Is My Tube Amp Unfixable? Help Needed

If it only happens in your system, could it be something upstream?  Try shorting plugs on the input and see if there is still popping sound from your speakers.  Does the sound increase when you increase the volume from your linestage?  If it does, it is not the amp, but something upstream.  How efficient are your speakers?  If they are very efficient you may hear low level sounds that all those other repair folks would not find to be as loud as you hear it.  
 

Hi Larry,

Thanks for your suggestions, but the amp exhibits noise with no preamp interconnect attached...just the amp and speaker load. And it does this attached to multiple AC outlets in different states! Additionally replacing the noisy amp with another QS Mono 120 in the same position (same everything else) results in a perfectly quiet amp. It is 100% the amp itself (a component, transformer, etc). 

Wow.

If it is under warrantee can you get your money back? I mean they can’t fix it. Take a video of it not being connected and the sound it is making for proof.

Yep, did that as well. Sent QS a video of the amp making the noise that was very clearly audible. I offered the same same file to the second repair shop but was told he did not need to see it because "the scope would catch anything wrong." Btw, my speakers are 89dB sensitive (Spendor 1/2s), so it's not like I have crazy sensitive horns, etc. 

Have you tried tightening the sockets for the left channel?  It could be a loose or bad socket causing that kind of noise.  If you are handy, you can try reflowing solder joints for the left channel.  A friend hunts down noise by touching joints with a pencil eraser to see which joints are noisy.  This is something one has to take extreme care when doing this because the amp is on and connected to speakers.

Hi Larry,

My very first guess was also a bad solder joint somewhere, but Mike personally went though the amp and could not locate any. And the binding posts have always been fine to my knowledge...nice and tight. The guitar amp shop actually did a very nice job on the new posts (proper soldering using new QS posts). I actually suspect that it might be a bad output transformer, but that is a crazy guess. I am not an EE, so I am relying on others to actually find the issue. I'm not about to reflow any joints myself because I would very likely introduce a new problem.

I'd guess it isn't the amp making the noises.

Carlsbad,

This amp exhibits noise when not connected to a preamp, and I have tried it with 4 completely different tube sets. What do you suggest may be generating the noise?

Cables and speakers would be on my list.

This is an odd situation -- the manufacturer and an independent repair shop both swear the amp is running fine with no noise, yet it is noisy in your system, even with no input.  Just a thought, but do you have any friends with decent speakers (or perhaps a local audio group) where you could try the amp in their system to see what happens? 

I'm also assuming you're not using weird speaker cables and have checked the AC in your house for problems, along with making sure there is nothing near the amp that could be an RFI/EMF source.

Damn Sad,

Made me think of her final reply

https://www.youtube.com/watch?v=jRvNg4zQ_14

No RFI/EMF sources and no AC issues, and I have tried this amp in 2 completely different systems w/ 3 pairs of speakers, in FL and now in SC. I have been in audio for 35 years and never encountered an issue like this. Amp bias is dead on as well, so no biasing issues.

I'm not sure if this will help at all, but I used to own a pair of  QS mid monos and one of them just stopped working. I sent it to Mike and he said there was nothing wrong with it and sent it back.It did work for a few days, then died again. The same as you did,I took it to a shop that mostly repairs guitar amps,restores vintage equipment,along with some home audio equipment.As soon as he saw it was a Quick Silver he told me they are notorious for the capacitors to fail.That turned out to be the problem along with a couple of other small issues.I had him replace the capacitors in both amps and check both out thoroughly.

The mystery is particularly odd because the noise cannot be detected by the repair shops, but you hear it when the amp is in different systems and when you were located in different states.  Because you hear the sound immediately, it is not the case that that those shops missed it by not having it on long enough.  I don’t expect the problem to be downstream, but you can try switch the left and right speaker wires and then the speakers to see if the problem stays in the left channel.  Perhaps a cap in one speaker is acting up.

