shielded cable from RCA to pot?

[AllanMarcus]

Hello,

I showed my Crack to some very experienced DIYers and I discussed with them some humming I'm getting. They suggested that my input wires aren't tight enough, and that shielded cable might be better. I happen to have some unused Canare L-2T2S cable, so I was thinking of using that. It's 2 conductor shielded cable with two 23 AWG wires and a braided shield. I was thinking of using on cable run for each channel, and soldering both 23 AWG wires of each cable to the signal, and the shield to the ground, then two the two shielded cables (one for each channel) to the pot.

I realize that I should try just braiding the cables tighter, but I nicked one of the cables with the iron and melted the insolation, and that a tighter braid might make the cables too short. I happen to have the L-2T2S cable on hand, so I'm wondering what people's thoughts are on just using it for the input runs.

Thanks,

Allan

 

[fullheadofnothing]

Is the hum affected by the volume pot?

 

[AllanMarcus]

nope.

 

[fullheadofnothing]

Then its not the braid.

 

[AllanMarcus]

Crud. Ok, other ideas? I've reflowed all the joints, and the experienced DIYers I showed the Crack to felt the joints were pretty good. Given the hum is only on the right, any ideas?

 

[fullheadofnothing]

Here are some common hum culprits: mismounted hardware, bad solder joints (don't put to much stock in the "experienced" opinion; their last diagnosis ignored the facts of the situation), poorly twisted heat wires, poorly routed heater wires, tube noise, environmental issues (i.e. poor outlet grounding or physical proximity of amplifier to a electromagnetic field), poorly routed signal connections, damaged components.

 

[Doc B]

Yes it's unfortunate that your experienced friends didn't understand that the braid being ahead of the volume pot would immediately exclude it from being something that would affect a hum that is constant at any volume setting.

It's likely to be a bad solder joint that is only in the right channel circuitry. So look around the connections that would be exclusive to the right channel. It would be a great exercise to use the schematic and trace back from the right side connections of headphone jack (it could be the connections of the black wires at the headphone jack terminal closest to the chassis - very common issue) through the right channel output cap to the tube socket connection for the right channel and reflow solder joints along the way. There is also a slim possibility that it could be related to a tube. But that is typically heard in both channels, and cannot be considered until you have eliminated the much higher probability that it's just a bad connection somewhere - the cause of 99% of the issues we hear about.

 

[AllanMarcus]

Thanks. I'll check all those things.

as for my friends, they really are extremely accomplished DIYers. They comments on almost all the items you all touched on. they didn't catch the volume pot issue because the issue I mentioned to them was a volume balance issue, so they first thought of the pot, and then the wiring all over the amp, not just before the pot. they guys have built some of the best amps I've ever heard. Again, they solved a number of odd noise issues with about 5 minutes of discussion and moving a few wires a few millimeters.

That all said, I'm at a conference this week, but when I get back I will reexamine every suggestion in this form, and post pictures if I cannot resolve the hum, which is pretty minimal.

Oh, one more thing that I discovered this morning. I turned the amp on and let it warm up, then with no music playing, I listened to the hum. Again, volume was not an issue, but when I tapped on the chassis, the hum went away, then slowly came back. Does that help indicate anything?

Thanks,

Allan

 

[Doc B]

That tapping issue indicates it's a bad connection, most likely in the wiring, much less likely in a tube. Solving hum problems by moving a wire a few millimeters generally means the actual problem is (once again) bad connections. If the wires have been routed as shown in the manual there will not be any significant hum pickup.

 

[AllanMarcus]

So I got me some Chatham 6080 tubes; one 6080 and one 6080WA. All is great now! After reflowing joints, separating wires, caps, and resistors, changing the RCA to pot wires to shielded wire, swapping the stock pot with an Alps blue velvet ($11 from eBay <http://www.ebay.com/itm/320816794126>), and changed tubes, I think my crack finally has a respectable noise floor! Oh, I also got a nice Machined Solid Aluminum volume knob from Amazon <https://www.amazon.com/gp/product/B00X773QEU>.

Very probably some of what I didn't might not have made a difference, but it works great now. Happy camper here.