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Thup-thup-thup and static coming from my power amp

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Posted on December 12, 2016 at 14:17:41
Wilbur Pan
Audiophile

Posts: 16
Location: New Jersey
Joined: February 18, 2008



This isn't really a from-scratch DIY tube amp, but I'm trying to rehab my tube amp, so I thought this would be a good place to get some guidance.

I have an old Anthem preamp/power amp combo. It's about 18 years old, and seems to be your typical EL-34 based push-pull power amp. A while back, static started coming from the speakers. When I checked it out, one of the resistors in the power amp was cracked, which was an issue I've had in the past with this amp. I've attached a picture of the schematic for this amp. It's R54 that failed. I'm pretty sure the purpose of the resistor is to provide a way to check the bias on the power tubes.

I sent the power amp to Parts Connexion, which is affiliated with Anthem/Sonic Frontiers back in the day. They replaced the resistors (R53-R56), the power tubes, and some of the driver tubes.

When I got my amp back, it was working well, but over a two week period it's developed this fast thup-thup-thup-thup-thup-thup sound, with occasional static. This is a new sound to me. I've swapped all the tubes I can between the left and right channels except for a 12AX7 that's the first input channel tube, and the sound still persists.

Any hints as to what may be wrong? By process of elimination, it looks like if it's a tube problem, it's that first 12AX7. Or could something wrong have happened when the resistors were replaced?

 

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RE: Thup-thup-thup and static coming from my power amp, posted on December 12, 2016 at 14:22:47
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 My guess would be the output stage coupling caps (C11,C12,C13,C14) or a decoupling cap in the power supply.


 

RE: Thup-thup-thup and static coming from my power amp, posted on December 12, 2016 at 15:46:21
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 R54 is a cathode resistor and can be damaged if it gets overheated - one way this can happen is if the tube loses bias and passes excessive current.

Is the noise in both channels or just one? That will help to narrow it down.

But as soon as you finish posting the answer to my question go thoroughly CLEAN AND RETENSION THE POWER TUBE SOCKETS! Or replace them if they are worn out or damaged. Poor pin-to-socket contact could be the cause of the resistor problem and the noise!

 

RE: Thup-thup-thup and static maybe an oscillation, posted on December 12, 2016 at 19:20:48
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Unfortunately, that could be a tricky one. Since it came on after awhile.Best to try the other remedies first. Excessive feed back is one of the causes of amp oscillating. Failing power supply caps can also cause that. good luck

 

RE: Thup-thup-thup and static maybe an oscillation, posted on December 12, 2016 at 19:21:29
Wilbur Pan
Audiophile

Posts: 16
Location: New Jersey
Joined: February 18, 2008
 The noise was just in the right channel at first, which is why I was trying to narrow it down by swapping the left and right power tubes and then the left and right driver tubes.

But now it looks like the noise is coming out of both channels equally.

The power tube sockets are in good shape. I just checked them out again.

 

How old are the main power supply caps? (n/t), posted on December 12, 2016 at 19:46:23
FenderLover
 mt

 

Static and occasional thup...., posted on December 13, 2016 at 02:08:42
andy evans
Audiophile

Posts: 4378
Joined: October 20, 2000
 Had static and occasional thup in a preamp. Swapped parts until I located it in a pcb I had re-worked. Bad contact.

If it's in both channels it could be the power supply..... not sure why it migrated.

In my case it was in a sensitive part of the circuit - the preamp. Noise might be more obtrusive in the input, but who knows with these things.

Bad contact is my guess.

 

the 12ax7, posted on December 13, 2016 at 06:53:47
altecboy
Audiophile

Posts: 404
Location: Northern NJ
Joined: May 29, 2004
 Hi Wilbur.
You haven't changed out the 12ax7 from what I can tell. With the amp off, pull that first tube and turn the amp on. It would be best, whatever you do, to use a pair of cheap speakers just in case something pops.

 

RE: Thup-thup-thup and static maybe an oscillation, posted on December 13, 2016 at 08:49:02
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 I don't mean to be a pest or be insulting about this - but how did you check the sockets? And even if you did check them clean 'em well anyway!! You might be amazed at the dirt in there. It wouldn't hurt to clean the 9 pin sockets either.

If the noise is from both channels it has to be either something common to both channels (like the 12AX7) or the same fault showing up in both channels.

 

RE: How old are the main power supply caps? (n/t), posted on December 13, 2016 at 17:34:59
Wilbur Pan
Audiophile

Posts: 16
Location: New Jersey
Joined: February 18, 2008
 They're original to the amp. The whole amp is as it was when I first got it, except for the tubes, and the resistors which were replaced with the service I had done.

 

RE: Thup-thup-thup and static maybe an oscillation, posted on December 13, 2016 at 17:37:45
Wilbur Pan
Audiophile

Posts: 16
Location: New Jersey
Joined: February 18, 2008
 No worries. I used a toothpick to see if there was any dust in a few of the holes where the pins go. There wasn't. I took a hobby brush, dipped it in some isopropyl alcohol, and swabbed out the holes anyway.

I've played guitar through a tube amp for a long time, so I'm somewhat familiar with basic upkeep.

If you have a better way of cleaning out the sockets, I'm all ears.

 

RE: How old are the main power supply caps? (n/t), posted on December 14, 2016 at 01:16:01
FenderLover
 If that amp is older than 20-25 years --- those filter caps maybe drying out and not adequately filtering between stages. You could be getting some coupling and motorboating.

 

RE: How old are the main power supply caps? (n/t), posted on December 16, 2016 at 09:39:48
ZENTISH
Audiophile

Posts: 57
Joined: December 16, 2009
 I had a weird noise like that coming from my speakers, turned out it was my wifi signal getting in my system somehow. So now I turn off my wifi when listening and there is no noise.
Tish

 

RE: Thup-thup-thup and static maybe an oscillation, posted on December 18, 2016 at 02:36:48
altecboy
Audiophile

Posts: 404
Location: Northern NJ
Joined: May 29, 2004
 You might consider using Caig Deoxit (Partsexpress carries it). It attacks the corrosion that can build up on the tube pins and the tube socket contacts.

 

RE: How old are the main power supply caps? (n/t), posted on February 13, 2017 at 07:31:17
Wilbur Pan
Audiophile

Posts: 16
Location: New Jersey
Joined: February 18, 2008
 Resurrecting this thread: as it turned out, it was my wifi system causing the weird noise and static. Moving my router around fixed that issue.

 

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