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Valve Amp Problem - Any ideas?


lostwin
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3 minutes ago, ESK said:

In all my years of searching and swapping older 6SN7s I never found a set that was totally quiet.  They were either microphonic or gave that pop/ting type noise as they warmed up.

I've got around 50 that are both quiet and make no odd noises. Just have to select out which luckily was done when valves were cheap and duffers were just binned rather than eBayed!

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Just now, lostwin said:

Yes, good suggestion.  Actually grabbed back my back up amplifier from my neighbour this morning with intention to eliminate every other possible cause in the chain 

Many hours spent at parties where people have been struggling to get the sound system going.  I would invariably get called over after half a dozen people had descended to try and sort the problem and they'd be pulling cables out at random, flicking switches on the mixer etc.

I would always suggest stripping it down and starting again and they'd always insist on faffing around for another hour or so before calling me over again.  Within 10 mins it would be stripped down and set back up in a logical order to locate the 'fault'.  It was usually something simple :)

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Super Wammer
5 minutes ago, ESK said:

In all my years of searching and swapping older 6SN7s I never found a set that was totally quiet.  They were either microphonic or gave that pop/ting type noise as they warmed up.

Interesting - the warm up thing seems consistent with what I am experiencing. I will try and switch these over to see if the problem changes channel.

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4 minutes ago, Jazid said:

I've got around 50 that are both quiet and make no odd noises. Just have to select out which luckily was done when valves were cheap and duffers were just binned rather than eBayed!

Lucky you.

I needed a quad and with each batch there was usually a duffer.

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It could be a component that has developed a temperature sensitive fault. Most often either a resistor or semiconductor. The usual fault finding method is to use an aerosol freezer spray directed at the suspected component. This operation needs to be performed in the operational mode so must be performed by a qualified tech. Probably not the answer you wanted under the circumstances.

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I agree with Rabski - most likely a connection issue. I get it from time to time in stuff I build and it's mostly a dry solder joint. Joints can degrade with age (as if we didn't know!!). I go over the wiring with a magnifying glass with a light in it, pulling on wires with a small pliers to see if any are loose. Obviously turn everything off, pull the power cord out (don't forget this), and measure the high voltage with a multimeter before doing anything. Don't attempt this if you don't have a good understanding of how valve amps work. And in any case you may want to re-solder anything that looks dodgy so you need to know your way around a circuit. 

If you're lucky it will be a valve base or socket, so try those first. I wouldn't use any kind of spray. I'd use 99% isopropyl alcohol on an inter-tooth dental brush. They do different sizes, useful for valve bases. Some steel wool can help for valve pins if they're very tarnished. Or even a rubber can sometimes help.  

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Super Wammer

Bit of progress made last night trying to pin this issue down;

- I cleaned all the KT88 pins and socket holders.  I did find a pipe cleaner but they are not made in the same way as 'when I were a lad', the IPA dissolves the glue holding the bristles resulting in them shedding all over the part you have cleaned.  The inter-dental brush suggestion sounds a great alternative.

- Swapped speaker cables around on the back of the amp and the start up noise (after just 30 seconds) now in right hand speaker, also then switched off pre & phono with no difference to the noise issue.  The amp power lead is directly plugged to a wall socket on the end of a dedicated mains spur. Conclusion - it is one channel of the amp causing the problem. Amp was powered up for only 2 min or so from cold before switching off.

- Turned amp off for about 20 mins. Swapped the ECC81's around, still got the issue from right hand speaker but took about 5 mins before it started.

- Turned amp off for about 45 mins.  Swapped the 6SN7's around, no start up noise in the 10 mins it was on.

It is obviously proving a little difficult to test for as the issue is either a little sporadic or relates solely to a warm up problem made worse when the amp is turned on absolutely cold.  I guess I will see when trying again tonight from another proper cold start up if the 6SN7 swap has made a difference, if not then I will take a look inside again and check for any obvious connection faults.  The amp is fully hard wired with no obvious PCB's.

Appreciate all the suggestions so far :^

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This sounds like a dry joint issue. A good way  to locate the  problem ( but you need to be careful) is to switch it off and unplug the amp. Then place it on its side and remove the bottom cover.Then plug it back in with the amp connected to the system as normal but with the volume very low.Then with one had behind your back use a wooden chopstick in your other hand to carefully poke /touch all the soldered joints.You will hear a distinct click,pop through the speakers when you’ve found the culprit.Be very careful not to touch anything other than the chopstick which of course,is insulated !Then switch off, unplug and wait 4 hours for the capacitors to fully discharge before attempting to re solder the failed joint.

Edited by Hifimad1
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Super Wammer

Well I think I have fixed it! Thanks for the useful suggestions from all.

In the end I didn’t dare go down the  prodding a live amp with a chopstick route :nerves:but I did get out the magnifying glass and took a good look all round particularly focusing on the valve base that I suspected to be the cause. There was a joint where the solder didn’t look as though it was fully capturing the wire, a quick re-solder and now had two separate cold switch ons without issue.

Great to have sounds back for lockdown 👍🏻

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37 minutes ago, lostwin said:

Well I think I have fixed it! Thanks for the useful suggestions from all. I did get out the magnifying glass and took a good look all round particularly focusing on the valve base that I suspected to be the cause. There was a joint where the solder didn’t look as though it was fully capturing the wire, a quick re-solder and now had two separate cold switch ons without issue. Great to have sounds back for lockdown 👍🏻

Well done!!!

A good and well lit magnifying glass is great for trouble shooting valve amps. 

Edited by pmcuk
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I had a similar problem with my Berning ZH-270. 

I changed the ON/OFF switch which solved the problem. It had become 'furry' over the years.... 

worth a try

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