• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Help with Quad II hum

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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.
 
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?
 
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 :)
 
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
 
Hi,

Have you tried swapping the amps?

I have the same problem of 50 Hz hum from the left channel on a 22/II+II. Swapping the IIs shows the 22 to be the culprit.

Haven't had time to look into it, but in my case the 22 left channel was not used until recently, so it wouldn't be surprising if a capacitor needs replacement.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.