Crack with Speedball - Loud Humming Help [resolved]

[Deke609]

Based on your pics, it looks like the washers on the input jacks are in the wrong order. The washer with the metal ground tab should be sandwiched between the plastic washer and the nut. It looks like you have this reversed, with the ground tab washer sandwiched between the chassis and the plastic washer. If so, the ground tabs on both Red and Black inputs may be isolated from the shell of the input jacks or only making intermittent contact.

cheers,

Derek

 

[alihill77]

It is 2AM and I thought I would check the forum to see if anyone had posted any other ideas and when I saw your washer comment I could not go to sleep without seeing if this was the problem. It was 100% the problem and I don't even have buzzing at max volume now! I can't believe I made such a stupid mistake.

The real question now is why my amp worked for a year without having this buzzing issue.

I have tidied up my cabling anyway as suggested.

Thanks for your help

 

[Deke609]

Good stuff!

The amp could sound fine even with the washers reversed if the inside edge of solder washer was pulled up against the outside edge input jack shell - but then sound lousy if moved just a fraction of a millimeter.  Simply removing or inserting RCA interconnects might break the contact.

 

[alihill77]

I wrote this post because I managed to make my amp worse when trying to fix another problem though.

This problem is that there is a low but noticeable when there is silence in movies etc hum in the left channel only.

This does not change in intensity when changing volume and is present with the volume is set to 0. This is the hum I can make go away sometimes if I hit the top of the large tube amp.

Should I still try and touch up the areas circled to try and fix this? Should I create a new thread or is this one fine?

 

[Deke609]

I would definitely do as PB suggested and try to get those leads crimped to (or at least bent tightly around) the terminals/lugs and resolder them.

A hum that goes away for a while when you bang the amp may be a less than perfect ground connection. I would take a screwdriver and wrench or pliers and go around and tighten all the nuts/bolts that are used as ground connections -- i.e., any spot that connects a wire to the chassis. I solved a hum problem that way in my Crack.

If crimping/resoldering and tightening ground connections doesn't solve the problem, I would look at whether it is a problem with the left interconnect (try switching left and right interconnects) or maybe a function of the amp or even just the left interconnect being too close to the tv or some other source of interference. 

But that's all I can think of -- I'm just a novice audio  DIYer. PB is the expert on these things.  He will have more ideas.

 

[Deke609]

Another idea: if you're whacking the large tube and that makes the hum go away, the tube socket may have a flaky connection with one of the tube pins. Doc B. has posted numerous times about the potential benefits of cleaning and polishing the tube pins.  But even repeatedly removing and inserting the tube could do the trick - essentially you;d be scraping some of the tarnish off the points of contact to expose clean metal.

If it's a loose tube pin socket, you can tighten the sockets (after powering down and waiting a bit) -- I can;t remember the structure of the Crack tube sockets. Depending on structure, you'll either have to GENTLY pry a contact plate towards the center of the pin hole or GENTLY crimp the metal socket to make a more snug fit for the pin.  Either way, the goal is to make the socket grab the pin a bit more tightly.

Good luck!

 

[Paul Birkeland]

This is the hum I can make go away sometimes if I hit the top of the large tube amp.

This is still a loose connection.

 

[alihill77]

I ordered a Tung Sol 7236 tube because I was pretty sure the tube was causing the humming. As it turns out the humming has gone with the new tube and I have not heard any background noise in the hours that I have listened today.

One weird thing I noticed was that this tube seems to pick up interference from my mobile phone from further away than the old tube. Is this normal? Before I would not have any noise when my phone was also on my desk but now it needs to be around a metre away or I occasionally get bleeps.

 

[Jameson]

I rolled in a 6AS7G today and picked up interference from a cell phone I hadn't before so I don't think that's too strange.

 

[Doc B]

Yes that is normal. Cell phones radiate pretty strong signals and it is best to keep them some distance from tube electronics. The 7236 has somewhat less negative gain in a cathode follower (higher mu) than a 6080 and thus the output level will be somewhat higher for any signal, noise or music.

 

[alihill77]

Thanks for the help. I've set this thread to resolved now.