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Old 25th June 2004, 03:09 PM   #1
gary f is offline gary f  Canada
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Default Amp, Stange temporary buzz, what is that?

Hi

This is my first post on tube forum. I have a Dynaco St-70 Serie 2 from SoundValves. This amp has 2 LED on the front pannel to adjust bias on each channel.

I had this strange problem yesterday.

On startup, in normal conditions, only one of the two LED light up and gradually, when tubes get hot, the second one slowly light up and get the same luminosity as the other. this take about 1 minute.

Yesterday, I turned on the amp, everything was fine, then after 30 sec, there was a strange noise, like a soft noise for a couple of seconds, and one of the LED went off. I waited and after 10-20 seconds, the amp went back to normal. the sound was OK. I repeated the startup procedure and it did the same thing.

What is that? is that dangerous for speakers? I thought the amp was going to blow. But it worked perfectly after that.

Thanks
F
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Old 25th June 2004, 06:15 PM   #2
gary f is offline gary f  Canada
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Well, can it be a defective tube or is it more likely to be the circuit. I doubt it is a tube because the problem happen on both channels... How can I tell

F
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Old 25th June 2004, 06:39 PM   #3
dhaen is offline dhaen  Europe
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I've been waiting to see if someone posted, who had experience with this particular amp. Since no one has, I'll give some general guidance:
Try swapping the output valves (tubes) between the channels, and see if the other LED is affected instead. If so, then it's a valve problem.
Output valves are the least reliable components used in amps, so there is a good likelyhood that one is the cause. I believe they are the EL34's in this model. When you locate the faulty one, replace it's partner also.
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Old 25th June 2004, 07:21 PM   #4
gary f is offline gary f  Canada
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The tubes (valves) are a brand new set of JJ E34L, less than 100 hours. But as i said, both channels had problems. I will change to the old Sovtek to see if the same problem arise.

Thanks for help.
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Old 25th June 2004, 07:29 PM   #5
dhaen is offline dhaen  Europe
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Ah, I thought you said that only 1 LED was affected, but more careful reading of your post shows it's a bit more complicated. Sometimes a fault on one side can affect sound in both sides. Valves do sometimes fail prematurely.
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Old 25th June 2004, 11:06 PM   #6
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Hi,

Quote:
I waited and after 10-20 seconds, the amp went back to normal. the sound was OK.
From reading this I'm not convinced the tube(s) are at fault here.
Rather the softstart procedure seems to have a problem, if it's controlled by a cap charging then this may be faulty...or maybe a relay is on the blink...?

Cheers,
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