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PostPosted: Fri Oct 04, 2019 12:11 am 
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Hey guys,

Been rocking my OSD for a while now and it has been going great... but just this evening I was playing fairly loud at a jam and suddenly the volume dropped out and I could smell the lovely smell of burnt electronics. I opened it up and saw that the power tube type selector switch got fried. The wires look a bit melted too. I’m running 6l6s and they should be properly biased. I’m wondering if a connection got loose or if maybe the switch just failed? The speaker amp impedance was matched at 16 ohms.

Any thoughts on what might have happened? Think replacing the switch is enough? Or should I check elsewhere in the amp?

Thanks ahead of time. Here is a lovely picture of the burnt switch. Happy to upload any additional pictures if necessary.

-Ralf


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PostPosted: Fri Oct 04, 2019 11:57 am 
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Check and read the Tramp board, page 1, "OT Impedance Switch Arcing" and "Installing Switch for EL34 Use!" posts. They cover arcing issues with the small Carling switch arcing under heavy loads when used to switch OT loads. Hope this helps. :) Seems very similar to what you're seeing and the conditions it occurred under.

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PostPosted: Fri Oct 04, 2019 12:44 pm 
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Thanks Hankules! This is very helpful information and seems like what happened. I did not try the amp again, and was worried that something else was also damaged. Sounds like I just need to replace the switch, use some heat shrink tubing, and keep all everything as far apart as possible.


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PostPosted: Fri Oct 04, 2019 1:42 pm 
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You could try replacing the switch with one that's physically bigger and also rated for higher voltage and current than the fried one, as well as insulating the terminals.

Meanwhile, if you need to use the amp or you want to make sure it's still working, you can bypass the switch and connect the OT directly to the output tube sockets. Disconnect all the wires from the switch, insulate the blue and brown wires coming from the OT, and connect blue-white and brown-white wires from the OT to the tubes for 6L6.


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PostPosted: Fri Oct 04, 2019 5:14 pm 
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I'm a big fan of the big switch https://www.mouser.com/ProductDetail/Ca ... 5mpnEos%3D, however, if you decide to go that route, make sure you have enough real estate on the chassis to install it (see Pharmrock's link on Tramp board) and you will have to enlarge the hole(carefully, drill bit will have a strong tendency to grab and pull into hole). If you use the small red one, Coco recommends using liquid electrical insulation (recommend Permatex brand, available on Amazon or possibly electrical supply store). I haven't had a problem w/ the big switch on the Tramp. PS Apologies to Mitch for stepping on his post. :oops:

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PostPosted: Fri Oct 04, 2019 5:55 pm 
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We all have useful information to contribute. Nobody is stepping on anything.

Step bits are a good way to enlarge holes. They cut sideways instead of up and down so they make a neat job and don't grab. They are good for drilling holes in chassis for tube sockets, too.

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PostPosted: Fri Oct 04, 2019 8:08 pm 
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Thanks guys for the responses. So it sounds like it just arced and hopefully the only thing that got damaged is the switch itself. I was worried there might have been other damage. So I’ll just replace the switch, insulate it and test it out. I don’t think I’ll be able to fit that big switch in the layout.


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PostPosted: Sat Oct 05, 2019 12:47 pm 
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Was reading about "Howard" Alexander Dumble and he started coating his circuits w/ an opaque resin, but anyway, have you thought about coating the switch terminals w/ some sort of epoxy coating? :?:

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PostPosted: Sun Oct 06, 2019 6:17 pm 
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Was looking @ Jim1's build; looks like there's plenty of room in the OSD chassis for the larger switch. More space between terminals and higher ampere rating. 15A @125/250V for the big one and 5A @ 125V/ 2A @ 250V for the small one.

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PostPosted: Mon Oct 07, 2019 12:17 am 
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There might be space. It’s just the impedance selector wires and switch make it look a bit tight, and I didn’t give myself a lot of wiggle room there. I haven’t had much time to work on this yet. I’m not quite sure what I will do. When I have more time I’ll remove my old switch and see what I can do. I did think about an insulated coating, but I’m a bit concerned about how difficult that might make future repairs. Will that stuff melt away with a solder iron?


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PostPosted: Wed Oct 09, 2019 1:16 am 
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Ok so I replaced the switch with a similar switch which is a Taiway, same rating as the original, but I wrapped the two center lugs with heat shrink tubing. I then suddenly had my doubts if I had the switch in the correct position originally... could I have screwed up there? Oh I don’t know now. To be safe I checked it against the schematic and set it to the 6l6 position and verified that I had connectivity to the blue/white and brown/white wires. Rebiased the tubes and hooked it to my UA OX and cranked it with headphones to not wake the wife. Everything seemed fine.... of course when it arched it was an hour in, so I won’t know till the next jam session.

But one thing is for sure... the drive channel purrs something wicked. Great amp! I’ll update if everything goes smooth at the next jam.

Though I was thinking, if Stephen is reading this it would be cool to add a sticker or something that shows which tube set is selected on the chassis. Just to make it even more foolproof and prevent people from selecting the wrong tube type.


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PostPosted: Wed Oct 09, 2019 5:16 pm 
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If it arcs again, look for a heavier switch. Sound like you're not in North America. But, if it never arcs again and you never have to change it again :thumbsup: Glad you got it working again.

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