Is this tube hum/hiss?

[FairDinkum]

Hey everyone I've successfully built the kit and the ohm and voltage checks are all within specification. My shoutout to Grainger for helping me sort out the tools. The sound out of the Crack is excellent!

However, in between tracks when it's quiet, I can hear a background noise like a hum or hiss.
I did some testing and discovered that this background noise doesn't come on immediately after the amp is switched on; it comes on gradually after several seconds (presumably when the tubes get warmer). This background noise doesn't fluctuate with the volume control. The Crack is hooked up to my CD player. FYI I'm using the 240v transformer.

What do you think this is?

Also, should I expect the Crack to have totally no background noise at all? I'll be installing the Speedball in the next few days which apparently has totally silent background.

Your thoughts please?

 

[Doc B]

What headphones are you using?

 

[jboehle]

Have you tried hooking it up to a different power outlet (not on the same circuit)?  Or tried moving the Crack around the room to see if it always has the same hiss?  How about with no source hooked up?

 

[FairDinkum]

What headphones are you using?

Hi Doc I'm using the HD650.

 

[Doc B]

OK, then it's not overly sensitive headphones. If it is hiss let the amp cook for a while, as sometimes the cathodes of the tubes need to "wake up" and get their emission level up to par. If it is hum try moving the amp around a bit to see if it is picking up interference from a nearby transformer.

 

[FairDinkum]

Thanks Doc!  What's the difference between a hiss and  a hum? 

Can I go ahead with the installation of Speedball?

 

[grausch]

Also, should I expect the Crack to have totally no background noise at all?

My stock Crack is dead-quiet with no hissing or hum. I do have it plugged into its own outlet and I am at least 3 metres away from the nearest electrical appliance (except for the iphone which I use as source). To be honest, I can't imagine that the noise floor can go lower with the Speedball, but I am no expert.

 

[FairDinkum]

Oh I forgot to mention that I followed the revision in the manual on pg.19 and connected the cable from the transformer terminal 4 to terminal 14U (instead of terminal 22L). Would that have any effect?

 

[Grainger49]

You can leave the Crack on for 4 days to give the cathodes a good "forming."  You can listen whenever you want but you don't need music playing all the time for the cathodes to form.

Hiss is the sound you hear when an old style tuner is off station.  It says "SSSS" or "SHHH".

Hum is either 60 Hz. or 120 Hz.  It says "MMMMMM".  But a very low sound.

 

[FairDinkum]

Thanks everyone for your input. I've tried another power outlet with the amp just by itself and that seems to have reduced the hum (yup not a hiss) a bit. I have to listen hard for that hum. Sometimes I get microphonics which disappear when I tap on the output tube.

Just for completeness, I'll do the following:
1) reflow all my joints
2) run the tubes longer

In the mean time I'll build the speedball.

 

[Grainger49]

Good troubleshooting steps. 

To give the Crack a fair test play something at normal listening levels.  Then turn it off and wait 60s for the caps to discharge.  Remove the cables from the inputs. 

Then put a jumper (standard thing to have, you will find them so handy you will wish you invented them) and jump each input RCA jack from the center wire to the outer ground lug.  This eliminates everything feeding it as a source for hum.

If there is still hum try moving anything that is within 2 feet (0.609060M, I looked it up) away from the Crack.  Some things don't turn off just go into standby.  So unplugging everything within 2 feet will also work.

Now you know how much hum is inside the Crack.

On the output tube.  You are saying that your output tube gives microphonics while listening.  And if you tap the output tube it stops, right?  I'm going to think about that one.

On the Speedball, don't do what I did.  Read, then re-read the sticky about the new transistors.  Then have a spot of tea and read them again.  I put them in backward. 

 

[FairDinkum]

Helpful as always,  Grainger! Thanks a bunch! This jumper business sounds promising. These are the ones with alligator clips on each end?

 

[Grainger49]

Exactly.  And they last a lifetime unless you put them where they pass too much current.  I did burn one up.

 

[FairDinkum]

Well... This is what I've done so far :
1) reflow every joints
2) jump the RCA jacks to ground lug per  Grainger's suggestion.
3) tried a different wall outlet

Results:
Using the wall outlet (with a  surge protected board) at my listening station and hooked up to a source , there's a slight hum.
I jumped the RCA center pins again and hooked the Crack up to a different wall outlet by itself and with no source connected: I could barely make out a hum, if there's one at all!

This means I either have noisy power running through the first wall outlet, and /or a noisy RCA input.
I'm going to go ahead with the Speedball installation!