Quick question about tube shielding?

[Electricblue]

I'm close to finishing my DIY hybrid mini-amp and I would like to know where to place the 12AU7 preamp tube. My original idea was to mount it on top of my aluminium enclosure (similar to pic 1) but would this pick up more noise than having it inside the enclosure? Most internal wires will be shielded, I'm trying to make this amp as quiet as possible. I'm using a Hammond 1590D box (pic 2)

I do not own these pictures.

Pic 1

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Pic 2

Thanks!

 

[beans-on-toast]

What you do in terms of the placement of the tube depends on your design, how much heat you need to dissipate. A tube that isn't running hot can go in the box but will need some space around it and possibly venting.

A tube shield such as an IERC 6020B (intended for a medium sized 9-pin tube) will shield the tube, help dissipate heat, and protect it. These particular shields push onto the tube and are held in place with tiny fingers that act as a heatsink. There are a variety of different shields available in different sizes and with different features but you have to get one that will fit your tube. They can be difficult to find. Since you only need one, try Invalid Link Removed. They don't list one in stock but may be able to find one.

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Check out ebay:
http://www.ebay.com/itm/2-X-US-IERC...098?pt=LH_DefaultDomain_0&hash=item3a9fdddb42

Other styles of shield that connect onto the tube socket are readily available.

To protect the tube, there is the possibility of adding a tube cage on top of the box.

A lot of single tube designs don't use any form of shielding and they are quiet. Best to mock up your design and see what you need, then get into something more elaborate.

If you google single tube amp and click on images, you will see a number of ideas as to how you can mount the tube such as the one below. The tube can have different amounts of shielding depending on its position in the box. This way, it could have a smaller mesh cover over the top of the glass envelope and allow cooling .

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[Electricblue]

Awesome, thanks! I have a metal frame that can go over the tube to protect it if it's mounted on the top, so that's one thing covered. The tube is part of the Matsumin Valvecaster booster I've used in the amp. It's a 12v design, so it's not really a 'full-on' tube circuit and it doesn't build up too much heat.

The only reason I would mount the tube on top is for aesthetics, it would be a shame to hide it away in the box I just don't want to introduce a potential noise issue just so I can look at it. If I were to use a guard like you mentioned, that would defeat the purpose of having in on top of the box

 

[beans-on-toast]

It sounds like a fun project. I wonder if it has some similarities to the Mesa V-Twin pedal. It also runs on 12VAC. The schematic is available online.

It would be interesting to see how the sound in your pedal would compare if the B+ were boosted to a higher voltage like they do in the Mesa pedal. Of course, it takes more space to do that.

At a low voltage such as 12V, you aren't going to have any issues that require tube shielding.

 

[Electricblue]

I just had a look at the V-twin schematic, it's far more complicated than this! This original version can be build right on the back of a 9-pin valve socket! I've made a pcb based on it, plus a few of my own tweaks. It started off as a standard Valvecaster circuit and over time it's been part of many pedals/preamps I've cobbled together, so I've decided to combine it with a 40w power amp module to make a mini guitar head. It will be mostly used through my hand-made 2x12 guitar cabinet. It also sounds pretty good for bass at room volume (Ashdown 410 cab).

I've tested it already, but my hand is healing from a previous injury so I gotta take it easy with drilling the enclosure holes. I will post pics of the head and cabinet when I'm done, I might even include a few sound clips

 

[beans-on-toast]

I also took at look at the Matsumin Valvecaster booster circuit, it is about as basic as basic can get for a tube.

Did you design and have a PCB made for it or did you make it yourself? A lot of people are getting into doing their own PCB boards. Quite a learning curve. Sounds like the 40W is going to be good! Even better when you make it yourself.

 

[Electricblue]

Sorry, it's not a PCB, it's Veroboard. I just remembered the P in PCB means printed... My bad!

I don't think I have the patience to design a proper PCB, it's a one-off project so as long as it works, that's good enough for me

 

[beans-on-toast]

...it's Veroboard...

Veroboard, point-to-point, or PCB, they all get the job done. If you have a chance, let me know how it turns out when you are done.

 

[Electricblue]

Will do! I'll make a fresh thread about it and direct you there, I will also include my 2x12 cabinet. I started a thread ages ago when I was half way through building it, and I never posted the results I drilled a few more holes in the enclosure last night, so progress is happening! I'm waiting for a tube guard to arrive from China, 7-14 days shipping. Lovely...

The only other thing to worry about now is shielding all the insides. I'll buy some copper tape, and wrap every wire I can. Even the shielded wires are getting wrapped Almost done though!

 

[beans-on-toast]

Nice tube cages.

Wrapping the shielded wires will add extra protection. Some wires come with a wrap over the shield.

With shielded wires, one end of the shield is grounded, the other isn't. If you ground both ends you can get a ground loop and hum is induced. I buy mine, the 1 conductor shielded PTFE (teflon) from this place.

 

[BassmanPaul]

For a project such as this I would have considered using a miniature wire in tube instead of a nine pin.

Love them there cages too, would you provide a link??

 

[Electricblue]

Not a problem, it's on Ebay and I think it was this one. I don't like ordering over the pond, but there's no UK-based suppliers that I could find.

There's a similar circuit to my boost/od that uses a 6111 sub-mini tube, but I already have a nice JAN Philips 12AU7 It's not too tight inside, I've precisely 'jiggled' everything so it fits. It's pretty much a tweaked Valvecaster with a power amp module strapped on the end.

I'm just waiting for that tube guard, so I can drill the holes for the valve socket etc. As soon as that comes I can start screwing everything in!
I've received my 10cm x 4m copper tape, shieldied my Strat to pass some time.