Puzzling issue with Quad II valve amplifier? - UK Vintage Radio Repair and Restoration Discussion Forum

Lacan07 · 2nd May 2019, 8:35 pm

Hi all

I seem to have a puzzling problem with my home audio set up which, through a process of elimination, I think I can attribute to one of my vintage Quad II amplifiers. Both amplifiers were recently acquired and restored by myself.

When I first noticed the issue some weeks back it occurred quite infrequently but has steadily become more prominent. At first I thought it may have been an issue with my preamp. Now it occurs usually after 10 - 20 minutes of playing music (before it occurred after 2 -3 hours) - everything is fine and then suddenly the level of gain / volume on one of the speakers rises (sometimes accompanied by a soft pop like a build up of energy) and the soundstage disappears, or more accurately, veers to the side of this speaker due to the extra gain. Switching the amp off for a while sometimes seems to reset it.

Can any experts out there offer an explanation? I'm curious to know what might be causing this. I've checked the valves by swapping out with the other Quad and they seem pretty solid. All the resistors and capacitors were updated by me 2 or 3 months back with the exception of C4 and C6. Next I'll focus on the solder joints, I hope it may be a dodgy connection or bad component. Just my luck it'll be a problem with the transformer

Cheers L7

rontech · 2nd May 2019, 8:47 pm

Years ago I had problems with dry solder joints on Quad II valve amps causing strange behaviour

Lacan07 · 2nd May 2019, 8:57 pm

What kind of problems rontech?

My amps are early 60's I think, so all the joints look dry lol. I was thinking maybe I should renew the joints associated with the output valves.

audiomm · 2nd May 2019, 10:10 pm

Dry joint on R10 and R11 (feedback) would cause volume to rise significantly. But check & double check all joints seeing as you have replaced all the components on the circuit board. Mike.

Michael Maurice · 2nd May 2019, 10:15 pm

I would concur with Mike that the problem is likely to be in the feedback circuit. I think its highly unlikely that the transformer is faulty.

Lacan07 · 2nd May 2019, 10:28 pm

Quote:

Originally Posted by audiomm

Dry joint on R10 and R11 (feedback) would cause volume to rise significantly. But check & double check all joints seeing as you have replaced all the components on the circuit board. Mike.

Thanks Mike, I'm on it

Diabolical Artificer · 3rd May 2019, 6:08 am

Does sound like a dry joint but it also sounds like one amp OP going out 180 deg out of phase possibly, odd. If a visual inspection doesn't reveal anything try poking it with a stick, a non conductive one obviously.

Andy.

Lacan07 · 5th May 2019, 8:01 am

Quote:

Originally Posted by audiomm

Dry joint on R10 and R11 (feedback) would cause volume to rise significantly. But check & double check all joints seeing as you have replaced all the components on the circuit board. Mike.

Without wanting to tempt fate this seems to have solved the issue, so big thanks again to Mike

2nd May 2019, 8:35 pm	#1
Lacan07 Triode Join Date: Apr 2011 Location: Surrey, UK. Posts: 49	Puzzling issue with Quad II valve amplifier? Hi all I seem to have a puzzling problem with my home audio set up which, through a process of elimination, I think I can attribute to one of my vintage Quad II amplifiers. Both amplifiers were recently acquired and restored by myself. When I first noticed the issue some weeks back it occurred quite infrequently but has steadily become more prominent. At first I thought it may have been an issue with my preamp. Now it occurs usually after 10 - 20 minutes of playing music (before it occurred after 2 -3 hours) - everything is fine and then suddenly the level of gain / volume on one of the speakers rises (sometimes accompanied by a soft pop like a build up of energy) and the soundstage disappears, or more accurately, veers to the side of this speaker due to the extra gain. Switching the amp off for a while sometimes seems to reset it. Can any experts out there offer an explanation? I'm curious to know what might be causing this. I've checked the valves by swapping out with the other Quad and they seem pretty solid. All the resistors and capacitors were updated by me 2 or 3 months back with the exception of C4 and C6. Next I'll focus on the solder joints, I hope it may be a dodgy connection or bad component. Just my luck it'll be a problem with the transformer Cheers L7

2nd May 2019, 8:47 pm	#2
rontech Heptode Join Date: Jun 2017 Location: Southport, Merseyside, UK. Posts: 646	Re: Puzzling issue with Quad II valve amplifier? Years ago I had problems with dry solder joints on Quad II valve amps causing strange behaviour __________________ Time flies like an arrow, fruit flies like a banana

2nd May 2019, 8:57 pm	#3
Lacan07 Triode Join Date: Apr 2011 Location: Surrey, UK. Posts: 49	Re: Puzzling issue with Quad II valve amplifier? What kind of problems rontech? My amps are early 60's I think, so all the joints look dry lol. I was thinking maybe I should renew the joints associated with the output valves.

2nd May 2019, 10:10 pm	#4
audiomm Pentode Join Date: Sep 2007 Location: Ranfurly, New Zealand Posts: 170	Re: Puzzling issue with Quad II valve amplifier? Dry joint on R10 and R11 (feedback) would cause volume to rise significantly. But check & double check all joints seeing as you have replaced all the components on the circuit board. Mike.

2nd May 2019, 10:15 pm	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	Re: Puzzling issue with Quad II valve amplifier? I would concur with Mike that the problem is likely to be in the feedback circuit. I think its highly unlikely that the transformer is faulty. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

3rd May 2019, 6:08 am	#7
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,661	Re: Puzzling issue with Quad II valve amplifier? Does sound like a dry joint but it also sounds like one amp OP going out 180 deg out of phase possibly, odd. If a visual inspection doesn't reveal anything try poking it with a stick, a non conductive one obviously. Andy. __________________ Curiosity hasn't killed this cat...so far.