After 10 years, my Yaqin MC-100B failed

ChopperChas

Super Member
I bought it in September 2007, and it's August 2017 now. That's 9 years 11 months and some change.

I powered the amp up, hit play on the reel to reel, and walked into the kitchen. I heard a loud crackle, and then the music began to slowly fade out (as if the power switch was turned off while a source was still playing).

There was a slight smell of burning electronics, and I checked the tubes. Three were warm, and one was stone cold. I'll take the amp apart and take a look at it soon.... but before I do, any ideas what could have transpired here?

Charles.
 
Take a look under the cover: probably a dead resistor or cap and eventually dead output tube...

Post some pics. Are you handy with a voltmeter?
 
Pray it did not take out your output transformer. Part of the fun for buying a Chinese amp.
 
Fuse in the back is blown, scorch marks evident after removal of bottom panel. Upon close inspection, the 180 ohm 3W resistor next to the V2 socket (on the right side of the socket, if looking at the bottom of the amp with the front of the unit facing towards your body) fried, and even melted some wire insulation. It did not appear to melt all the way through, though. V2 happens to be the cold tube.

So what happened here? Bad tube? It was a NOS Teslovak tube, not a new tube, so it's quite possible the tube failed.

What else could have potentially fried from this?

You think I can replace that 180 ohm resistor, fit a new tube, and be on my merry way?

Charles.
 

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You think I can replace that 180 ohm resistor, fit a new tube, and be on my merry way?

That fried 180ohm resistor is a current limiting device for the screen grid. See schematic here: http://www.monstercore.nl/100B2.jpg. You can read about what can happen in ultralinear operation, which is hard on tubes, here: http://www.diyaudio.com/forums/tubes-valves/189803-ultralinear-operation.html

I'd also check that there is the expected resistances in the primaries of the related output transformer.
 
I replaced the 180 ohm resistor, another small burned resistor (100k ohm) on the lower left of one of those pics, and all the 1 ohm 3W resistors in the amp. Powered it back up with a replacement tube and all is good! Is Ultralinear really that much worse on tubes? This is the first failure I've had in the decade I've owned the MC-100B, and I've run ultralinear the entire time. Doesn't sound any different than triode, and gives me more power to drive my vandersteens.

Charles.
 
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