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Quad II Valve amplifier worries

Discussion in 'Hi-Fi Stereo Systems & Separates' started by Simeon Lloyd, Dec 15, 2004.

  1. Simeon Lloyd

    Simeon Lloyd
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    :hiya: Hi people, glad I found a decent forum, I hope someone can help me out :thumbsup:

    I have a pair of old Quad II amplifiers, they sound the dogs dangly bits, but .....one of the monobloc's transformers get quite hot and one of the KT66's heater element is glowing way too bright on the other monobloc.
    I've tried swapping valves but the heater in the same socket is too bright. I've looked at the circuit diagram and in the chassis but there seems no obvious cause to this as both EF86's and KT66's come off the same 6.3v transformer tap!!!!
    When I measure the heater voltage, it is 6.3 volts!!!! infact, all voltages seem correct.

    Does anybody have any ideas?

    :beer: Cheers in advance
     
  2. alexs2

    alexs2
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    Interesting question....it's well known that the mains transformer in the Quad II's is rund very near it's absolute limits,especially if the amps are powering the preamp and other units as well.
    I also have a set of Quad II's,but the transformers run moderately warm only,and they don't power any other components.

    If you're running things like that off one amp,that may explain the mains transformer getting very hot on one,but the heater problem does sound more serious.
    You say you;ve tested the heater voltage....was this done under load,or via the socket with the valve removed?
    It could well be that a resistor in the pathway to that heater has gone out of spec and is allowing a higher current or voltage to be placed across the heater than should be.

    In the absence of anything more useful to say(I'm not an expert on the internals of these) my advice would be to contact the Emporium,who are not only very helpful,but have a good stock of Quad spares,and as far as i can remember,will repair them as well.

    http://www.emporiumhifi.com

    The following may also be of use to you....

    http://www.geocities.com/ResearchTriangle/Lab/6722/links.html

    New transformers are available for these amps from both Woodside Electronics and Sowter Transformers,and all other components are readily replaceable.
    Do beware of "modifying" or "upgrading them,as it will destroy their value,and often the sound quality as well.
     
  3. alexs2

    alexs2
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    Also forgot to add...circuit diagrams can be found here....

    http://www.tritet.demon.co.uk/q2p_amp.htm

    And just seen that you already have them....apologies!

    It's also worth mentioning that Quad,even under the current management(IAG) are still able to repair and service these,and some of the older engineers are extremely knowledgeable and helpful as well.
     
  4. Simeon Lloyd

    Simeon Lloyd
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    Thanks for the fast response Alexs2, fantastic :thumbsup:

    Thanks for the links..

    The Quads are on their own so I wouldn't have thought the transformers are under too much strain. I believe the amps to be original except for two EF86's which were Mullard replacements anyway. I originally tested the voltages with the valves removed, I will be testing again tonight with the valves in their sockets, testing the current will require desoldering the heater supply to the socket. As far as I can tell, there are no resistors between the heaters and the transformer, the 6.3 volts comes straight off the transformer to all four valves

    Thanks again.
     
  5. pcavelle

    pcavelle
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    I used to have a pair of these. They sounded great but crackled and popped a lot. Eventually I sent them back to Quad for a full service - £130 inc. shipping from memory - that sorted them out properly although they sent them back with some nasty little modern valves instead of the original KT66's, which you can now get full size replica's of btw.

    GT Audio / Graham Tricker, Quad or a number of other people can sort them out for you. Best place to seek advice is HiFi World mag, at the back.

    As they are of quite high value (I sold mine to a guy in Hong Kong for £750) it's actually worth, in my opinion, getting Quad to sort them out and supply the paperwork. Many of the components, capactitors especially, will be stuffed but if one of your transformers has gone it will be hard to replace unless (I think) you get a single scrap unit to supply the transformer from or unless Quad have them. Personally I wouldn't try to repaiur them yourself because of the the HT inside ie you really need to know what you're doing.

    regards

    Paul
     
  6. alexs2

    alexs2
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    No problem....I also just had a look at the circuit,and can see no reason for the heater problem,and run on their own,the transformers in each amp shouldn't be under excess load.
    One other thing that has been a problem over the years is KT66 copies,some of which draw excess heater currents,and have been known to cause transformer problems as a result.
    I've used both GEC KT66's,and also Golden Dragon KT66 (the retro styled tubes)with no problems,and the latter are actually very good sounding tubes.
     
