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Redfox777

Installed new tubes and blew my HT fuse, what gives?!!?

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I just installed 4 brand new jj el84s in my laney vc30 and turned it on (let it warm for a few mins first) and as soon as I started playing the sound abruptly stopped. I looked at the back of the amp and the tubes were still glowing fine so I turned the amp off and unplugged the power. Then I checked the HT fuse and, what do you know, its blown to {censored}. What does this mean, does the amp need to be rebiased? The old tubes were slovaks like the jjs so i figured the bias would match up. Any help would be great.

 

side note: I checked the manual and it says the tubes need to match up in grade. What is the grade? The only numbers on the jj tubes are el84.

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if it's misbiased the plates may glow red. the black plates aren't supposed to glow... just the little filaments in side.

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It would be a stretch (though possible) for the bias to be that far off. The HT fuse can handle the current from all of the power tubes running at full power. It would have to be way off for the HT fuse to blow...especially that quickly. Watch the tubes while you turn the standby and power on at the same time. As the tubes heat up watch for an unusual blue glow from one or more of the tubes. If any of the large metal plates start glowing red shut it off. Try another set and see what happens. Make sure you do NOT use a larger value fuse for the HT.

 

Good luck.

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The fuses the amp needs are 250v T 315mA L all radioshack had was was 250v 315mA. The radio shack fuse's filiment is really thin. Is it ok to use these fuses or should I try somewhere else?

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the "T" means slow-blow, which means that momentary surges above 315mA will not blow the fuse. the fuse with no "T" will be safe to use but should be replaced soon with the proper type because it could easily blow.

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It destroyed the HT fuse agian. When I turned the amp on the first tube flashed really bright and then went to normal. They all looked fairly normal after a second and there was no red or blue. Could the mains have gone out too?

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remove the first tube and try again. that's what i'd do. you can remove two tubes and run it at half power, or just remove the one and not play very loud until you've had a chance to replace it.

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do you still have the old tubes???

does it blow fuses with the old tubes in???

 

try and get your hands on some slow blow fuses as well because the fast blow ones may be blowing when a slow blow one won't.

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Yeah I tried it with the old tubes. They blew the fuse as well. Where should I go to look for the slow blow fuses?

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I just had a similar problem trying to find a T250mA fuse. I live in Australia though.

 

mouser.com have some. I think this is the catalog no 504-GMD-315MA but just check the physical size of the fuse (it is 5mm diameter and 20mm long)

 

Try them with your old tubes and if they keep blowing then it is something other than tubes, as edster said I would be surprised if it was the bias causing fuses to blow that quickly but if you can eliminate that then it means that you can start looking for something else

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Yeah its going to the shop tomorrow. Its going to suck if I can't find a way to get ahold of the good tech in town (haven't talked to him in a year) Cuz other than him there is nowhere to go in this part of Texas. :mad::cry:

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I hate taking stuff to a tech but I always recommend doing so if it isn't fixed with something simple and it looks like you've given it a bit of a go.

 

I hope you find the good tech

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Looking at the schematic, your amp is cathode biased. Not adjustable.

Was your amp working ok before you changed the tubes? If so it is likely that one of your new tubes has it's plate shorted (or at least arcing) to another element. This could have drawn enough current through the cathode bypass cap to short it's plates. That seems to me to be the most likely scenario anyway.

In any case, stop experimenting and take it to a tech as recommended. Every time that fuse blows you are drawing a surge through your OT that is probably near it's max.

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Pull ALL of the tubes and see if you lose the fuse.

 

Most likely cause of losing the HT fuse is an element short in a tube.

 

I would have said that one of your new tubes has an element short (yes, new tubes can be defective), but you said you put the old ones back in and it happened again.

 

This time pull all of the preamp tubes, as well. One of them may have an element short.

 

If the fuse doesn't blow with all the tubes out, put them back in one at a time (powering down each time, of course) until you find the bad one.

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^^You may want to use a smaller fuse for this ^^.

 

Maybe 150ma or so. No need to beatup your OT.

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