HELP!: my SET amp was turned off without a load and now it sound veiled! - diyAudio
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Old 22nd October 2010, 03:45 PM   #1
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Default HELP!: my SET amp was turned off without a load and now it sound veiled!

i had no idea hifi amps were subjected to same precaution as guitar amps, when i turned on my SET amp without speakers connected to it. the amp is Spark 743D, and has slow start indicator- red then green. the amp went to green after usual warm up period. a minute later the 300b tubes suddenly went bright blue then the amp went back to red- protection mode. i turned off the amp immediately when i saw that. there was no smell of any kind and the transformers were cool to the touch. but now the amp sounds veiled and muddy- everything sounding like an AM radio broadcast. did i fry the 300b tubes? oh boy.. please let me know.
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Old 22nd October 2010, 04:00 PM   #2
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Hi, from what I've been reading on the net. It looks like you might have fried your output transformers.

How long was the amp on without the speakers connected to it?
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Old 22nd October 2010, 04:19 PM   #3
M Gregg is offline M Gregg  United Kingdom
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Hi,

you need to check your bias. Do you have a schematic that you can scan and put on the site! Others will be able to advise when we have something to work from. You can then take measurements.

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Old 22nd October 2010, 04:34 PM   #4
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Were you feeding audio into the amplifier when you powered it up without a load?
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Old 22nd October 2010, 04:59 PM   #5
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no there was no audio to the amp. input cables weren't even connected. is it really possible that the transformers are fried? i smelled nothing burning and they were cool to the touch. i have no means of checking the bias. though this problem makes me regret not getting a multimeter sooner. the amp was on green for less than a minute when it kicked back into protection mode. more inputs appreciated. much thanks, gents.

p.s. the 300b tubes are unequal blue glow between the pair. one has a barely noticeable blue glow and in different location- the bottom plate. the other one glows mainly around the top plate... just a clueless info since i have no knowledge about these things...

Last edited by PreSapian; 22nd October 2010 at 05:08 PM.
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Old 22nd October 2010, 05:09 PM   #6
M Gregg is offline M Gregg  United Kingdom
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Hi,

You need to take a step back here! You have a problem so you need to fault find, put emotion on hold until you have proved what is wrong.

Do you have a schematic?
Are there any instructions with your amp?

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Old 22nd October 2010, 05:11 PM   #7
M Gregg is offline M Gregg  United Kingdom
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Is the fault on both channels?

When your amp was running O.K. did you have a blue glow in the tubes?

Last edited by M Gregg; 22nd October 2010 at 05:16 PM.
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Old 22nd October 2010, 06:11 PM   #8
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I've seen 300B tubes with the blue glow. That doesn't mean that the tubes are necessarily bad.
If the amplifier had no input, it seems unlikely that the output transformers were damaged.
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Old 22nd October 2010, 06:14 PM   #9
M Gregg is offline M Gregg  United Kingdom
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Is there a set up procedure when output tubes are changed?
Or is it plug and play?

If there is a procedure try doing a setup as if the tubes were new!

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Old 22nd October 2010, 06:24 PM   #10
DF96 is offline DF96  England
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Is this parasitic oscillation? I guess this could happen without a load, but it would not persist once the load was back in place unless the high voltage has caused insulation breakdown in the OPT so now it has shorted turns.
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