Mojotone NC3015 Debugging help

[_ej_]

In OP, in the pictures;

1. On 1st preamp tube socket, you have a black wire from pin 2 (?) that goes to a ground lug bolted to the chassis? And what's next to it, on pin 3? 

2. Did you ever solder up the white wire by the PT, power cord wire?

Everything going to that first half of the V1 has been disconnected.

I'm not sure what wire you're referring to.

[tubeswell]

This behavior is new since I replaced the tube socket. Before I had a much louder hum that was eliminated by unplugging the guitar cable.

So it seems there's 2 issues

As far as the new socket goes, I haven't had a close look at your wiring layout, but if you only replaced the socket and didn't touch the layout, maybe try re-tensioning the socket pin clamps

As far as the loud hum without a jack plugged in goes, check you haven't goosed up the tip switch contact on the input jack. The tip lug might not be connecting to the tip switch with the jack plug removed, leaving a floating input connection.

[HotBluePlates]

Unplug everything from the Input jack.  No guitar, no pedals.  I think there's a 99.99999999% chance all the "hum" goes away.


The only noise I hear in the video comes from "Guitar Pickup" ---> "Pedal Gain."

Yeah that's the guitar straight in. It's an ES335 with humbuckers.

So the hum remains when the guitar is unplugged. It's disappears completely when the V1b plate is grounded via a cap.
[/quote]

Cable too?  Nothing connected at input jack.

Personally, I think you're chasing your tail at this point.  I didn't hear anything to "fix" in the video, unless you want to use a noise-gate between pedals & the amp to kill pedal/guitar hum.

[_ej_]

Cable too?  Nothing connected at input jack.

Personally, I think you're chasing your tail at this point.  I didn't hear anything to "fix" in the video, unless you want to use a noise-gate between pedals & the amp to kill pedal/guitar hum.

Yeah, if nothing is plugged in I have a hum. If I ground the plate of the input stage the noise is completely eliminated. I'm going to try a low noise tube in V1 and see if I can get rid of it.

You might be right about chasing my tail though.

[shooter]

my "acceptable level" test;
set all knobs on amp to "5"
set guitar vol to zero
roll up guitar volume to 1 as you're playing
did noise "go away" at 1? 
IF yes, there is no problem

[Willabe]

I'm not sure what wire you're referring to.

There's a white wire twisted with a black wire by the PT, going to a switch on the front panel, attached to a front panel light?

Doesn't look soldered in place to the switch.

[Willabe]

Very carefully try putting a metal sheet on top off the open chassis. See if that helps any, it should.

The metal sheet will act as shielding. Use a cookie sheet. 

A member did this here a few weeks ago, killed off most of the last humm.

[_ej_]

I'm not sure what wire you're referring to.

There's a white wire twisted with a black wire by the PT, going to a switch on the front panel, attached to a front panel light?

Doesn't look soldered in place to the switch.

Oh yeah, that's the LED. I just have it tacked on there since I'll need to remove it when I install the face panel.

[_ej_]

The metal sheet will act as shielding. Use a cookie sheet. 

A member did this here a few weeks ago, killed off most of the last humm.

I'll try it tonight!

[tubeswell]

This behavior is new since I replaced the tube socket. Before I had a much louder hum that was eliminated by unplugging the guitar cable.

check you haven't goosed up the tip switch contact on the input jack. The tip lug might not be connecting to the tip switch with the jack plug removed, leaving a floating input connection.

[_ej_]

This behavior is new since I replaced the tube socket. Before I had a much louder hum that was eliminated by unplugging the guitar cable.

check you haven't goosed up the tip switch contact on the input jack. The tip lug might not be connecting to the tip switch with the jack plug removed, leaving a floating input connection.

If I understand you correctly I already tested this. I grounded the input directly with a jumper wire and it didn't affect the hum.

[tubeswell]

When the jack is unplugged, the tip-switch contact (on the socket) needs to be in contact with the tip contact (to ground the tip). The tip switch contact should be permanently grounded.

[_ej_]

When the jack is unplugged, the tip-switch contact (on the socket) needs to be in contact with the tip contact (to ground the tip). The tip switch contact should be permanently grounded.

Yeah, I understand that and my jack is wired that way.

I was worried that there was in issue with my input wiring so with nothing plugged in I attached a jumper going from my input wire directly to ground. It made no difference.

[_ej_]

I wanted to give a bit of a summary of where I'm at now as one issue was fixed but I'm still working through a second.

Originally there was a really obnoxious hum. That was the first video. Basically grounding any part of the signal path, from input to V3 would eliminate the hum. I traced it to an issue with the V1 tube socket (there was a crackle/pop when I touched the grid pin). I replaced the socket with a new one. That hum is gone.

Now I have a new different hum. It sounds like a guitar with a bad ground. However it is ever present, even when there in nothing plugged in.

I have confirmed the input is wired correctly and the switching contact has a good connection to ground. When I ground the grid of the first gain stage I get a pop and the hum changes but does not go away. If I ground the plate (via a capacitor) the hum is gone completely.

This indicates to me its an issue with the first gain stage. I replaced the grid stoppers and the wire coming from the input with no change.

I'm wondering if its a tube issue so I ordered a low noise tube and I'll see if that helps.

It is at a level that it isn't a show stopper for the amp but its loud enough that it will never stop bothering me. If there are any other thoughts as to what it might be let me know.

[Willabe]

The metal sheet has to completely cover the opening in the chassis.

That's only partial covered, wont work. 

[_ej_]

The metal sheet has to completely cover the opening in the chassis.

That's only partial covered, wont work.

Yeah, i understand that, I just grabbed the wrong sheet and didnt want to go back upstairs 

[tubeswell]

Don’t make any further changes until you’ve swapped out V1 with a different tube

[_ej_]

Don’t make any further changes until you’ve swapped out V1 with a different tube

Yup, tube is on the way. I'm hoping it's just that.

[Willabe]

The metal sheet has to completely cover the opening in the chassis.

That's only partial covered, wont work.

Yeah, i understand that, I just grabbed the wrong sheet and didnt want to go back upstairs 

     

[_ej_]

So I think the new tube has sorted everything. You can still hear a quiet hum in the room but it doesn't really come through on a recording.

I start this recording with nothing plugged into the amp, previously you would have been able to clearly hear the hum. Then I just play a chunk of a Militarie Gun song. Theres no limiter or mastering, this is just the raw track so crank it up. I'm really happy with how the amp sounds, thanks for the help everyone.

https://soundcloud.com/ejendres/nc3015

[tubeswell]

With tube amps, makes life easier if you acquire a stash of spare tubes so you have something on hand to quickly sub in at the start of the problem diagnosis - especially generic preamp tubes.


So when you need to get a new one, order 2 or more, or 1/2 dozen - and save on postage. You don’t know what you’re missing until you’ve got boxes of tubes coming out your ears.