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Old 1st August 2011, 04:24 AM   #1
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Unhappy Help with Quad II hum

One moment it wasn't there... and then it was. You have to press your ear very close to the speaker cone to hear it, but it's there on the left channel; the right channel is 'inky black' silent the way it has always been. The only thing different about the two Quad II monoblocks is that the left channel uses a 50+50uF for C4 and C6, whereas the right channel is closer to the original (16+16uF) with 33+33uF. They've run fine this way for nearly a year. I'll replace the 50+50uF with 33+33uF in case the poor old tranny can't hack it, but failing that, anything else I should check? I've swapped the tubes, etc. The hum coming out of the left speaker is there regardless of whether an input source is connected or not. Apart from the hum, it sounds fine, although it does run much, much hotter than its partnering monoblock.
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Old 1st August 2011, 05:56 AM   #2
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So next step was to replace the 50+50uF dual electrolytic caps with the original ones (16+16uF) -- the hum is still present, even stronger if anything, and both the mains transformer and the output transformer get extremely hot (I'd thought this was due to an overspec'd power supply capacitor)... both the hum and overheating continue. Any ideas?
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Old 1st August 2011, 08:08 AM   #3
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Sounds like one or both KT66s are taking excess current, possibly due to a fauty valve or bias fault.
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Old 1st August 2011, 09:18 AM   #4
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Or maybe leaky coupling caps from the EF86s
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Old 1st August 2011, 03:22 PM   #5
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I think your first hunch was correct. On closer inspection there's a hotspot on V3 which doesn't go away when the tubes are swapped, and it's dissipating a lot of heat. So now I hunt for what's causing it to be incorrectly biased...
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Old 1st August 2011, 03:30 PM   #6
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Well,it`s more or less got to be the coupling cap to the grid or a faulty gridleak since most of the cathode bias circuit is common to both valves. (from memory)
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Old 1st August 2011, 06:54 PM   #7
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Your memory is excellent. Except that the previous owner modded it so that each valve has its own cathode resistor and bypass capacitor. When I swapped the caps out for cheap Maplin ones I found that the negative lead was connected to the cathode and the positive to the OPT/ground. Can't be right, can it?

I should frisk up the cash for some Nichicons or Panasonics. Meanwhile it's playing music... fingers crossed! Many thanks
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Old 1st August 2011, 08:40 PM   #8
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Good work, it`s always more difficult sorting things out if someone else has been there first. I would be inclined to put it back to the original common resitor, it puts much less strain on the decoupling cap and anyway Peter Walker knew a thing or two about amplifiers.
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Old 25th April 2012, 02:15 PM   #9
ezr1 is offline ezr1  Israel
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hello
i need some help
i have quad classic ll s.n above 62,000
when i tried to replace the new tubes 6l6gc x4only one side is lighting
its the same prob. with the 2 mono block
i noticed the old tube have 5 legs and its supos. to be like a mirr. as the push-pull
where do i start to look legs 2-7 the heater or maybe some change done before inside
the bor-socket ... now the amp work but with one new tube and one old tube(same left and right)
thanks a lot
ezra
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Old 3rd May 2012, 02:31 PM   #10
ezr1 is offline ezr1  Israel
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hello
i just fix it
but now alots of"pops" -"crash" noise came out from my left amp
any ideas?
thanks
ezra
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