Tubelab SE buzzing noise from the speaker - diyAudio
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Old 17th July 2009, 03:22 PM   #1
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Default Tubelab SE buzzing noise from the speaker

I have built a monoblock Tubelab SE as discussed here.

Two days ago, I accidentally sent a test tone signal right after turning it on (not warmed up yet) and saw sparks flying inside of rectifier tube and the fuse blew. By the way, is this normal? So I replaced the fuse and turned it back on. This time with no signal being sent. Well, within couple seconds, there was a spark somewhere inside the chassis (not the rectifier tube) and the fuse blew.

I opened up the chassis and found the outside leg of D2 (FRED diode) partially melted and charred surface around it on the board. I replaced both D2, D3 and cleaned up the board. Then I went through the standard "check out" process and all voltages are fine with all the tubes in. I even replaced the rectifier tube (5AR4) with a spare one. I thought, "Whew, that wasn't so bad" until I noticed a buzzing noise from the speaker. It wasn't there before the damage and only D2, D3, rectifier tube and the fuse were replaced. I'm guessing that something else got damaged during the accident. Can someone take a guess on what it could be?

Thanks in advance.
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Old 17th July 2009, 03:35 PM   #2
rknize is offline rknize  United States
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Probably killed a capacitor, mostly likely one of the filter caps in the bias power supply.
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Old 17th July 2009, 04:37 PM   #3
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You mean C6 and C7?
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Old 17th July 2009, 04:48 PM   #4
rknize is offline rknize  United States
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That would be my first guess. Also check R5 and R6.
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Old 17th July 2009, 07:49 PM   #5
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Default oops!

It turns out, those caps and resistors are fine. While moving things around the circuit board, I must have bent the ground wire to the volume pot in a way that it broke the solder. It wasn't noticeable visually but when I physically checked, it was detached, thus the loss of ground connection. Oh well...

My bigger concern is, is it normal for this amp's rectifier tube to spark when input signal is sent while the tubes are warming up? I have Dynaco ST-70 amp and have built a 2A3 SET amp based on Angela Electronics schematic and they just don't produce sound in the same situation. The sound comes out gradually as they warm up without the violence I've seen with my TSE. Could it be that my rectifier tube was already bad?
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Old 17th July 2009, 08:06 PM   #6
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I do that all the time. In the TSE, the input tubes come up way after the outputs, since they are directly heated. I'm going to guess that it was probably a coincidence with the test signal. For both the B+ and B- diodes to scream bloody murder, it sounds like there was a short somewhere.

Look carefully through the tops of the output tubes. Does the cathode wire look nice and straight or does it come close to a grid wire anywhere?
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Old 17th July 2009, 09:59 PM   #7
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They are straight.

If yours doesn't have a problem with that kind of start up then I'm guessing my rectifier tube was bad. It's one of the cheaper ones in the market (current JJ model).
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Old 18th July 2009, 02:08 AM   #8
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I finally augmented diode to rectifier plate mentioned here. Something I should've done long ago.

My TSE mono is now back to normal.
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