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 Post subject: K-502 Tube Amplifier Kit
PostPosted: 16 Jun 2009, 09:12 
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Joined: 08 Jun 2009, 20:19
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I started the build on a K-502 kit yesterday. Things went pretty smoothly. I soldered all of the resisters on the top and all of the taller components on the bottom, taking careful note of polarities and such. I bought a nice box that house some really old alarm system. It fits the kit perfectly. It has enough room on the inside for all of the components and stuff. I was wondering if my wiring setup would work.
I bought an AC plug jack that will take one of those AC computer cords. It has the three prongs and I'm not sure what to do with the ground. Should I leave that floating or solder that somewhere, such as where the output transformers are grounded.
One of the leads of the input jack goes to a fuse and then a 120 V rated DPDT toggle switch. It has 4 pins and I'm pretty sure it was labeled DPDT. After the switch, it goes to the input transformer. The other lead goes directly to the input transformer.
Then, I want to use a wall wart to condition the AC signal to power an LED and a computer fan. The fan and LED would be hooked up to the other part of my switch.
Does this seem alright? Or is there a DC voltage on the board that I could use instead of putting a wall wart in there? I'll post some pictures of what I did yesterday and what I do tonight later. I added a picture to help explain what I'm thinking about doing.


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PostPosted: 16 Jun 2009, 17:57 
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Hi Dandeman,

the wire / pin from your AC plug you asked for, is to connect the earth from your mains to the chassis. The "earth" from mains has nothing to do with any "ground" connections from the circuit you build. If i understood your question right - the "earth" and the "ground" are two different things!
Normally the "earth" connection must be connected to an coatless chassis / enclosure point (when the chassis / enclosure is made of metal). It is to protect you from current if i.e. a lead is broken and has contact to the chassis. Feel free to ask if you have any questions to this subject!

I also would suggest to connect the wallwart like you want to connect the "input transformer". In your schematic the wallwart is connected to mains all the time without using the dpdt switch. So it is always powered. Even if the switch is "turned off".
The rest seems to be OK! Take the wallwart if you already have it there.

For further informations you also might find some help here: http://diyaudioprojects.com/Tubes/K-502-Tube-Amp-Kit/

I hope i could give you some help and it was good to understand.

Cheers,
Tom.

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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 16 Jun 2009, 18:19 
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Thanks a Bunch! Yeah I figured that the other pin was supposed to go to the housing but I'm using a wooden box with not metal. The kit discusses soldering two wires from the ground of the PCB board and sliding them under the metal bits of the output transformer where they are mounted. Thanks for the link. I've studied his project quite a bit already :)


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PostPosted: 16 Jun 2009, 19:00 
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Location: Winnipeg, CANADA
Hi,
Assuming the tube will protrude from the enclosure and the power transformer is located in teh outside of the enclosure, where do you plan on putting the fan?

As for the switch, I would go from the socket to a fuse and then use the switch for both the hot and neutral then go to the amp and walwart.

Cheers, Gio

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[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 16 Jun 2009, 19:51 
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I was thinking about using a small fan. I've seen some computer fans that were only 2-3 inches tall. I was just going to drill a port out the back for it and fit it flush. I'm not sure if I will do that though. I've run into a problem with my circuit board coming down into the box. The sides are almost 3/4 inch thick and I need to grind away quite a bit of particle board with my dremel so I'm not sure I'll be in the mood to grind away another big hole in the back for a fan. The transformers are going to be mounted on top, out side of the box. The tubes will protrude 1-2 inches out of the lid.

Thanks for the tips on the switch. That makes more sense.

I would post another picture of the progress but all I could do before the battery died on my dremel was grind away a 3/4 x 3 inch rectangle :/


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PostPosted: 16 Jun 2009, 22:36 
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Is it possible to get the tube completely outside the enclosure? That would really help with potential heat issues.

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PostPosted: 17 Jun 2009, 03:25 
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Hi Dandeman,

in my opinion it is very curious to slide a gnd connected wire under the tranny :?
I can't imagine why this should be done. The reason to think about that is the fact that E / EI core transformers are producing a mass of stray field inductions that may slightly go to the wire. As the electrons flow is from ground(!) to the tubes and not from UB+ / B+ to the tube i have no idea what this wiring should provide... It would make a sense if one "earth" lead is connected to one transformers mounting screw. Within this there's a chance to have a kind of protection if an short occures. Mounting the "earth" to ground is (as written) normally unusual if metal chassis are used. With metal chassis the earth is on the chassis. OK - you use wood. For an basic circuit and health protection you could connect the earth to ground - but with an small seperation circuit. This is not a "must be" and my opinion - but worth to think about. See:
Attachment:
earth_to_gnd.JPG
This seperation is to prevent potential differences and loops between the audio devices / circuits and the earth connection.

With the mounting from the tubes i must agree to Gio. This is an "very hot" thing. Your enclosure looks fine and really massive. Really - if you have a chance that the tubes protrude more out of the enclosure then do it. You could i.e. also mount the portentiometer and the RCA plugs seperately at the enclosure to mount the board closer to the lid. Otherwise you could increase the diameter from the tube holes that there is more air around them. If they are very tight shielded all around from the enclosure they will run hot and not living that long. A bit more air around the tubes within the fan to get an chimney effect should be one more solution.

Now i hope that helped a little bit. Have fun with your build. I would be happy to see the progress ;-)

Cheers,
Tom.


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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 17 Jun 2009, 08:57 
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Yeah, I understand these things run pretty hot. I was thinking that if I was to make a hole in the back and put a fan in, it could either draw air into the box from around the tubes or suck in air through the back and come out around the tubes. Which way do you guys think would be better? I think that sucking cool air in from the back and pushing it up through the holes around the tubes would be better than sucking the hot air around the tubes down into the box.

For the front components, I was going to have to grind all of the wood away to mount components anyways, so I decided to just leave them on the board. For the outputs, there is a little slot out the back that was for the outputs of the alarm system that was originally in the box. I cut and sanded a piece of plexiglass to cover the hole and I'm going to mount the four output plugs the kit came with to that. Then, I'll wire those up to the board.

Right now, I'm limited by my dremel tool's battery. With the heavy sanding/grinding I'm doing, the battery dies in a very short time. Today, I'll go to home depot and maybe buy a cable for it. I was pretty close to getting all of the material in the front carved out. I just need to finish that, grind a hole for the switch, grind a hole for the on/off light, grind a hole for the input voltage, and widen the output holes. Once all of that grinding is done, I think I can finish up the construction. I'll probably post some pictures later tonight, as long as it doesn't take too long.


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PostPosted: 17 Jun 2009, 16:33 
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Hi Dandeman,

it is better to blow cool air from the outside into the chassis and along the tubes. Otherwise you heat up the chassis inside.

Cheers,
Tom.

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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 18 Jun 2009, 10:55 
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Yeah, that's what I was thinking.


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