Background hum

[Doc B.]

Hi Viktor,

Does the hum go away when you turn the volume down all the way? What headphones are you using?

[Viktor]

Hi! No the hum doesn't go away when I turn it down, it is basically constant regardless of volume level. I'm not using vinyl at the moment, feeding it from the computer with a music streamer II. Also, I'm using HD650 headphones. Maybe it is something with the tubes, I don't know if I can muster going through all the solderings either.....And since all the measurements were correct it is a bit confusing!

[Doc B.]

Low hum levels aren't always directly correlated to bad solder joints. I would suggest trying different tubes if you have not done so already, as my hunch is that is the most likely source if the hum.  There is also a small chance that the amp is picking up the hum from a power transformer on a nearby piece of gear - if there is another power trans nearby.

[Viktor]

Ahh, okay! Unfortunaly I don't have any other tubes nearby, I'll see if it is possible to pick some up here in Sweden and try them out as well!

[Grainger49]

Viktor,

Doc supplies nice and new tube sockets with his kits.  But who knows when they were made.  There might be a little dirt/tarnish in the tube sockets.  Try removing and re-inserting the tubes 6 or 7 times to see if this cleans both the tube pins and socket.  It is easy, quick and often effective for noise.

[Viktor]

You have given me that advice several times no and I still haven't done it properly.... Of course I have taken them out and put them back in I think at least four times in total. Maybe I should also try with some de-oxidation/grease/super-duper oil as well. I have one here at home that I've used on contacts in my computers with success so maybe that could be worth a try also. I'll report back with my findings!

Thanks for all the help, most helpfull forum ever!

[Laudanum]

Viktor, Ive never been to Sweden but I would suspect that it's possible that one of the musical instrument retail stores would sell some new production tubes ... if there are any close to you.  12AX7 seems to be most popular pre-amp tube in newer production tube guitar amps but you never know.  Maybe worth a shot to make some phone calls.   

[Viktor]

Yesterday I took the big tube out and cleaned the pins for a bit then put it back in. The background hum was the same but I still listened to some music then all of a sudden all I can here is static, I was very afraid I'd destroyed my HD650 but fortunaly they seem ok. Then when I tried to turn the amp back on and listened with my SR60 ( my test headphones from here on ) there were no static at all and everything worked as before. I'm getting very vexed by this irregular behaviour, today I'm going to go trough the solderings once again as there is one part that I did a bit weir, its two of the connections for the 6080 where I put one wire to the bottom hole and the other one above, I don't think this should be a problem by maybe it is worth a try to put the both trough the same hole.... We'll see how it goes.

[Grainger49]

Viktor,

You have two problems, the background hum and the crackling noise.

There are a number of things that can cause hum.  A ground that is not well soldered becomes an "Ohmic" connection.  That can be checked by touching up all the ground connections.  

Another thing that might cause the background hum is the AC heaters to the tubes.  They should be twisted well and flat against the top plate.  The heater wires should be routed so they cross the signal wires at a 90 degree angle.

A loose ground or signal wire in your interconnects can cause hum... see the list goes on.  Have you checked with the inputs grounded?  (I think, can't remember all the posts in this thread, I think that your hum doesn't go away when the volume is all the way down.  That eliminates anything upstream of the volume control.)

The noise might be the tube, swapping it for another, one at a time, can eliminate that.  You will most likely roll tubes soon anyway so check the "Tube Rolling w/Crack" thread.  It is always good to eliminate the tubes as the source of a problem.

[Viktor]

Greatful for all the advice! I did a small checkup today and resoldered some connections including the first ones I did, seems a bit lower now but not completely gone. Are the ground connections the ones with black cable? I know that the ground often is black but not completely sure in this case!

Okay the cracking popping sound is still there though and the background hum is there as well. I think I'll order some new tubes and test those before doing anything further so I can rule that part out at least.

[Grainger49]

Another source of noise could be the source you feed into your Crack.  If it is too close it might inject noise/hum into the circuit downstream of the volume control.  Checking this costs nothing just move the source or any other electronic device away from the Crack.

[Viktor]

I have done that to no avail sadly. Right now I'm listening to some music and tapped the 6080-tube slightly and the sound went away completely in the right channel. I know about microphonics but didn't know it it should be this apparent. I will report back when I get the tubes, ordered a 6AS7GA (mostly based on looks...) and a TAD 12AU7WA. Also found a store in germany so I should get them fairly fast!

Cheers

[Laudanum]

 It sounds promising that tapping the tube remedied the noise, atleast in the one channel.  Promising in that you just may simply have a bad tube.  I learned a good while ago that it's always a good idea to have a few spare tubes on hand.  Even if they are inexpensive newer production tubes or cheaper used or NOS tubes.  The spares may not be your tubes of choice but atleast they get you up and running if a tube goes south.  Reading through the forum threads regarding troubleshooting, seems like Doc nails it on the head most every time.  If he thinks it's a problem tube, it's probably a problem tube.

[Grainger49]

Let's second Laudanum's suggestion.  For trouble shooting purposes a pair of tubes you hate are a thousand times better than a questionable tube you love.

[Viktor]

Laudanum: The cracking noise didn't go away, the noise went away completely which further strenghtens my and also Doc's thoughts that it is a bad tube. Found a store located in germany so I'll probably have them next week or so!