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Aaahh! It's Blowing Up! (Little Dot MK III thread)

post #1 of 8
6/18/11 at 2:38pm
Thread Starter 

Hello, all.  I haven't posted on this site for quite some time.

 

Anyway, a few months ago my Little Dot MK III tube amp started acting a bit weird in that it seemed like it was lowering the volume of the right channel - but, it was very subtle.  A few weeks ago, though, it became a significant problem.  I tried swapping the tube's sides and even trying different tube but the problem remained.  It seemed like after the amp got warm enough it would start dimming the right channel's volume.

 

Well, a few days ago I opened the amp up and took a look at the board and components (I am tech savvy, so no worries there) and saw absolutely nothing out of the ordinary; I put the unit back together, without having done anything to it, and used it as normal for a couple of days.  Then just yesterday I'm sitting at my desk with the amp on for a few hours and BAM!... the pre-amp tubes blow.  I though, "What the hell was that?!" realizing it was the tubes.  I turned the unit off, let it cool down, switched out the tubes, turned it back on for about an hour and KAPLOW, it blows another tube (fairly new).

 

So, obviously the unit came to the end of it's life.  It still turned on an works but once it gets hot it blows the pre-amp tubes.  Does anyone know if this is something that could be fixed or should I start looking for a new amp before I waste time and money fooling around with it more (and potentially blowing more tubes)?

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post #2 of 8
6/18/11 at 9:08pm

You may want to make sure that the dip switches has not changed positions that you were not aware of. There could be some DC leakage coming in or elsewhere.

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post #3 of 8
6/19/11 at 6:00pm
Thread Starter 

Nah, everything is exactly as it was.  Either the unit simply came to the end of its life or I messed something up putting it back together (which would be odd).

 

I was going to look into getting a new amp anyway, so now I have a great excuse to spend the money.

 

Of course threads for, "Which amp is the best for the AKG K701" seems to be the most asked question around here with no definitive answer beyond, "Get a stupidly expensive tube amp with an output transformer."  So... there goes my bank account!

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post #4 of 8
6/19/11 at 6:11pm

by "blowing" does it look like the heater is glowing too bright and then burning out or the plate voltage is too high and arcing?

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post #5 of 8
6/19/11 at 9:37pm
Thread Starter 
Quote:
Originally Posted by LiqTenExp View Post

by "blowing" does it look like the heater is glowing too bright and then burning out or the plate voltage is too high and arcing?


The little filament thingies that bridge the pins at the button stem to their corresponding component bellow the insulating spacer completely vaporize.  The tube does glow much brighter than usual before "blowing".

 

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post #6 of 8
6/20/11 at 1:10am
You have a bad cap or resistor (or several) in there. Components change value when they get hot, so one of them is opening wide up at operating temperature. Taking a guess, I'd blame a resistor in the power supply. If you're handy with a soldering iron, replace the resistors in the power supply.

Yeah, a transformer-coupled tube amp would be a better choice, especially one built point-to-point. Not inexpensive, but it'll hold up a lot better than a cheapie on a PCB.

Or you could go solid state. Solid state might not be as trendy as tubes, but you'll get a nice, low output impedance and sound good.
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post #7 of 8
6/20/11 at 5:31pm
Thread Starter 
Quote:
Originally Posted by Uncle Erik View Post

You have a bad cap or resistor (or several) in there. Components change value when they get hot, so one of them is opening wide up at operating temperature. Taking a guess, I'd blame a resistor in the power supply. If you're handy with a soldering iron, replace the resistors in the power supply.

Yeah, a transformer-coupled tube amp would be a better choice, especially one built point-to-point. Not inexpensive, but it'll hold up a lot better than a cheapie on a PCB.

Or you could go solid state. Solid state might not be as trendy as tubes, but you'll get a nice, low output impedance and sound good.
 
 


Yeah, either way I'm going to get a new amp.  The MK III really isn't designed for good use with the K701s.  I want to get the Heed Canamp but I'll be damned if it's not impossible to find.  The Matrix M-Stage is cheap and super easy to find but I'm not sure if it'll be up to snuff considering the suspiciously price point and Chinese manufacturing.

 

I'm still going to try to fix the MK III.  It'll be good for a different set of cans in the future.

 

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post #8 of 8
6/20/11 at 11:01pm
I didn't know the Canamp was out of production. It was very popular here for awhile.

Give some thought to the Dynalo/Gilmore Lite. It's a really good circuit that's been around for years, built by a lot of DIY people, and has never had any serious faults found. Also consider the M^3, which has a nice active ground. Another great design that's been popular with geeks who build their own.
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