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Help with Mechanical "knocking"Noise in ARC Classic 60 Amp

I have had a modified Classic 60 for about 25 years. It worked fine with an occasional retube until a few years ago. One day after moving components around on my rack, there was a massive flash over with the expected horrendous sounds from both speakers. I shut everything off as quickly as possible.

I have always used a Tice Power Block for my entire system. It turned out that when I had plugged my CAT preamp into the Tice, I apparently did not fully insert the plug into the socket. I corrected this and everything worked fine but now there was a light mechanical "knocking" sound coming from near the back, left corner of the amp after being on for a few minutes. It was like what I would expect if I tapped lightly on the chassis with a plastic screwdriver handle. The noise continued to happen for varying periods every time I used the amp.

About a week later, there was another massive flash over and sure enough the preamp plug was loose again. Maybe my wife had bumped it with the vacuum cleaner. I tightened the pins on the preamp cord plug and pushed it tightly into its TICE outlet. Once again, everything worked fine but the knocking noise was still happening from time to time.

Then, about an hour after I restarted the system, there was another smaller flash over in one tube only and it's grid resister popped. I pulled all the tubes and checked them on my Weston 981. One KT-88 was dead.

I sent the amp to ARC and asked them to check the entire amp out, repair as necessary and diagnose the knocking noise. They replaced all of the power tubes with 6550 Russian =C=, two damaged resistors, and the 4 main filter caps one of which they said was completely open. Altogether, they charged me over $1600 plus made me buy a new box ($150) because they said the old one was "weakened" and they absolutely would not return my amp in it. However, they only bench tested the amp and failed to play it to try to diagnose the knocking noise.

So, does anyone have any ideas on what might be causing the knocking noise?
 
You paid $1750 for ARC to repair the amp and it is still knocking?

$1200 for the power tubes and filter caps. $100 per part. I was pissed that they seemed to ignore the issue I sent it to them for. I could have replaced the tubes and caps myself. Of course, I would have had to buy a capacitor / ESR tester but that would have been a smarter move if I had known how it was all going to go down.The real insult to injury was when they refused to send the amp back unless I bought their new box. It was "buy the box or pay us $350 to dispose of the amp (plus some crazy evaluation fee). Did I mention the $95 it cost to send it to them? I will never send anything back to them again.
 

Duke58

Member
2009-01-03 8:00 pm
I've owned one ARC amp in my 62 years. I had a module fail on the amp I don't remember what model it was. Couldn't buy a module from ARC because they were no longer made. They told me to buy their newest model, which cost more than $5000. My car wasn't worth $5000 back then. I sold the amp, the car and now own only 12 amplifiers. Not one is an ARC amp.

Good that audiowize will help you out. Good luck with the amp.
 
If you'd like to make the short drive to the Seattle area, I would be happy to take a look at it. Hearing a tapping noise while the amp is just sitting there running is very concerning and absolutely worth looking into.

Thanks. I might have to take you up on the offer if no one comes up with a possible cause that turns out to be a fix.
 
To track down the cause, you would have to methodically unload the amplifier. This has to be done in such an order that you don't cause any damage when powering the unit up. Eventually you will remove some section and the noise will go away, then you can focus on that section specifically.

It might also be that when running the amp on the bench with the cover off that the culprit can be identified and repaired without those efforts.
 
Just throwing this out there.

Do those amps use a muting or speaker protection circuit/relay?

I don't think so. There has always been a thump in the speakers when it is powered down. I have always powered the system down sequentially: source, then preamp, then amp last. This sequence results in the least loud speaker thump.

It seems to have a slow start circuit though. When it is powered on there is click sound sound at the front after about 10 seconds when the low voltage circuit indicator light comes on. That sound is similar to the one I am concerned about but it is distinctly different and is clearly in a different location. It has always happened and Iam very familiar with it's sound.
 
To track down the cause, you would have to methodically unload the amplifier. This has to be done in such an order that you don't cause any damage when powering the unit up. Eventually you will remove some section and the noise will go away, then you can focus on that section specifically.

It might also be that when running the amp on the bench with the cover off that the culprit can be identified and repaired without those efforts.

That sounds like a professional's method that is way beyond my capabilities. LOL
 
<snip> I have always powered the system down sequentially: source, then preamp, then amp last. This sequence results in the least loud speaker thump.

<snip>

That seems counter-intuitive, generally doing the reverse would be considered the correct order for shut down.

The one thing I would look for is signs of low frequency instability on that channel (or both) - a loud periodic tick coming from the output transformer would indicate the presence of a very low frequency pulse. Should be able to see this with a digital storage oscilloscope. Note that this tick could also be generated by your pre-amplifier so check both.
 
That seems counter-intuitive, generally doing the reverse would be considered the correct order for shut down.

The one thing I would look for is signs of low frequency instability on that channel (or both) - a loud periodic tick coming from the output transformer would indicate the presence of a very low frequency pulse. Should be able to see this with a digital storage oscilloscope. Note that this tick could also be generated by your pre-amplifier so check both.

Yep. I agree and used the reverse for my solid state amps, When I first began using tube amps I did the same but got very loud thumps in my Martin Logan Sequels. I tried the order I now use and the thump when I turn off the various ARC amps I have used is much "softer". I have no idea why.

On start up, I sequence: amp first, preamp second (the CAT has a much longer soft start time than any amp I have used) and source last.