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DisasterArea 16th December 2010 02:03 PM

K12G left channel dead: HELP!
Well... I managed to screw up my amp already One of the wires got pulled out of my speaker on the left channel. After that one of the tubes on the left channel started glowing really bright, red hot. I got a replacement set of tubes, and all four light up properly, but I have no sound on the left channel, just a quiet hum. I checked all my solder joints and nothing seems to be shorted, I'm thinking it's got to be the OPT on the left channel blew when the speaker came unplugged, does this sound about right for the symptoms?:confused:

DigitalJunkie 16th December 2010 03:27 PM

The OPT could certainly be fried. Disconnect the primary from the circuit and test it with a DMM,That should at least give you a clue as to whether or not it's open. It's possible that it has shorted turns also,that might be a bit harder to determine.Checking the DCR of the bad transformer against the other channel's good transformer should shed some light on that.

Soonerorlater 16th December 2010 04:25 PM

If it turns out to be your output transformer, and you can't get a replacement from whoever supplied the amp kit, then it might be a good time to upgrade. Hammonds shouldn't be too dear in your part of the world, Edcors would be nice too.

Good luck


funkytek 16th December 2010 09:21 PM

Bummer. Sorry to hear about that man.
Yup, OPT's don't like to run with out a load on the secondary. Although it usually takes a bit to do this. It doesn't usually happen instantly.

The usual failure mode is shorted windings on the primary due to insulation breakdown. DJ is correct about checking the primary DCR against the other transformer. You should be able to do this in circuit by pulling the output tubes. Power off of course!
Although if it is a PP amp then you should disconnect the center tap(s).
If it has two center taps (separate primary windings) then test them separately.

If the windings look ok at DC, then try connecting a small AC voltage source (a signal generator or a filament transformer secondary) to the primary and measure the AC voltage at the secondary. Do this for both transformers and compare the difference.

It is probably a bad short or open circuit though and will show up at DC.

I rarely find OPT that fail open though. Usually you hear something but it is really distorted (shorted windings). Or you can visually see that the transformer melted down. I have seen a couple that got so hot they melted the solder on the flying leads. I figure the owners must not have been in the room to shut it down before it went into melt down.

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