I’ve got an Audio Note kit version of the M3 phono stage (allegedly) and getting an occasional nasty cracking sound, i think through both channels.
It sounds like a really nasty record scratch but isn’t.
There’s no sign of arcing or component damage as far as I can see.
I’m thinking of just replacing all the valves but would appreciate any advice if it sounds like something you’ve come across before please.
(Valve rectifier based) power supply issue?
Worn out unicorn pubes?
I’ve got a couple of old stock RCA 6SN7s that I tried briefly in my Puresound 2a3 and had similar issues to what you described.
An occasional cracking sound that was very distracting and irritating so I pulled them out and used the Chinese ones that Guy sends the amp out with and no problems with them.
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Thanks Christov, I’ll get shopping
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The thing with phono stages is that they’re so sensitive that a single problem anywhere can quite easily cause noise everywhere. The most reliable way of finding out if the noise is in both channels is to unplug the lead from either the L or R output of the phono stage and run the system in mono (strictly half-stereo) for as long as it takes to establish whether the cracking is still audible. Then do the same for the other channel. If it’s only in one channel, or if it’s a lot louder in one channel, then that might help track it down. If it’s equally loud in both then that points the finger at something which is common to both channels (e.g. the rectifier circuitry, or maybe a grounding issue). If a complete valve replacement fixes it then it’s the valves, but that would be a pricey test especially if it turns out not to be the valves or even if it’s just one of them. Can you borrow some valves to try ? Which valves does it use ?
The phono stage has three 12AY7’s.
I’ve found the old ones and can put them back in to try.
Having difficulty identifying the valves in the power supply as they’re Russian.
I think this is a 6X5?
It looks to say 6U5C on it.
Ithe other one, 6430?
Another marking on it is OTK74.
The K may be cyrillic lokking at the style/font.
The second valve is, I think, a ‘6 phi 3 pi’ which transliterates (?) to 6F3P 6Ф3П, Tube 6Ф3П; Röhre 6Ф3П ID18916, Triode-Beam Power Tube | Radiomuseum.org. They’re the Russian equivalent of an ECL82 which was a common TV valve over here (actually the PCL82 was perhaps more common but that just has a different heater).
Anyway since an ECL82 contains two separate valves - a triode and a pentode - it can make a simple one-bottle voltage stabiliser and that can be very handy in a unit which is otherwise sensitive to supply rail noise. Until it breaks, of course.
Personally if I suspected a valve then I might start with replacing that one. Langrex have Svetlana NOS ones for £12 plus P&P. Anyone who’s ever worked on old tellies will have a bunch of used ECL82s in a box somewhere. I’ve never worked on them yet somehow I must have 20 old ECL82s. They’re like EF80s. They multiply when you’re not watching them.
I’m less familiar with that rectifier but if it says 6U5C then that will be a 6X5. I might have one or two of those in the loft. Again they’re not expensive though.
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Thanks Graeme, really appreciare the benefit of your knowledge and experience.
The power supply valves haven’t been changed since i bought it about 10 years ago.
On that basis I think it makes sense to change them anyway as a start.
As you said they’re not expensive.
I’ll check out langrex as Watford valves don’t appear to have any 6X5’s.
You can either deal directly with Langrex or you can buy through their eBay shop (yitry) https://www.ebay.co.uk/itm/264359377683. Oddly sometimes their P&P can be cheaper via eBay.
You’re welcome to try a used ECL82 if you like. I’m just slightly nervous because quite a lot of NOS Russian valves are actually ex-military and the Russians tended to ‘ruggedize’ their military valves. So they will survive more of a beating than the European originals on which they’re based. I don’t know the M3 circuit but if it pushes the voltage stabiliser hard then an ECL82 might fall over when a 6F3P would be fine.
The svetlana one on langrex says its a 6F3P made in the USSR, which bodes well.
Probably out of a Mig. 
The 6X5’s on there are 6X5GT/G or 6X5GT.
Do the suffixes make a difference?
They’ll be the same. The g stands for glass and gt is glass tubular.
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Valves ordered, fingers crossed that fixes things.
If not at least I’ll have some spares. 
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For what it’s worth the 6X5GT will be physically the smallest. Just occasionally space is very tight and only the GT will fit and/or allow you to screw the lid down.
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