Minor channel imbalance

[kill_surf_city]

I’m planning on finally building my Speedball upgrade this weekend but before I do I wanna resolve a slight channel imbalance. The right channel is about 1dB louder than the left. Sometimes I don’t notice it but I notice it enough to where it bothers me. Probably best to get it out of the way before I upgrade.

What can I do to either bring the right channel down 1 dB or bring the left channel up 1 dB?

 

[Paul Birkeland]

Is this the same at all volume levels on the pot?

 

[kill_surf_city]

Is this the same at all volume levels on the pot?

I believe so. I know there’s an imbalance at all levels but it think last time I tested it the imbalance stayed consistent with the increased volume.

 

[Paul Birkeland]

Have you tried a different 12AU7? 

 

[kill_surf_city]

I think I may see where the problem is. I took new readings. It's showing the left channel as having slightly higher voltage though. And after re-looking at my solder job it possibly could need re-done at the top middle joint on the pot (left channel joint). I've attached a photo.

I'm testing at AC Volts (V~). Amp is powered off, tubes are still in.

At volume in the middle I get 0.104V for the left channel (white) and 0.094V for the right channel (red).
At full volume I get 0.634V for the left channel (white) and 0.633V for the right channel (red)

The reading is taken at the middle lugs on the volume pot.

The reading at the RCA inputs is 0.633V (white) and 0.634V (red)

I notice when i put the clip on the top middle lug (white) I get a less consistent output. it shoots up to 0.186 to 0.201V sort of randomly unless I adjust the clip. I've attached a photo. Looks like maybe that middle top lug could use re-soldering?

 

[kill_surf_city]

photo

 

[kill_surf_city]

Have you tried a different 12AU7?

Ok, I re-soldered that joint and it didn't change anything.

 

[Paul Birkeland]

Yes, in your photo the white wire in the top middle lug of the volume pot isn't soldered. 

If there are any other joints in the amp like this, they need to be reflowed.

In the meantime, if you turn the volume pot all the way up, what AC voltages do you get at the middle lugs on the volume pot?

 

[kill_surf_city]

Yes, in your photo the white wire in the top middle lug of the volume pot isn't soldered. 

If there are any other joints in the amp like this, they need to be reflowed.

In the meantime, if you turn the volume pot all the way up, what AC voltages do you get at the middle lugs on the volume pot?

at full volume, I get 0.634V on both joints.

 

[Paul Birkeland]

You could spray some electronics cleaner into the inside of the pots just to be sure they are clean, otherwise you could consider just replacing the pot.  While it is normal to have some imbalance at very low levels, that's not usually the case beyond the first 1/4 of a turn or so.

 

[kill_surf_city]

You could spray some electronics cleaner into the inside of the pots just to be sure they are clean, otherwise you could consider just replacing the pot.  While it is normal to have some imbalance at very low levels, that's not usually the case beyond the first 1/4 of a turn or so.

Okay. Yeah I checked all the other joints and they all looked good. I even re-soldered the red and white wires that lead down to the pot and i still get the same readings. 0.0100 mismatch - almost exactly 1dB. Also shouldn't the louder channel be putting out more voltage? I'm getting the opposite. Either that or my ears are off.

 

[Paul Birkeland]

Yes, the louder channel should put out more voltage. You can also check the AC voltage at the headphone jack with headphones plugged in.

 

[kill_surf_city]

Yes, the louder channel should put out more voltage. You can also check the AC voltage at the headphone jack with headphones plugged in.

I’m starting to think it’s my ears. I played a mono album for my wife and she says the left channel (the higher voltage one) is clearly louder. But for me it’s still the right channel that seems louder. But I’ll check out the voltage with headphones plugged in and see what it says. When I do that, do I need to have the amp turned on? Or just the same as before but with headphones plugged in?

 

[Deke609]

I’m starting to think it’s my ears.

You could test that by swapping red and white wires at the headphone jack.

cheers, Derek

 

[kill_surf_city]

You could test that by swapping red and white wires at the headphone jack.

cheers, Derek

Good idea. I’ll try that later. Thanks!

 

[Deluk]

Can you just put your cans on the wrong way round or swap the cables at the earpiece end. Easier than changing stuff on the chassis.

 

[Deke609]

Good point! I'm a bit embarrassed for not thinking of and suggesting that.  :-[