2. amps-preamps
  3. 189661-190234

**Smoking integrated tube amp, advice needed:

Hey folks, first time tube amp owner, first time poster here. 

I just received my first tube amplifier - a Mistral MT34 integrated tube amp (35 wpc), and am excited to experience the warm analogue sound that I’ve been saving toward for such a long time. After plugging it into my system, however, it almost immediately started smoking from what looked to be the preamp section/tubes, so I immediately switched it off and disconnected it. The filaments were **glowing** red, and the room filled with electrical smelling smoke.

The component chain looks like this:

Pro-Ject Perspective turntable > Bellari VP129 Phono preamp (100 ohms output impedance) via RCA to the AUX port of > Mistral MT34 integrated amplifier (100k ohms input impedance) > KEF Corelli speakers (8 ohms).

I connected the black side of each speaker cable to the white amp input, and the red side to the red 8 ohm tap of the amplifier.

Is there a catastrophic mismatch between my equipment specs that could have caused me to fry the amp? Have I set it up incorrectly? There’s a new electronic safety test sticker from April 2024 on it as I bought the amp from a charity shop on eBay, so this is throwing me. The description said it was turned on and functioning before the sale, however, so if the seller plugged it into power without connecting to speakers first, could this have damaged the circuitry/caused damage that might have led to this smoking? Would a single blown tube lead to this outcome?

I’m worried I may have fried or permanently damaged the amp, and now face a hefty spend on investigating this with a local AV repair specialist - a cost which I didn’t anticipate, and which has dampened my excitement, honestly. 

I’d also liked to know what caused the issue before I try to integrate it back into my set up and do the same thing again.

Any thoughts or advice appreciated. Thanks so much!

fugazikid1991
immatthewj
2,198 posts
 

I used to own a pair of ARCs that one or the other would routinely blow a grid resistor, but it was never as bad as what you have described (no smoking and such).  If you are feeling adventurous, unplug it and put it on a work area and pull the bottom off and see if you can identify any smoked electrical components.  Be careful around the capacitors, you might want to get a hold of a shunt to manually make sure they are discharged (although if it has been turned 'off' for a couple of days I would think that they would be).

Someone mentioned "runaway voltage" and I now have a Cary amp and a coupling cap went out and i did get a run away bias going on.  I imagine the tubes got pretty hot in the short time it operated without me catching it, but again:  the tubes were not "glowing red" and no smell of smoke.  If I hadn't have caught it, maybe they would have after a short while, so perhaps you have a run away bias going on, but I don't know anything about bias and preamp tubes and I wouldn't think it would happen "almost imediately."

When you say:

I connected the black side of each speaker cable to the white amp input, and the red side to the red 8 ohm tap of the amplifier.

are those the ONLY four binding posts for speaker cables that are on back of that unit?

 

pindac's avatar
pindac
2,864 posts
 

Your Story is very much in keeping with adice given a long time ago,  in relation to be disciplined towards a safety attitude around tube equipment.

I was informed of Red Valves and the need to have eyes on Tube Equipment at Power On and in use and double check shortly after Power Off.

I am vigilant about this attention given.

You are lucky to have discovered this condition developing, and getting it stopped asap.

The repair is beyond the usual skill set and will need a EE, preferably one that has Valve Experience.  

dynacohum
647 posts
 

Other than misusing the term input when you meant output, your setup sounds perfectly all right…assuming the white speaker terminal is labeled negative or 0.  If the amp is set up correctly for your voltage, it was defective.  You should return it.

atmasphere's avatar
atmasphere
11,896 posts
 

@fugazikid1991 Sounds like the amp was damaged prior to your placing it in your system. Was the shipping box damaged?

immatthewj
2,198 posts
 

assuming the white speaker terminal is labeled negative or 0

I was wondering about that myself.

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