[Fishline]

Apparently I can't just paste the flikr link into the image tag... See corrected post on the next page.

 

[n_maher]

Quote:

Originally Posted by GeckoSlayer /img/forum/go_quote.gif I apologize for not wanting to read through all 284 pages of this thread, but a question to someone who has followed this thread, can you use alternate tubes? Namely ones off this page? (And preferably the cheaper ones off that page..?) Thanks.

Start by reading this.
http://www.head-fi.org/forums/6024902-post3873.html

Fishline, I'm not seeing a picture?

 

[Fishline]

OK. I've gone through what I have several times and couldn't see anything wrong, so I'm putting a pic of the build and hope some of you will be able to tell me what's wrong with my brain. (Been having a cold lately so progress is s-l-o-w and head couldn't think too straight.)

I've desoldered the output part for now, and I still get the short as soon as I turn the power on (power drop from 48v down to 0v). Would very much appreciate any pointer.

 

[n_maher]

Quote:

Originally Posted by Fishline /img/forum/go_quote.gif I've desoldered the output part for now, and I still get the short as soon as I turn the power on (power drop from 48v down to 0v). Would very much appreciate any pointer.

What do you get when you measure resistance between the MOSFET tab and your case? While I can see that you have used isolation pads I've found that those pads tear quite easily (on rough hole edges for example) and it takes next to nothing to create a shorted condition.

 

[Fishline]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif What do you get when you measure resistance between the MOSFET tab and your case? While I can see that you have used isolation pads I've found that those pads tear quite easily (on rough hole edges for example) and it takes next to nothing to create a shorted condition.

I get 0F for both. Sorry for the idiotic question: Does that mean they are shorting?

 

[tomb]

Quote:

Originally Posted by Fishline /img/forum/go_quote.gif I get 0F for both. Sorry for the idiotic question: Does that mean they are shorting?

Hmm ... maybe Nate's distracted, so I'll chime in -

0.0 resistance = short.

BTW, it's really hard to tell from your pic, but even if the pads are not torn - if you haven't used an insulating shoulder washer, they'll short for sure.

 

[n_maher]

Quote:

Originally Posted by Fishline /img/forum/go_quote.gif I get 0F for both. Sorry for the idiotic question: Does that mean they are shorting?

It depends on your meter and what 0F means, it sounds weird to me. So the real question is what do you get when you touch the two probes of your meter together? I would have expected that if you had a direct short you'd still measure some resistance, the value of which could vary depending on your meter. Example: with the probes shorted together my Fluke 179 reads .1ohm.

 

[Forte]

Quote:

Originally Posted by GeckoSlayer /img/forum/go_quote.gif I apologize for not wanting to read through all 284 pages of this thread, but a question to someone who has followed this thread, can you use alternate tubes? Namely ones off this page? (And preferably the cheaper ones off that page..?) Thanks.

Try searching for 12SR7 in this thread, there is a version you can build using those. They are available from the store you linked, just not on that page, but it may be cheaper to order from somewhere like Vacuum Tubes Radio Tubes - 5,000 different tubes in stock - Over 10 million tubes! even after shipping.

 

[Fishline]

This is the meter I have:
http://www.radioshack.com/graphics/u...0813_PM_EN.pdf
Doesn't say what 0F means when measuring resistance.

I tried the open/close test on the meter. If I test the mosfet tab and the case, I get "open". If I test the mosfet tab and the star ground, I get "close". Does that mean much?

 

[Fishline]

I did have the insulating washers between the nuts and the mosfet tabs.

The meter starts at 0F, and stay that way when I test between the case and the mosfet tabs. However, I get 0.2ohm when I test between the mosfet tabs and the star ground. Does that mean anything?

[edit: Apologies for the near double-post. I keep getting the message about can't post within 15 seconds or something like that, and can never be sure if the post made it...]

 

[n_maher]

Quote:

Originally Posted by Fishline /img/forum/go_quote.gif If I test the mosfet tab and the star ground, I get "close". Does that mean much?

That would be bad, yes, but would also be very strange unless your star ground is not connected to the chassis.

 

[Fishline]

You guys are lifesavers! I loosen both mosfets and cleaned the mounting holes, remounted them carefully, and now no short!

With the output still disconnected, I'm gettting 21.3v on pin 3 of the tube sockets (tubes not installed). Does that sound right? The schematic says 19v there...

 

[n_maher]

I don't think you can get an accurate reading without the tube in there.

 

[Fishline]

It's all done! It played music! Hallelujah!

Having succeeded with my el cheapo build in a mooncake box and mostly "stock" (and cheap) parts, I tried to make this one as nice as I can manage. The enclosure is actually really tight, and I'm lucky that everything actually fits. There's however, no room for bypass caps. The "upgrades" are really nicer jacks and nichicon kw 470uF output caps and panasonic fc 680uF power caps. The heatsink is an old CPU heatsink that a friend donated.

I did get the 3-piece set of stepped drill bits from amazon, which made drilling all the holes rather easy. I don't have a drill press, but did inherit a vise, which was really handy for holding the case while I drill the holes.

Thanks to all the folks for helping with my really newbie questions and wonderful suggestions and recommendations!

 

[tomb]

Looks great! Those are some wild looking heat sinks.