Blown fuse

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Kcz5o
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Blown fuse

Post by Kcz5o »

Quick question. I have a JTM 45 kit that I put together and occasionally it will blown a fuse if I turn both power switches on at the same time or nearly the same time. I can use the amp multiple times before that happens. It calls for a 2 amp fuse. I am not sure if this is related but the same thing happens on a plush head I have. Since it's happened on a couple different heads I thought it might be something in my power supply board so I switched that out but it just did it again. Any thoughts? Thanks in advance.
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Re: Blown fuse

Post by JMPGuitars »

Are you sure it's the correct type of fuse? Is it possible you're using fast blow instead of slow blow? Check the amp or manual for the correct type. If that's not the problem, you may have another issue.
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Re: Blown fuse

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It calls for a slo blow and that is what's in there. A dim bulb limiter doesn't indicate a short.
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Re: Blown fuse

Post by TriodeLuvr »

Does your kit have a 5AR4 rectifier like the original?

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Re: Blown fuse

Post by crgfrench »

If you can't get to the bottom of it, put a thermistor between the fuse and the on/off switch.
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Re: Blown fuse

Post by crgfrench »

Which kit is it?
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Kcz5o
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Re: Blown fuse

Post by Kcz5o »

Its a JTM 45+ kit from tube depot. I posted a transformer question earlier about the same amp and included the schematic. It has 5AR4 rectifier and a switch to change to solid state rectification. I'm not sure what a thermistor is. I will have to look that up. Thanks
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Re: Blown fuse

Post by TriodeLuvr »

Does it only happen when it's switched to use solid-state rectification? Have you contacted Tube Depot to see if other owners have reported the same thing? This could be caused by filter caps with a particularly low ESR, higher than average line voltage or any number of things that aren't actual defects. Difficult to know for sure. I'd be tempted to install a 2.5A fuse (but no larger) to see if it's just marginally overloading the 2A fuse during the turn-on surge. But if it's doing it even when the 5AR4 is switched in, that's a different thing.

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Re: Blown fuse

Post by Kcz5o »

I usually only use it with tube rectification. I didn't purchase it from Tube Depot just used the documents and sourced the parts myself. So I'm hesitant to call and ask for advice :-) In a previous post on this forum I mentioned that I initially started the amp up with a dim bulb limiter and had left a resister hooked to ground. I was using it to drain the filter caps between startup points for initial testing. It ended up blowing the rectifier diodes but everything else seemed to work. Is it possible that the filter cap is compromised in some way? The voltages seem ok with the exception of the plate voltage being 460 when the schematic suggests it should be 480. That's only about a 5% difference so I didn't think it was a big deal. I used a different PT than was called for by the instructions and just chalked it up to the different PT. I thought about putting in a 2.5 amp fuse but wasn't sure if that was safe to do.
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Re: Blown fuse

Post by JMPGuitars »

Kcz5o wrote:
Tue 01/19/21 3:02 pm
The voltages seem ok with the exception of the plate voltage being 460 when the schematic suggests it should be 480. That's only about a 5% difference so I didn't think it was a big deal. I used a different PT than was called for by the instructions and just chalked it up to the different PT. I thought about putting in a 2.5 amp fuse but wasn't sure if that was safe to do.
This sounds logical to me.

The filter cap drain resistor is a good idea. We typically use 220K 2W from the first filter cap to ground. That shouldn't cause any harm. If there was a bad solder connection, faulty resistor, or other issue that caused the problem, it could have messed up the filter cap.

The other question is what is the value of the first filter cap?
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Re: Blown fuse

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The first cap is a can cap with two 50uf/500v capacitors.
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Re: Blown fuse

Post by Kcz5o »

I probably didn't clarify. I powered the amp on with the resister still hooked to ground (forgot to remove it). The light bulb started to glow when the caps started to charge and I shut it off. It did toast the diodes though.
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Re: Blown fuse

Post by JMPGuitars »

Kcz5o wrote:
Tue 01/19/21 4:42 pm
I probably didn't clarify. I powered the amp on with the resister still hooked to ground (forgot to remove it). The light bulb started to glow when the caps started to charge and I shut it off. It did toast the diodes though.
Take a look at the cap can in this layout: files/JMPGuitars_18_Watt_Modern_Classic_Layout.pdf

That's what I'm talking about with the 220K resistor connected to ground where it says "A."

If you toasted the diodes, then you may have destroyed the filter cap, and both the diodes and cap can would need to be replaced.
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Re: Blown fuse

Post by TriodeLuvr »

JMPGuitars wrote:
Tue 01/19/21 3:09 pm

The filter cap drain resistor is a good idea. We typically use 220K 2W from the first filter cap to ground. That shouldn't cause any harm.
If I remember correctly, it wasn't a bleeder resistor per se. It was a temporary resistor of a few hundred ohms (?) that he forgot to disconnect when he powered up the amp. That's a good point about the shorted rectifiers possibly damaging the first filter cap(s). I wouldn't use the shotgun approach on this though, especially considering the cost of parts nowadays. If it was mine, I'd insert a handheld ammeter in line with the B+, just after the rectifier, to measure the current draw into the caps and beyond. A lot of things can cause a problem like this. It could also be the rectifier tube.

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Re: Blown fuse

Post by Kcz5o »

Thanks for all the help so far. Everyone has been very gracious and helpful. I have definitely had a good experience on this forum.
It was a drain resister that was hook to the first filter cap and then to group. If the filter cap was damaged would I have some hum occurring. The amp is nice and quite.
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Re: Blown fuse

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filter cap to ground. Should have proof read my post.
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Re: Blown fuse

Post by TriodeLuvr »

Kcz5o wrote:
Wed 01/20/21 10:47 am
If the filter cap was damaged would I have some hum occurring. The amp is nice and quite.
Not necessarily. And I'll add that you're not likely to resolve this by guessing. When we design a new product at work, we use a peak-hold AC current meter to confirm that we've sized the input fuse correctly. This tells us the inrush current and the steady-state current. If your meter can do this, it's the next logical step (and/or monitoring B+ current like I suggested earlier). Going to a slightly larger fuse is also a helpful technique to distinguish between a moderate overload and what might be an intermittent short.

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Re: Blown fuse

Post by Kcz5o »

Makes sense. I will put in the 2.5 amp fuse next time it pops a fuse and see if that makes a difference. I just have a standard DMM so would not be able to make the other measurements. I agree that throwing money at it isn't the best approach but did order another can cap just in case. It was only $15.00 with shipping and is probably worth it if the other one is compromised in any way. Thanks
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Re: Blown fuse

Post by Kcz5o »

Not sure if I can ask another question that has to do with the same amp. If I need to post the new question under a new heading let me know. I noticed this. If I turn the treble volume control close to full about 85%, the volume drops to about 1/2 and it gets real farty sounding. I can turn the normal channel up all the way and it is fine but if I am plugged into the normal channel and turn the treble volume control up to 85% the normal does the same thing. The amp has a master volume that works for both channels. I took the amp out of the cabinet and checked the treble volume pot and it checks out ok. Any ideas?
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Re: Blown fuse

Post by Kcz5o »

Can disregard this. Its something to do with my input jacks. Thanks
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