Crack Re-Build: remove the hum.

[Chris65]

I built my Crack about 4 years ago, it was one of my first DIY builds & only had some minor issues at the start & has worked well. Since then I've built a few more tube projects & everything has gone well.
Recently finished the Reduction & the BeePre & both are very quiet. But my Crack has always had a hum, so I am going to re-build it in the hope that I can get it quieter.

So I ordered a new top-plate from the Queen, so it matches the new style of the Reduction & BeePre. I will use the current Speedball boards but upgrade most of the standard parts - Nichicon caps in the power supply, Tocos volume pot, JB JFX film caps on the output,
Vishay/Dale resistors.

The hum I have is constant with volume, comes just after the heaters come on.Checked all the usual things - changed location, cables, tubes, shorted the inputs but always there. Many people report very quiet Crack amps, so I should be able to get it better.
Headphones are Beyer DT990 Premium 250Ω.

Any suggestions for areas to be careful, or changes that may help? Shielded input wiring?

 

[Paul Birkeland]

Any suggestions for areas to be careful, or changes that may help? Shielded input wiring?

Being really careful about wire dress, routing, and length will be the most important consideration to getting a quiet Crack amp.

Changing out parts and wires will only confuse what the actual issue might be, so we don't generally recommend it. 

-PB

 

[Grainger49]

I would advise that you should match the stock wiring of the manual as closely as possible.  Increasing the distance from AC and DC would be advisable.  The same thing goes for increasing the distance between the signal wires and any AC. 

Just my two cents worth.

 

[Doc B]

Have you tried different tubes?

 

[Chris65]

Thanks gents. It is built stock & following the manual. As I said it was my first build & maybe some of the connections/soldering are not as good as I can do now, but the wiring was as per the manual.

Yes Doc, many different tubes: 6080 different brands, 6H13C, 5998. Various 12AU7's, 13D5, even 6F8G.

 

[Paul Birkeland]

The next thing to look at would be gain structure.  For example, if you're using power hungry headphones with your Crack, and a weak source, then you can end up with some issues.

To test your source level, you can play a 60 Hz tone through it, then measure the AC voltage coming out of your RCA cable at full source volume. 

-PB

 

[Chris65]

As mentioned, headphones are Beyer DT990 Premium 250Ω. Only listen to vinyl, through Reduction recently or solid state phono stages. Tried direct to Crack & through pre-amp (BeePre recently).

 

[Chris65]

Ok, did a lot of tube rolling & found some significant differences between 12AU7 types. RCA clear top was one of the quietest.
But I found with some tubes, if I touched one or two 12AU7 it would hum quite a bit more, rocking others would produce slightly more hum, whereas with others there was no change.
I kind of suspect a funky joint on the 9-pin socket, although nothing shows up on the chopstick test.

 

[Strikkflypilot]

Maybe the solder joints on the 9 pin are ok but one or more of the sockets are not completely
grasping one or more tube pins? Maybe less of a problem with the stock ceramic but I certainly had this issue with a teflon replacement.

 

[Doc B]

If it's one tube it's far more likely that the tube has dirty pins.

 

[Chris65]

Thanks for the replies Gents. Will see how it is after the rebuild.