Channel imbalance even at max volume

[pharaohMagnetic]

First off, it was a great joy to build the kit (my first headphone amplifier) which has really brought my HD650s to life! I have just ordered the speedball upgrade but I'd like to get to the bottom of this issue first:
There is next to no sound on the left channel until the last 25% of the pot's travel, and even at the maximum travel the left channel is not getting quite as much juice as the right. I have swapped out the 12au7 once but haven't tried another 6080 yet. The cans themselves are balanced when I plug them directly into my RME babyface. I'm wondering whether I need to upgrade the pot or if perhaps I damaged a component during testing (I accidentally bridged either A4-A5/A5-A6/ or A6-A7 (can't remember) while voltage testing the first time around, resulting in a loud pop).

I retested voltages today and it showed this:

1 75.7
2 172.8
3 0
4 173.4
5 84.8
6 0
7 100.7
8 0
9 108.1
10 0
11 0
12 0
13 173
14 0
15 193.3
20 0
21 216

A1 82
A2 0
A4 0
A5 0
A6 75
A7 0
A9 0

B1 75.7
B2 174.3
B3 101
B4 84
B5 173.6
B6 108.3
B7 0
B8 0


Thanks in advance!

[Paul Birkeland]

Try temporarily connecting the two middle lugs on the volume pot to each other, then let us know if this changes anything.

-PB

[pharaohMagnetic]

Thanks CB -

Did as you said and retested voltages as well. No major variance to first voltage readout, but when the cans were plugged in, volume was balanced between channels throughout the range, despite being mono as expected.

[Paul Birkeland]

Well, that tells you that the issue is between the volume pot and the RCA jacks.  I'd check your RCA jacks to be sure there's no solder partially shorting the left jack.

-PB

[pharaohMagnetic]

Thanks PB -

So I've inspected the area and there doesn't appear to be any wayward solder - I have resoldered the joints in question and the issue remains.
In the first test I temporarily connected the middle lugs on the pot as you recommended- are these "down signal" from the pot? Would you advise I try temporarily connecting the two "outside" lugs that connect from the RCA jacks to isolate the issue to the pot? Or am I confusing the signal path?

[Paul Birkeland]

Sure, you can give that a shot.  If you have a shorted jack, you will lose both channels.

[Grainger49]

Did you try swapping right and left input cables?  It could be in your source. 

[pharaohMagnetic]

Apologies for the delay in responding - have had a couple busy weeks.

Grainger - I have tried cable swapping etc but no luck.

PB- I connected the outside lugs which result in a mono signal but with the channel imbalance issues mentioned at the beginning of the thread. This is in contrast to connecting the middle lugs which results in a mono & balanced sound on the headphones. This suggests to me an issue with the pot. What do you think?

Thanks
JG

[Paul Birkeland]

I think you have an issue with the jack on the low channel (maybe some debris between the shell and center conductor?).