Get another brand and model and put into your system for 1 to 2 months. If you don’t hear the issue, your problem and aggravation is gone. Get rid of the amp. Don’t let the problem and loss of money bother you. I had this same problem with a manufacturer and sold both preamp and amp back to him at a big loss. My situation was very similar to yours. I moved on from the brand entirely. The reason? There are many, many manufacturers of audio and thousands upon thousands of available equipment.... many of which is capable of bringing you joy. It's not worth the aggravation of trying to get one pair of amps working properly. IMHO.

Bad Jokes Aside:

I had a similar unresolved situation.

Fisher 80AZ Mono Blocks, made in 1958, dead silent performance for 60 years. A few resistors replaced over the years.

Then, a low volume intermittently crackling. Silent for about 30 minutes, the crackling began and lasted about 30 minutes, then gone, played silent until the cows come home.

Switch left/right/cables/speakers/ go nuts, nothing changed the 30min/30min cycle.

Took to well known tech, he played them in his shop for a week, checked tubes, changed side to side: never misbehaved. Home, problem the same. After a while tried my friend’s beloved tech, same, no noise there, but guessed and changed a few small things. Home, same 30/30 min cycle.

Had a friend/buyer audition them, no problem, loved them, sold them, never a problem at his place for 4 years so far.

Current Cayin AT88T, same cables, same speakers, nothing changed but the amp: dead silent. Hope I get 60 years out of the Cayin.

Fishers Amps came in my Fisher Console I inherited from my Uncle, so there were memories, hard to part with them, but I got remote volume out of the problem

I actually had a similar situation years back with a AudioResearch unit. And it drove me NUTS trying to get it worked out. In my case it was what the tech called a "leakey cap" meaning the behaivoir would be interrmittent - which is the hardest to diagnose. Replacing the caps fixed the issue, but I will forever wonder why the manufacturer and electronics shops I went to either missed it, or flat did not want to deal with a cap replacement. I hope this helps in some small way. Nothing to match the anxiety of the few seconds before you turn your amps on..... - Steve

Assuming this amp uses a removable (IEC) power cord:

You mentioned power cord -- are you (still) trying different power cords? I ask because I actually had a very bad IEC power cord -- bending the cord made the problem (noise/static) even worse.

In the months before discovery, my experience was pretty much what you describe. When sent/taken to techs, my bad power cord stayed at home, and the amp was perfectly noiseless on test benches.

Is the issue apparent almost immediately after power up, or does it take a while for it to start acting up?

I had a somewhat similar issue with my little tube amp recently and it turned out to be a solder point on one of the resistors, inside. I went through and added a bit of solder to all the locations that looked like they had virtually no solder and the issue is gone.  Before that, the static would take about 20-30 minutes before it started and the amp was fully heated up. I guess the heat caused some slight movement through expansion inside and was revealing a bad solder joint. 
 

If all other options have been checked a bad solder point seems like a likely culprit. Also the faulty piece of gear that works fine in the shop is pretty standard I've been through that before!

@bojack Don't panic! This is solvable. 

The first thing to do in a situation like this is move the monoblock to the other channel and see if it does it there. Apparently the other monoblock is fine, right?

If its quiet in the other location, then its a pretty good bet that something about this amp is more sensitive than others to noise that is present at the first location, like a digital device such as a wifi router, DAC and so on.

In any event this sounds like an RFI issue, which might also be caused by an oscillation in the amplifier itself. It might be just on the edge of oscillation and so behaves in other locations. Hypothesis of course.

So if the amp still makes the noise in the other channel, then what you do is remove the input tube (while the amp is off of course). It should not damage the amp to run it without this tube. The question is, is the noise still there? If not then we have a very good idea of what circuit is causing the noise!

If the noise is still there, then the 2nd (driver) tube can be removed. The driver is capacitor coupled to the output section so this can be done without damage, but again the amp must be off when this is done.

