Right channel SLIGHTLY quieter than left channel

[starnerd67]

Just finished building the base Crack last night.
Sounded great initially. But over the course of time i noticed a slight bias towards the left channel over the right.
Hooked it up to a sescom A/B switch to another amp of mine and could tell that the right channel was slightly quieter.
The end result is that the mid point of sound in the headphones is like half a foot left of center.

Based on https://forum.bottlehead.com/index.php?topic=6931.0
I resoldered as many joints as i could solder but that didn't seem to change anything.

 

[Paul Birkeland]

How are your DC voltages?  If you plug your phone into the Crack and use the volume control on your phone but leave the level control all the way up on the Crack, how is the channel balance?

 

[starnerd67]

Resistance Checks:
1 * 2.9
2 * 2.94
3 0
4 * 2.18
5 * 1.94
6 0
7 2.92
8 0
9 2.95
10 0
12 0
13 *15.1
14 0
20 0
22 0
B3 2.92
B6 2.95
RCA L 94.4
RCA R 96.9
Ground 0

Voltage Check
s:
1 75.6 V
2 170.8  V
3 .4 mV
4 170.5 V
5 79.7 V
6 .3 mV
7 98.2 V
8 .6 mV
9 101.3 V
10 .3 mV


Turned the Crack all the way up (volume control from the computer) and there is still a left channel dominance. (that was really scary btw LOL)

 

[Paul Birkeland]

You may just need to let the kit run for a while to let the tubes settle in.  It's still possible that there's a flaky solder joint or a tube that's not 100% balanced, but letting the amp get 100 hours on it would be a good next step.

 

[starnerd67]

I guess i'll turn it on and let the tubes burn in
I'll take another pass on the joints later.

If it is an unbalanced tube, is that more common in the output or the pre-amp tube?

 

[Paul Birkeland]

I would tend to suggest the 12AU7 over the 6080, though it's pretty rare to have an audibly unbalanced tube. 

 

[starnerd67]

Was looking around the forum for additional advice.
I tried playing a 60 Hz tone at max volume and tested the voltages from the tip and ring of the jack.

tip: 10.83V
ring: 10.42V

I'm pretty sure its not the potentiometer cause in incoming and outgoing voltages on both L and R are 1.29V
A2 and A7 are both 1.29V

I kinda get lost on where the L and R go after they go into the tubes though (Im a software engineer not an EE lol)

My guess is that L and R are outputting to A6 and A1 respectively based on the voltages below, and those are the only two that seem to make their way to the output tube.

A1 14.76V
A6 16.44V
(Voltages on both go to zero when the 60 Hz tone stops so i'm fairly certain that these are the L and R)

If that's right. my guess is that it's an input tube imbalance?

 

[Paul Birkeland]

If your meter is able to resolve AC in the presence of DC, and you have 14.76V AC at A1 and 16.44V AC at A6, that's a difference of almost a dB.

At the jack, that difference drops to 1/3 of a dB. 

If anything your power tube may be masking some imbalance related to the 12AU7, which could be the tube itself or a connection that isn't 100% soldered.

 

[starnerd67]

Are there any joints in particular that i should check since we can narrow down the voltage difference between A1 and A6?

I tried reheating all the joints along the path leading in and out of A1 & A6.
Channel imbalance remains.
(attached images of the input and output tube joints)

Otherwise i guess i'll need to try a new input tube.

 

[Paul Birkeland]

4L and 5L could be filled with solder pretty easily.

A2 doesn't look soldered.

3L is also pretty crucial and I would want to see the joint reasonably full of solder.

 

[starnerd67]

I tried resoldering those joints. The jack, A1, and A6 voltages were basically the same as before...
So the channel imbalance continues

 

[starnerd67]

Just wanted to follow up if any of the previous joints seem off?
I feel like i've reheated and soldered all the joints multiple times now with no difference.
So either i'm overlooking a joint or one of the solders i think is correct isn't.

Voltage and resistance checks still pass.

 

[Paul Birkeland]

9L might benefit from some extra heat and extra solder.

 

[starnerd67]

That didn't do it either...  :'(

Well i have some replacement tubes coming on Monday. Hoping those will fix it.

 

[starnerd67]

So i just got replacement tubes.
Interesting results.

Replacing the input tubes had no noticeable effect.
Replacing the output tube (6AS7G) and right channel sounds right again.

Tested the voltages at the jack to make sure i was hearing things correctly. (60hz at max volume)

tip: 9.5V
ring: 9.9V

A1: 14.4V
A6: 16.05V

So now it's slightly Right Channel dominant. But i wasn't able to really perceive it until checking voltages.
The A1 and A6 terminals are about the same voltages as before. The output is still imbalanced, just in the opposite channel though less noticeable.

Is this an indication that it's the problem is in the Output tube?

 

[Paul Birkeland]

Only time will really tell.  Often we find that tube rolling can move connections and even the act of inserting the tube will scrape of oxidation on the pins, so down the road you may find that the channel balance shifts again and more investigation is warranted.  Often channel imbalance that is actually occurring within the tube will even out with some extended burn in time. 

 

[starnerd67]

Burn in it is then!

At least the channel imbalance now is mostly imperceptible unless i'm deliberately looking for it.

 

[starnerd67]

It's been a couple years and I ended up putting the amp away for awhile because of the channel imbalance.
Pretty sure its coming from the tubes cause the intensity of the channel imbalance changed with the tube. But its still pretty frustrating to deal with and I didn't want to buy more tubes without a way to correct for the imbalance.

So instead of buying another tube, I bought a Left/Right RCA volume controller:
https://www.sescom.com/products/view/product/productslug/ses-rca-lvl-lr-stereo-dual-rca-left-right-volume-control

I can't guarantee that any tube i get will not be slightly left or right leaning. So this is one way to control it without modding the amp.
Enjoying it much more now.