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Thread: Yaqin MS-12B valve preamp

  1. #21
    Join Date: Apr 2008

    Location: Cheltenham U.K

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    I'm John.

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    Quote Originally Posted by Mike Homar View Post
    .. am I right in thinking you bought it primarily for use as a phono stage? If so, I might be able to think of a few little 'tweaks' that might help it along.
    I did indeed, so I would be very interested to hear of tweak possibilities.

    I did some voltage checks, and all seems well. The mains trans. secondaries are marked 200V and 16V on the circuit diag., and I measured them to be spot on those values with an mains input voltage of 240V AC. So I think it's reasonable to conclude that this model was actually designed for 240V operation.The main supply caps are Nippon Chemicon rated at 400V.
    The heaters of the 12AU7's are being run somewhat high, at 13.8V. The 12AX7 heaters run off a 12v regulator - they are fractionally under 12V.

    Hey, I just found a 'dead bug' inside it - a 2N5551 transistor lying near one of the phono inputs, with its legs waving in the air, not connected to anything!
    Last edited by jonners; 09-05-2009 at 17:56. Reason: Added entomological observation.

  2. #22
    Join Date: Jan 2008

    Location: Wrexham, North Wales, UK

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    I'm AudioAl'sArbiterForPISHANTO.

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    Don't worry about Mike and I - we love yanking each other's chain!

    I did some voltage checks, and all seems well. The mains trans. secondaries are marked 200V and 16V on the circuit diag., and I measured them to be spot on those values with an mains input voltage of 240V AC. So I think it's reasonable to conclude that this model was actually designed for 240V operation.The main supply caps are Nippon Chemicon rated at 400V.
    The heaters of the 12AU7's are being run somewhat high, at 13.8V. The 12AX7 heaters run off a 12v regulator - they are fractionally under 12V.
    Glad it seems safe, John - mine (and a few others) certainly weren't!!

    Dunno about this 'bug', though... What was it for, I wonder?

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  3. #23
    Join Date: Apr 2008

    Location: Cheltenham U.K

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    I'm John.

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    Quote Originally Posted by Marco View Post

    Dunno about this 'bug', though... What was it for, I wonder?
    The circuit uses them for relay-switching between inputs, so I guess an extra one just got dropped inside and left there.

  4. #24
    Join Date: Feb 2008

    Location: North East UK

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    I'm InSpace.

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    Quote Originally Posted by jonners View Post
    I did indeed, so I would be very interested to hear of tweak possibilities.
    PM me with your address and I'll send you a couple of valves to try in place of the 12AX7's.
    Shian7
    --------------------------------------------------------

    Kudakutemo
    kudakutemo

    ari mizu-no tsuki

    Though it be be broken -
    broken again - still it's there:
    the moon on the water.

    - Choshu.

  5. #25
    Join Date: Apr 2008

    Location: Cheltenham U.K

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    I'm John.

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    Quote Originally Posted by Mike Homar View Post
    PM me with your address and I'll send you a couple of valves to try in place of the 12AX7's.
    Great! Many thanks - PM sent.

    Today I tried a pair of 5814's in the 12AU7 position, but got a load of hum. I know the 5814's take a bit more heater current but I don't see how that could account for this. Any ideas anyone?

  6. #26
    Join Date: Mar 2009

    Location: Sheffield, UK

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    I'm Dave.

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    That ebay seller has quite a lot of nice looking valve gear. I've been fancying tying out a valve pre-amp on the cheap, so I've just ordered one of the Marantz7 knock off's to try out. I think they look nice and a few people seem to think they work well from what I read on a quick google search. The price is fantastic so I think it's worth a punt.

    I think I'll try swapping the volume pot for an Alps motorised unit so I can control it via a remote. There seems to be plenty of spare room in the case to allow access for modding, so there's scope for future tinkering. I'm sure more performance can be obtained by swapping the valves too, so I look forward to doing some rolling.

    As I'm not in a rush to get it, I just went for the ground shipping, so it'll be a while until I get it. For some reason I find something quite appealing/romantic about getting it shipped on a slow boat from china.

    Thanks for the tip on the seller John.
    Source: Apple TV 4K - DAC: Beresford Bushmaster Mk II - Preamp: CI AudioPLC-1 Mk II - Power Amps: Musical Fidelity 550K mono blocks - Speakers: Wharfedale Opus 3 - Cables: Mark Grant etc - Misc: Belkin PF30 mains filters.

  7. #27
    Join Date: Apr 2008

    Location: Cheltenham U.K

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    Quote Originally Posted by HighFidelityGuy View Post
    I've been fancying tying out a valve pre-amp on the cheap, so I've just ordered one of the Marantz7 knock off's to try out.
    Yes I saw that after I ordered mine. I might have gone for it if I'd seen that first.
    Let us know what it's like when it arrives. Hope it doesn't get intercepted by audiophile pirates!

    EDIT: Initially I thought this was a phono preamp, but it seems not....
    Last edited by jonners; 11-05-2009 at 09:19. Reason: As above

  8. #28
    Join Date: Mar 2009

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    Yep, I'll make a new thread when it arrives so I can let everyone know what it's like and get some modding advice. Like you say it's not got a phono stage built in unfortunately which I would have preferred but as I don't have a TT yet I can get over that. It was mainly the line pre-amp part I was interesed in tinkering with for now anyway.

    The pirates problem did cross my mind. Hopefully it will arrive in one piece and without me having to pay a ransom. I've got visions of valve and bits of wire arriving in envelopes until I pay a million dollars.
    Source: Apple TV 4K - DAC: Beresford Bushmaster Mk II - Preamp: CI AudioPLC-1 Mk II - Power Amps: Musical Fidelity 550K mono blocks - Speakers: Wharfedale Opus 3 - Cables: Mark Grant etc - Misc: Belkin PF30 mains filters.

  9. #29
    Join Date: Feb 2008

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    Quote Originally Posted by HighFidelityGuy View Post
    Yep, I'll make a new thread when it arrives so I can let everyone know what it's like and get some modding advice.
    Start by making sure it's safe on 240V, and throw away the nasty Chinese valves...
    Shian7
    --------------------------------------------------------

    Kudakutemo
    kudakutemo

    ari mizu-no tsuki

    Though it be be broken -
    broken again - still it's there:
    the moon on the water.

    - Choshu.

  10. #30
    Join Date: May 2008

    Location: Lancaster(-ish), UK

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    I'm ChrisB.

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    Quote Originally Posted by HighFidelityGuy View Post
    The pirates problem did cross my mind. Hopefully it will arrive in one piece and without me having to pay a ransom. I've got visions of valve and bits of wire arriving in envelopes until I pay a million dollars.
    Hehe - Getting it kidknapped might be a good thing.......'cos they'd send you an EAR!! That might be preferable, no?

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