Music Angel XD-SE EL34 intermittent thump - diyAudio
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Old 3rd April 2011, 08:07 PM   #1
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Default Music Angel XD-SE EL34 intermittent thump

I've got an old music angel that i just got working, when i got it, one of the EL34 tubes was glowing red. since then it sat for a few years. Yesterday I pulled it out and found the problems. there were a few poor connections and one of the tubes was bad. I re soldered most of the connections in the board and replaced the tubes with a matched quad of JJ EL34, it works pretty well and sounds pretty good!

The problem that i'm having is, while the amp is on with or without music playing it emits a subtle thump of about mid-bass pitch from the right channel. kind of like a quiet bass drum being hit with a fuzzy mallet. The thump is very intermittent, sometimes i'll hear a thump once every couple seconds and sometimes it'll go for a couple minutes before i hear another.

I hooked up my multimeter to the speaker terminals each thump is about 1.27VAC and .168VDC. Other then the thump, the DC offset is no more than .01VDC, so that's good.

I switched the two 6N9P input tubes from left to right and that didn't change anything. The bias is set to about 38mA for all 4 tubes.

Any ideas?

Thanks, Kyle.
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Old 3rd April 2011, 08:25 PM   #2
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Also, here is a copy of the schematic for this amplifier.

Click the image to open in full size.

Last edited by kyleaudio; 3rd April 2011 at 08:28 PM.
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Old 3rd April 2011, 09:34 PM   #3
DF96 is offline DF96  England
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Could be a capacitor going leaky, or a nearby RF transmitter switching on and off.
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Old 4th April 2011, 02:28 AM   #4
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capacitors makes sense, i'll do some testing and replacing and see if that helps. thanks for the idea!
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Old 13th April 2011, 08:59 PM   #5
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i found the problem... i checked all of the capacitors and switched each one from left to right and the problem was still in the right channel. then one of the right channel tubes started glowing red again... after lots of metering around i found that the right output transformer has an open circuit on one of the primary windings... unless anyone out there has a good tranny that they want to donate, i think that this music angel is done for.

it is a good looking chassis tho, i'll hold onto it, maybe one of these days i'll build something else in there.
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