  7. CJROSS

    CJROSS
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  8. alexs2

    alexs2
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    I'd second most of that,and would add that NOS valves are still readily available at a price,including Mullards and GEC's,and the transformers are also available used(at around £120),with new ones being made also,and many of which will fit the original cans.

    Another useful set of repair related links here....
    http://www.geocities.com/ResearchTriangle/Lab/6722/quadspares.html
     
  9. Simeon Lloyd

    Simeon Lloyd
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    Thanks people, it looks like I'm going to be busy for a while with all this information.

    Paul, I'm a bit worried sending them back to Quad if they're going to replace my original valves with modern ones, especially as I don't think there's much value in these manky pair of amps!! Mind you, I have seen a single broken amp go for £181 on eBay :eek:


    Thanks again for your help
    Sim :beer:
     
  10. alexs2

    alexs2
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    The Quad replacement valves arent all that bad really,as mine came with a pair when I bought them,but there's no doubt that either NOS or Golden Dragon retro's are better.

    I would have no worries about sending them back to Quad,but by all means discuss it with them,and keep your valves at home,letting them use their own set whilst they fix the amps maybe.
    You may also find that if your original valves are a bit tired that the new replacements actually sound better also.
     
  11. Jules Tohpipi

    Jules Tohpipi
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    Why not telephone Quad service on 0845 4581122 and get some free advice off Ken, who has been repairing and servicing that amp for no less than 32 years - many of which were spent in the prescence of the late great Peter Walker ?
     
  12. alexs2

    alexs2
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    Thats the guy whose name I was trying to remember when I made my post above on that suggestion as well...wonderful to speak to,and excellent free advice.
    It's a real treat to find that such things are still available in this age of automated "helplines" etc.
     
  13. Simeon Lloyd

    Simeon Lloyd
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    I spoke to a nice lady at Quad. There is a standard labour charge of £45 plus any parts used. There is currently a 4 week turn around just for a repair quote. She also told me the tell tale sign of a transformer gone is tar leaking from them, the transformer being about £156 if it has gone. I guess the only way to see how much it's going to cost me is to send them in.

    Thanks for your help guys.
    Sim
     
  14. alexs2

    alexs2
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    I think for the standard labour charge as quoted,it's a good deal,and whilst leakage from the transformer pots is a good sign of something amiss,it's not a failsafe indicator.

    There are plenty of others out there servicing Quad II's but the original is still the best,and particularly so in terms of resale value.
     
  15. Simeon Lloyd

    Simeon Lloyd
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    I would like to hang onto these amps so I'm not too worried about resale value at the moment, just sound value.

    Well now I have some answers :clap:
    The monobloc getting hotter than normal is most likely to be caused by the standard under-rated 3W 180ohm resister going out of value, this should really be 5W as the amp is putting more than 3 Watts here anyway. Also the capacitor that is in parallel with this resistor is likely to be duff from heat.
    The monobloc with the brighter valve on one socket is caused by a duff capacitor, C2 or C3 looking at the circuit diagram above.
     
  16. alexs2

    alexs2
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    Excellent news.....should be easy enough to fix,and I suppose not surprising given the age of the components.

    Glad its all relatively straightforwards.
     
  17. Nic Rhodes

    Nic Rhodes
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    sorry a bit late in here, the 3W 180ohm is usually the culprit but they are buggers to find replacements for. 5W is much more suitable but the 180 ohm is an odd value to find. Stick with the NOS even if they are a bit used, much better than the chinese ones, MUCH.

    Real leaking transformer, look for wax coming out....
     

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