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Old 9th May 2007, 08:17 PM   #1
irbow is offline irbow  United Kingdom
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Default I think I've damaged my amp

Hi,
I'm new to this site have a Music Angel 300B amp which has been fine for sometime now. A friend visited to hear the new single driver speakers and cabinets I've made, to compare these with my previous Kef's (the amp had been on for hours) I unplugged the Rt channel only with the volume down *MISTAKE* and we had a listen for 30secs or so then I swapped back, the amp was still on. A few days later it started making loud rustling sounds 'under' the music (not volume dependent) on the right channel with some squeal when I tap the chassis, there is also a lot of noise when I power down from the Rt channel. I have tried swapping the valves (2x6sl7's and 2x300b) with no change and have checked inside for loose connections 'bulging' capacitors etc and can see nothing wrong. So were my earlier actions a probable cause for damage and what could it be? the problem is becoming noisier but only starts after the amp has warmed up an hour or more. My first thought is to replace the main capacitors under the transformer or the resistors. I don't have a schematic for it but does anyone have any views on this?
Thanks,
Iain
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Old 9th May 2007, 09:36 PM   #2
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Hi Ian,
If you changed speakers over with the volume down I doubt very much that have damaged the amplifier. I think it is sheer coincidence that this fault has cropped up.
It sounds very much like a noisy resistor to me, so don't go changing capacitors willy nilly that doesn't help.
If you need further assistance email me at john dot caswell at tesco dot net and we can sort out phone numbers etc.

John Caswell
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Old 9th May 2007, 10:57 PM   #3
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I don't know, running an output transformer without a load can cause it to arc and destroy itself. Doesn't mean it happened here, and maybe being turned all the way down is close enough to no signal, but it is a generally unsafe thing to do.

From Electra-Print: Do not operate transformer without a proper load (speaker or power resistor) with a signal into the amplifier. This will cause the signal to be at a very high voltage due to the infinite impedance and will arc across to the secondary. This will destroy the transformer.
http://electra-print.com/singleended.php
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Old 10th May 2007, 07:08 AM   #4
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Default Damaged amplifier?

Hi all,
Well from my limited experience it is quite safe to changes over speakers "with the volume turned down" as Iain says. It is really only when you get up into high levels ie 5-10 Watts or more do troubles arrive. I fully agree with what Elektra Print say but I don't think this applies to Iain.

John Caswell
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Old 10th May 2007, 08:44 AM   #5
irbow is offline irbow  United Kingdom
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Thanks for the replies, was thinking of resistors or capacitors but I suppose a resistor is more likely to become noisy as things heat up. I'm going to try letting the problem start and gently touching the resistors on that channel with a plastic probe to hear wich on it is. I can mount the amp on a wooden workbench so I can get underneath, I will be careful.

Thanks, Iain
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Old 10th May 2007, 09:26 AM   #6
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Iain,

It's just as likely to be a poor connection/dry solder joint, and nothing to do with your speaker experiment. One way to find out is to prod various points in the wiring and various parts, carefully, with a long plastic knitting needle. See if you get find where the noise is made worse by prodding.
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Old 10th May 2007, 05:08 PM   #7
kevinkr is offline kevinkr  United States
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IF you suspect solder joints I would inspect them with a magnifying loop and resolder any that looked the least bit suspicious.

I'm not a big fan of the poking/prodding approach to diagnostics because (amongst other reasons) one of my former clients used this approach in an amp he had recently constructed and the cold solder joint was on a grid resistor in a 300B output stage, with prodding the connection went open and the tube which was vintage WE went up in a blaze of glory. A simple (re)visit with a solder iron would have prevented this.
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Old 10th May 2007, 06:04 PM   #8
Seraph is offline Seraph  United States
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Output transformer is fine or you would have had one dead channel. No worries there.

1. I would make sure the tube sockets are clean and have positive grip on the tube pins. A loose connection can cause the noise you are hearing after warm up or during power down. Just slightly bend the contacts to make sure you have good contact on the tube pins but clean the sockets. Proceed to the next step if problem is not resolved.

2. I would as indicated visually reinspect the solder joints. If you wish to prod, power off then gently move the joints looking for slight movement at the joints. Note that a cold solder joint does not necessarily move. There should only be a few solder joints just reflow the solder to be sure. Proceed to the next step if problem is not resolved.

3. Check the plate and cathode resistors around the 6SL7 in the problem channel. Pins 2 and 5 are the plates, 3 & 6 are the cathodes. Faulty plate resistors in my experience can be innocent looking offenders in disguise. I would consider replacing them if the problem persists.

4. Consider writing the seller.

Hope you find the culprit.
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Old 10th May 2007, 07:02 PM   #9
irbow is offline irbow  United Kingdom
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Hi there,

The replies on this board are very informative and are appreciated, however I did try the gentle tapping thing when I got home from work before reading the post from 'kevinkr' but it was OK. Unfortunately the noise was there from start up this time, but didn't seem to change as I touched the resistors.

I did try removing the small 7 pin RCA tube above the volume control which gave me silence, I then put this back and removed the 6SL7 on the right channel and the noise appeared at a lower volume on the left channel, when this tube was replaced the noise was in the right channel and the left seemed to be silent, does this help? is it the RCA tube or components in its vicinity?
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