One possible way the amp can make this noise is if a certain part was omitted, called a 'stopping resistor'. It sits (or should) at the input of each tube in the amp. If not there, the tube can go into oscillation. Often people get away with not installing these resistors in their DIY project, but they are good practice. But if the amp is handwired, its the sort of thing that would be easy to forget on accident and might be really tricky to figure out later since the amp might behave, although not all the time. 

Try these tests and get back to us.

Sure glad Mr Ralph stepped in to set this in the right direction. Some real chicken little shite going on in this string. 

+1...Ralph's cool! 😀

Ralph,

Awesome advice, and thank you for these suggestions. I am going to try removing the input and driver tubes and see what happens...Stay tuned! Btw, the amp is noisy no matter where it's operated, so not an RFI issue from what I can tell.

Btw, the amp is noisy no matter where it's operated, so not an RFI issue from what I can tell.

@bojack The RFI could be generated by the amp itself due to oscillation.

Fascinating. It's like reading an Agatha Christie whodunit.

Maybe it was Colonel Mustard, in the dining room, with the candlestick?

OK, now we are getting somewhere because without the input tube (12AU7), the noise has vanished...gone. So Ralph, I'm assuming a good tech would simply check for bad components in the input stage? And I'm guessing this also eliminates things like the big filter caps, coupling caps, etc? 

Waiting for Ralph for the correct answer.

But in the meantime maybe try a different 12AU7 if you haven't already just for S & Gs

Same thing happened to my quicksilver Pre. It ended up being a loose Cap.

As it went through the mail, contact was made and it worked perfectly when Mike had it. He snt back to me and in the mail connections were jostled loose and same issue, it was sent back and forth a total of 6 times.

As I picked the pre up for the last time, after disconnecting everything, about to wrap it in its box forever, the right cap completely fell off. I explained this to mike and sent it back one last time. I asked his to check each every wire and secure them or recommend someone that would and I'll send it to them to fix. He went through it and it has be perfect ever since.

Then again....if you want to give it away???......😏

Bojack if the noise is gone without the 12au7 that tube probably has a dirty tube pin that can cause serious noise.

Again, waiting for Ralph.  But along the lines of @jond's train of thought, I've had more than one tube which did not function correctly, and I VERY slightly and gently bend the pins one way and the other to ensure a solid contact, and it worked every time.  Also, with a light gauge sandpaper gave the pins a couple of swipes.  Wiped them clean and voila.

Replace the output tube with a known good & then work your way back. It's also possible the tube sockets are wonky. Clean them with a drop of deoxit sprayed on a periodontal brush. Also clean the pins. Good luck.

OK, now we are getting somewhere because without the input tube (12AU7), the noise has vanished...gone. So Ralph, I'm assuming a good tech would simply check for bad components in the input stage? And I'm guessing this also eliminates things like the big filter caps, coupling caps, etc? 

@bojack Good work. Now we know the problem is associated with the input section. The big filter caps are likely off the hook. A loose connection is entirely possible so that should not be ignored.

If this amp is handwired (and as best I can make out, it is) then the thing to ask a local tech to look for is stopping resistors on each section of the socket of that 12AU7 (pins 2 and 7, for those keeping track). Loose solderjoints could also be an issue. But I doubt that on account of the amp has done some traveling so that should have shown up, but intermittents being the way they are, the are far more likely to show up in the customer's home than on the bench. 

If you can remove the bottom cover of both amps and place them side by side with the bottom facing you, the good amp can be a map of what the bad amp should look like inside. So you can look for differences between the two channels on that 12AU7 socket. You don't need to poke about the amp- this is just observing what is there.

Thanks to all for your suggestions, and Ralph, you are my new hero, seriously. Thank you! I really love the amp and did not want to do something nuts (i.e. toss it), so I think a solution is in sight now.

On my Primaluna the noises such as you describe were always due to 12au7 tubes acting up, or about to fail. Using the chopstick tapping method I found the tubes and replaced them.

Anything can be repaired.

Anything for which parts are available can be repaired. 😉