? Yes he replied.
It now has the quad of Shuagang I got last year, biased to the 'hotest tube' - matching spec not quite matching on test I believe. Ran for 45 minutes... and sent.
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