ValveKing Blowing Fuses

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johnny be good
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ValveKing Blowing Fuses

Post by johnny be good » Fri Jan 21, 2022 12:40 pm

Hello all. I am a novice trying to learn some trouble shooting tips on tube[valve] amp repairs. I have read Dave Hunter"s Guitar Tube Amp circuits and signals.
I do know the basics and DANGERS of filter caps I play around wit ham amps. I own a VK100 head and a VK 212 100w. and I saw an add for a Non working amp it was a VK 212 so I figured great learning project and if nothing else it has all new tubes pre and power 2 decent VK speakers and transformers.
Owner says that a shop put in all new tubes he played it a couple months put it away for a few years took it out and would not turn on so he let it go cheap. I first noticed the exterior fuse holder was not holding the fuse in tight so I got a direct replacement and thought to easy so I saw internal fuse had blown F203 T3A replaced it temporarily and it blew soon as amp turned on. looking at the board looks like the first four diodes may have gotten hot and marked the board. I left original blown fuse leads on for now but are not shorting to anything like may look like in picture.
So anyway just wonder if anyone has dealt with one of these and the issues some may have before I start removing and testing.Thanks for reading
20220102_112224_Film1.jpg
20220102_112224_Film1.jpg (3.92 MiB) Viewed 3456 times

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johnny be good
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Re: ValveKing Blowing Fuses

Post by johnny be good » Fri Jan 21, 2022 12:42 pm

Schematic VK 212
peavey_valveking_100_212-1.pdf
(301.41 KiB) Downloaded 137 times
peavey_valveking_100_212-1.pdf
(301.41 KiB) Downloaded 137 times

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Enzo
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Re: ValveKing Blowing Fuses

Post by Enzo » Fri Jan 21, 2022 1:55 pm

Does it blow fuses with the power tubes removed?

If you suspect those rectifiers, you have a meter? Are any shorted? Do this test with tubes removed, as these diodes are the DC heater supply. I suspect they run warm normally and over time discolored your board.

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Re: ValveKing Blowing Fuses

Post by johnny be good » Fri Jan 21, 2022 6:40 pm

Yes have meter all tubes are out I was going to remove diodes to get a good test Thanks for the reply

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Enzo
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Re: ValveKing Blowing Fuses

Post by Enzo » Fri Jan 21, 2022 8:51 pm

You need not remove the diodes, check if they are shorted where they sit. There is nothing in the rest of the circuit that will make a shorted part appear not to be shorted. If one seems to be shorted, THEN remove it for further test. Avoid excess parts removal and re-installation.

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Re: ValveKing Blowing Fuses

Post by johnny be good » Sat Jan 22, 2022 5:45 am

Thanks again Enzo,Yes it does blow fuses with all pre and power tubes out I will be checking those diodes today. I will keep you posted. As I said I bought this non working as a learning project. The amp looks brand new well taken care of not even dusty inside.

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Re: ValveKing Blowing Fuses

Post by dak » Sat Jan 22, 2022 9:26 am

johnny be good wrote:
Sat Jan 22, 2022 5:45 am
Thanks again Enzo,Yes it does blow fuses with all pre and power tubes out I will be checking those diodes today. I will keep you posted. As I said I bought this non working as a learning project. The amp looks brand new well taken care of not even dusty inside.
The amp looks brand new well taken care of not even dusty inside.

Uh oh. Valve amps have high voltage in them that attracts dust. Could be that it has just been collecting dust on the outside. The condition "not switched on, stored in a warm and dry place" is basically one of the worst things to happen to electrolytic capacitors: they need to be under voltage occasionally in order to maintain their isolation. That's not all that critical for caps in the signal path, but it can spell trouble for power supply caps. Under disuse, the isolation layer becomes thinner which makes the capacity grow but eventually leads to large leakage and/or breakthroughs at working voltage. Which is sort of an ugly popping sound.

I would not really worry all that much about the heated diodes though it's a good idea to check: to dissipate heat, there must be voltage across the diode and current through it. As long as the diode has not stopped heating altogether, it's a good bet that it is still operative. If it shorts out, there will be no voltage, if it cracks, there will be no current. Now the current has been enough to blow a fuse (essentially, there is not really much of an option for blowing the fuse that would not involve current through some power supply diode quartet) but the voltage drop across the diodes is 0.7V or so, while the voltage drop across whatever is blowing the fuse will have been quite more.

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Re: ValveKing Blowing Fuses

Post by johnny be good » Sat Jan 22, 2022 3:34 pm

Thanks dak appreciate the knowledge I am hoping just a diode will see tomorrow I will post results as I go.

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dak
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Re: ValveKing Blowing Fuses

Post by dak » Sat Jan 22, 2022 4:59 pm

Even if a diode is blown, there will almost certainly be a reason for that elsewhere. Rectifier diodes are simple devices that are easily dimensioned to withstand the maximum operating load permanently. If you replace a blown rectifier diode, it's very likely you'll keep blowing fuses and possibly diodes until you find the actual culprit. Power supply capacitors are what comes right after the rectifiers and what is most likely to suffer permanent failure after being unused for too long, with the avalaunching leakage current heating the diodes and a breakthrough blowing the fuse.

You better hope for that since they are easy to replace and usually don't cause followup damage (if the speakers survive the din). Other problems usually blow a fuse only after taking out at least half a dozen or more other components.

On your image to the right there is one such big capacitor. Check it and any like it for bulging or excess yuckiness on the circuit board: they can inflate due to gases when failing, or leak fluid, or outright explode. The latter is unlikely for a device called "ValveKing" but somewhat common with amps for which valves are contemporary enough not to make it into the name of the amp.

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Re: ValveKing Blowing Fuses

Post by johnny be good » Sat Jan 22, 2022 5:31 pm

Thanks again I will be going over all that

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Enzo
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Re: ValveKing Blowing Fuses

Post by Enzo » Sat Jan 22, 2022 8:01 pm

The fuse he is blowing is the heater fuse. Just a heads up.

If you do find a shorted diode, replace all four. When one leg of a bridge is shorted, it stresses the others. Diodes are cheap.

The schematic calls for 1N4003, but if you have the more common 1N4007 in your stock, that is perfectly fine to use here.


My suspicion is that a power tube is shorting to heater.

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Re: ValveKing Blowing Fuses

Post by johnny be good » Sun Jan 23, 2022 6:42 am

Thanks again Enzo the fuse is blowing with all pre and power tubes out of amp. I will be checking diodes today and keep you all posted

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Re: ValveKing Blowing Fuses

Post by johnny be good » Sun Jan 23, 2022 3:59 pm

Well diodes testing bad new ones ordered. Keep you posted

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Re: ValveKing Blowing Fuses

Post by johnny be good » Sun Jan 23, 2022 6:55 pm

Off Topic, where do I find the meaning of the small symbols next to thread titles mine has a circle with a flame inside and a small red star. Maybe the star means I am special :D

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dak
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Re: ValveKing Blowing Fuses

Post by dak » Sun Jan 23, 2022 7:08 pm

johnny be good wrote:
Sun Jan 23, 2022 6:55 pm
Off Topic, where do I find the meaning of the small symbols next to thread titles mine has a circle with a flame inside and a small red star. Maybe the star means I am special :D
Being fairly new here, I also tried finding that info without much luck. Best guess so far is that the star indicates threads where you already participated, and the flames mean the thread has seen recent action (is hot).

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