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ncdrawl
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Amp Malfunction. Acrid Smoke. Tube Thermal Runaway. Suggest a repairperson?

So I was watching CARS with my daughter today , enjoying a relaxing time when all of a sudden..BOOOOM.. on instinct I grabbed my daughter and dove behind the couch. Came up cautiously to see smoke pouring from the bottom of my Mastersound Due Venti(20 WPC SET (4x el34, 2x ecc82). and one of the tubes cherry red. evacuated daughter and pregnant wife from the house, went back in , unplugged the thing, and put it in a shipping box.

now what? the amps are made in italy and i aint sending no 60 lb boxes there. good tube gear repair folks in the USA?

websites? names? numbers?

mrlowry
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Re: Amp Malfunction. Acrid Smoke. Tube Thermal Runaway. Suggest

Did you check with Mastersound's American importer to see who they would recommend? Service is a big part of what an importer/distributor is supposed to do as part of their duties. Audio Classics( http://www.audioclassics.com/ )is well known for their repairs of old McIntosh tube gear. I've worked with them before in that respect and can say that their service was quick and thorough. I'd be surprised if they didn't work on other tube gear too.

wkhanna
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Re: Amp Malfunction. Acrid Smoke. Tube Thermal Runaway. Suggest


Quote:
......when all of a sudden..BOOOOM..


Not only does that suck, it can be more than a bit frightening!
I remember not too long ago watching my tweeter as it reached critical mass, spewing blue plasma out in all directions, and then fizzle to its death under a plume of white smoke. Fortunately it was no problem to find another one and replace it, I was back in business within a few days.

Hope you can get it fixed easily and quickly, and most of all, affordably!

Jan Vigne
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Re: Amp Malfunction. Acrid Smoke. Tube Thermal Runaway. Suggest

It's very difficult working on a piece of equipment without a schematic. There are numerous tube repair technicians around but if they don't know whether they are getting the right readings from a circuit or component, they might have a difficult time repairing the amplifier. Pentodes run as triodes makes this even more difficult.

Before you ship the amplifier anywhere, I would at least take a look at what happened inside the amp. A KABLOWIEEEE usually means a cap has exploded inside the amp. This could be just a bad cap that ended its life in a dramatic fashion - Italians like that sort of thing - or it could be a problem in the circuit in front of the cap or a bad tube behind the cap. If you're feeling adventurous and you have a spare tube, a load resistor or junk speaker and a source for parts, replace the cap and slowly bring the amp up to power. Use either a Variac or a regular household dimmer but don't just plug the amp into 120VAC. Placing a lamp (light bulb) in series between the dimmer and the amplifier will give some indication of current draw. If the lamp starts to glow brightly before the amp is powered up, shut the amp down immediately, it is drawing excessive current and will require further repairs. If the lamp and the amplifier come up to power normally, it might have just been a defective capacitor.

If you don't know how to drain power supply caps and where not to place both hands when working inside an amp, disregard this last bit of advice. You might only get a little nip but it's not worth killing yourself for an amplifier.

If you don't care to work on this yourself, I would also suggest you contact the amplifier's importer or US distributor.

ncdrawl
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Re: Amp Malfunction. Acrid Smoke. Tube Thermal Runaway. Suggest

commsysman
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Re: Amp Malfunction. Acrid Smoke. Tube Thermal Runaway. Suggest

First thing; I have had a lot of tube amps, and the first thing I always look for, when I see smoke, is a burned-out resistor at the cathode of a power tube. These are usually 2 watt resistors, and they are often designed so that they act as a sort of a fuse when a power tube shorts out internally, or conducts excessively because of a bias problem in the grid circuit. The fix may be to replace the resistor and the power tube, but there could be other problems too(the cathode is pin #8 on your EL34 power tubes). Another common problem is a failure of the DC grid bias due to a circuit problem there; loss of negative grid bias will cause excessive plate current in a tube that is not defective (until the excessive current overheats it, that is...).

The circuit you posted there is a very very simple circuit; any technician experienced with tubes should have no problem troubleshooting that unit.

Second; a good repair facility:

An excellent one on the east coast of the U.S. is Ben Jacoby's in Brooklyn...High-End Audio Repair; phone # is 718-377-8010.

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