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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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PostPosted: Mon Nov 25, 2019 11:41 pm 
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Ralfg wrote:
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.


Hi, I'm planning to build a Triwatt (having built a couple amps but nothing more complicated than a 5e3) and I'm trying to psych myself up by reading the instructions. One thing I don't get is how you're supposed to know which switch position is which tube type. Did I miss something in the manual or is this something you need to understand schematics/electronics better than I do to intuit? After searching the forum this seems like the main/most recent place the switch is discussed.


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PostPosted: Tue Nov 26, 2019 1:21 am 
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Elliot, check out Roar's post "New Tweed Build"in the Tweed forum, Tweed layout showing the connections for 4 ohm and 8 ohm in the main drawing and with the side view of the same switch, and Tramp build guide explaining and showing the bias switch connections. Looking at the bias switch setup for the Triwatt, it's pretty much the same as the Tramp. With center lug on DPDT switch only in the circuit you are only connecting one resistor (180K) for 6V6 operation. When the center lug AND the wire connected to the lugs w/ the second resistor are switched together you are connecting both resistors in parallel(180K and 620K) for KT66 operation. DPDT switches connect the center lugs and the lugs DIAGONAL to the toggle lever. I always attach labels for each tube position. Hope this isn't confusing the issue. :bugeye:

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Last edited by Hankules on Tue Nov 26, 2019 2:35 pm, edited 1 time in total.

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PostPosted: Tue Nov 26, 2019 1:38 am 
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Well, it helps to read the schematic.

Hank's explanation is a good one and he beat me to it again! So to put it another way:

The Triwatt schematic shows a switch that selects between 6V6 and KT66 tubes and it's marked which position is which. KT66 tubes need more B+ voltage and more bias than 6V6 so that's what the switch does.

In the 6V6 position, both poles of the switch are open so the B+ voltage is reduced by 18 volts because of Zener diode D5. Also the AC supply for the bias voltage circuit just passes through the 180K resistor before getting half-wave rectified by D6.

In the KT66 position, one pole of the switch bypasses the Zener diode so full B+ is supplied to the amp. The other pole switches a 620K resistor in parallel with the 180K. This reduces the resistance to about 140K and increases the bias.

The physical position the handle on the switch has to point towards for 6V6 or KT66 is shown on the layout drawing. The switch connects its center terminals to the outer terminals opposite to the way the handle points.


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PostPosted: Tue Nov 26, 2019 2:10 am 
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mitch m wrote:
Well, it helps to read the schematic.
The physical position the handle on the switch has to point towards for 6V6 or KT66 is shown on the layout drawing. The switch connects its center terminals to the outer terminals opposite to the way the handle points.


This is key... and is the most confusing thing, I think, about how switches work. Use your voltmeter at the continuity setting and connect it to one of the center lugs and the one directly the left or right of it. It helps visualize how the connectivity works.

For KT66 the toggle points to the preamp tubes. For 6v6 it points away.

The Triwatt is an amazing amp. Love mine.

Hankules wrote:
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.


I am in North America. Is it cause I said wicked? My wife is from Massachusetts and I'm from California so there is a lot of "wicked" and "dude" that flows around in our house. And thank you, yes it has been playing great. Sorry for the late reply, I must have missed it.


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PostPosted: Tue Nov 26, 2019 2:31 pm 
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Thanks for the wicked good answer, duude ;)

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PostPosted: Tue Nov 26, 2019 3:28 pm 
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Hankules wrote:
Thanks for the wicked good answer, duude ;)


Yah Brah! :D


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PostPosted: Tue Nov 26, 2019 8:06 pm 
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Side comment for Mitch; What else do I have to do? I'm refried, err, retired :)

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PostPosted: Tue Nov 26, 2019 8:14 pm 
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Thanks for the responses, everyone! I'm really champing at the bit to place an order and start building...but it won't be until the New Year most likely, so I'll just have to make do with reading the build guide over and over! I'll document my build on here when I get going :)


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