Right Channel Imbalance - RCA & Pot Voltages OK, Headphone Jack Bad

[dill3377]

Hello everyone, my crack w/ speedball has been working flawlessly for almost 3 years now. I recently moved and accidentally dropped the tubes, resulting in my left channel not working since the 12AU7 tube failed (internally popped and turned black). I got a new JJ 12AU7 tube and my left channel is back, but it is much quieter. I have measured these voltages using a 60hz sine wave and my DMM set @ 2V AC (or 20v when measuring headphone jack).

RCA L = .391v
RCA R = .390v

Pot ¼ = lower, 0.30v; upper, 0.29v
Pot ½ = lower, .60v; upper, 0.59v
Pot 1/1 = lower, 0.387v, upper, 0.384v
-All pot measurements are at the middle lug (though I measured the left/right lugs and upper and lower matched)


Headphone Jack (Pot Max) L (tip) = 3.63v
Headphone Jack (Pot Max) R (ring) = 4.33v


Everything measured from 12U.

-Swapping headphones makes no difference
-Swapping RCA jacks make no difference
-playing with the cables makes no difference
-tapping the case/tubes make no difference


I figure it might be my 6080 tube and will be ordering a new one, but was wondering if you guys had any suggestions. Thanks!

[Paul Birkeland]

I suspect you have a loose connection in the amp somewhere.  Your measurements demonstrate that this isn't between the RCA jacks and the volume pot, as that is very tightly matched. 

It's also possible that the tube you got has horrendous balance.  This could even out with usage.

[dill3377]

Thanks Paul. I'm ordering a new 6080 and another 12AU7 as a just in case. I'll see if those fix the issue, if not, then I'll reflow everything.

[dill3377]

I suspect you have a loose connection in the amp somewhere.  Your measurements demonstrate that this isn't between the RCA jacks and the volume pot, as that is very tightly matched. 

It's also possible that the tube you got has horrendous balance.  This could even out with usage.

I got two new tubes, both a 6080 and a 12AU7 (ECC82). I started by keeping the current 6080 in the amp and swapping out the 12AU7 tube; I got the exact same voltages as the other ECC82 (so these tubes are good).


I then swapped in the new 6080 and got these voltages:

Headphone Jack (Pot Max) L (tip) = 4.41v (originally 3.63v)
Headphone Jack (Pot Max) R (ring) = 4.73v (originally 4.33v)


I believe that's about a 0.6dB difference, much better than my prior 1.53dB, but still very noticeable. Nonetheless, there's still a bit of a difference with the pot maxed out. Do you have any thoughts on this discrepancy? Just a loose connection like you initially said? Thanks!


Edit: I am also hearing a VERY loud pop when in the left ear when I first turn on the amp. I'm playing around with how long I leave it off for to see if there's a correlation between time off and whether a pop occurs; it doesn't appear if I quickly power cycle and is now not appearing with longer breaks (but will leave it for sometime now).

Edit 2: popping seems to have stopped entirely, but imbalance is still there.

[Paul Birkeland]

How is the solder joint where the black wires meet on the headphone jack?  Is one of those wires loose? 

I do believe you have something loose or a cold solder joint that is causing your left channel to be a bit weak.  Swapping tubes can sometimes just move a flaky connection to make it work a bit better temporarily.

[dill3377]

How is the solder joint where the black wires meet on the headphone jack?  Is one of those wires loose? 

I do believe you have something loose or a cold solder joint that is causing your left channel to be a bit weak.  Swapping tubes can sometimes just move a flaky connection to make it work a bit better temporarily.

I have attached a poor (sorry about that) picture of the solder joint where the black wire meets the headphone jack. The solder looks similar on the other end of the black wire and also where the bare wire stretches across the jack; I wanted to take more pictures but my phone was being difficult. None of the wires seemed loose.

I left my soldering station at my parents and so will need sometime before I can reflow things, but I will do that once I get it all back to my place. I did poke around with a wooden chop stick, and with my hand, and found no loose wires - everything was surprisingly solid - except the B4 terminal itself (not the wire) felt a little loose; the rest of the octal socket terminals felt great.

Thank you so much again!