Wrong Voltages - Solved

[Bigtone]

Hi,

This is my first big electrical project, so please bear with me. I'm from a mechanical engineering background, so this whizz bang electrical stuff was all mystical to me. This was my first foray to broaden my knowledge.

 

I'm getting incorrect voltages. Though this is intermittent. 1 side of the 12A7 lights up intermittently (with the corresponding LED), though mostly it stays off.

This is frustrating, as I'll turn it on, it'll not light up. So I'll flip it over to troubleshoot, turn it on again, and magically the LED lights up... Then without touching anything, goes out again.

I've poked around with a wooden chopstick as suggested and to no avail.

The voltages are spot on when the LED lights up. So I suspect it's still a dodgy solder joint?


Here are the voltages when the LED isn't lit. I'm hoping to isolate which joints to look at based on these? as I'm guessing 1 channel isn't correctly wired?

1. 148
2. 164
3. 0
4. 164
5. 77
6. 0
7. 147
8. 0
9. 100
10. 0

I'm on 240V AC if that makes any difference.

Thanks,

[Paul Birkeland]

The solder joint at A4/A5 almost has to be at least one of the issues you're having.  That joint not being 100% will not allow both halves of the 12AU7 to light up, and one dark side means one side won't conduct, and that's what you're observing.

Be sure not to fixate on the LED itself, as you can destroy that pretty easily in the process of reheating the solder joints on either end.

[Bigtone]

Thanks Paul for the quick reply.

I've re-soldered A4/A5, which did look a bit wonky.

Voltages are the same as before however. Any ideas?

[Bigtone]

I resoldered some joints and success! Thanks Paul!

I had an odd occurance though. But this might be naivety given this is my first Tube based amp.

I let it warm up then plugged in a set of HD6XXs.

The sound was thin at the bass and treble, and oddly hollow sounding for about the first 10 minutes or so.


Then, part way through, there was a crackle and then quite a loud pop, and then suddenly all the bass and treble came in, similar in effect to having a pair of headphones only partially plugged in, then seating all the way.
Is this anything to be worried about? Or was I just premature in plugging in?


I have to say, the sound is bloody gorgeous. Like Melody Gardot rolled in maple syrup. But almost inexplicably, with lots of detail as well.

I can't believe the sound actually gets better than this? I can't wait for the Speedball upgrade.

[Paul Birkeland]

It sounds like you still have an iffy joint.  One of the super common ones I see in repairs is where the black wires meet at the headphone jack.  If in doubt, you could also post some photos of the build for us to look over.

[Bigtone]

Thanks Paul.

I re-flowed those joints and the pops and crackle all disappeared, as well as the ground loop hum. All fixed!