Bottlehead Crack Channel Imbalance Issue

[afrank]

Hello!

As the title says, I have been having an issue with my right channel audio being about 9% quieter than the left channel.

I have carefully inspected all of my wiring and soldering, and although it would be difficult to rule it out completely, I do not believe the issue is related to a cold solder after multiple careful inspections by myself and several friends.

I am wondering if this issue could be due to a tube related issue?

Here are my resistance checks:

1. *
2. *
3. 0
4. *
5. *
6. 0
7. 2.93K
8. 0
9. 2.95K
10. 0
12. 0
13. *
14. 0
20. 0
22. 0
B3. 2.93K
B6. 2.95K
RCA L: 89.0K
RCA R: 99.8K
Ground: 0

And here are my voltage checks:

1. 80.9V
2. 173.4V
3. 0
4. 173.0V
5. 84.3V
6. 0
7. 107.8V
8. 0
9. 108.6V
10. 0

Any thoughts on what the issue might be would be greatly appreciated. Happy to provide additional measurements / data if this would help.

Best,

Andrew

[Paul Joppa]

A 9% difference is less than a decibel. The two likeliest causes of small differences like this are the volume control and the tubes; both are +/- 20% parts.

Volume potentiometers are most unbalanced at lower settings - at what setting did you determine the 9% difference?

Better balanced pots are available. Better balanced tubes are also available from most dealers. At added cost in both cases, of course  :^)

[afrank]

Hi Paul, thanks for the quick response.

A note on how I arrived at the 9% level, using the windows left / right channel balance, when the left channel is at 89-91% and the right channel is at 100% the amp sounds balanced.

There is a bit of unbalance that goes in the other direction at very low volume levels (the right channel is much closer to the left) but much past the 7 or 8 o'clock position the imbalance increases to and remains constant as it is turned up. I have noticed low level imbalance issues on many amps, which goes away once the volume level is turned up to a certain amount. This seems to be a different issue.

Is there any way to identify if this is a pot issue or tube issue?

I did try turning up the volume up all the way (with the amp OFF) and measured the resistance through both channels of the pot, and it was equal. Not sure if this is an accurate way to determine if this could be causing the in balance.

[Paul Joppa]

Hi Paul, thanks for the quick response.

A note on how I arrived at the 9% level, using the windows left / right channel balance, when the left channel is at 89-91% and the right channel is at 100% the amp sounds balanced.

There is a bit of unbalance that goes in the other direction at very low volume levels (the right channel is much closer to the left) but much past the 7 or 8 o'clock position the imbalance increases to and remains constant as it is turned up. I have noticed low level imbalance issues on many amps, which goes away once the volume level is turned up to a certain amount. This seems to be a different issue.

Is there any way to identify if this is a pot issue or tube issue?

Measuring the effect of pot position, as you have done, is a pretty good test. Looks like it's a tube; most likely the 12AU7.

I did try turning up the volume up all the way (with the amp OFF) and measured the resistance through both channels of the pot, and it was equal. Not sure if this is an accurate way to determine if this could be causing the in balance.

No, you need to measure all three pins of each channel to determine balance.

[afrank]

Paul, thank you for the input, it was helpful, here is what I was able to do:

I have a friend who also likes playing around with amps and happened to have two old 12U7 tubes (I think from a guitar amp?), we tried both of them, and the imbalance persisted.  :'(

Next, I tested the pot at several different positions, and here are the resulting resistances (in OHMs):

Vol.  pos.   Left          Right
100%          1.4               1.3
2:00  pos.   54.82K       58.45K
12:00 pos.  73.8K          83.3K
9:00 pos:    82.55K        92.2K
7:30 pos:    86.2K          96.1K


Based on the above, it looks like the issue may actually be with the pot (the right channel has 7-13% higher resistance than the left), I assume this is likely the culprit of the channel imbalance? Is this a reasonable assumption?

[Paul Joppa]

Well then, it's likely to be the 6080.

The attenuation of a pot is the ratio of two resistance measurements, wiper-to-ground divided by hot-to-ground. You seem to have measured wiper-to-hot. Assuming 7:30 is all the way off, then it represents hot-to-ground, so subtract your measurement from the 7:30 measurement to get wiper-to-ground, then divide that by the 7:30 value.

Example: left channel 9:00 measured 82.55K; subtract that from 86.2K to get 4.65K wiper-to-ground. Divide that by 86.2K to get 0.0423.

Do the same on the right channel to get 0.041.

Those are the attenuation factors at the 9:00 position. Comparing the two channels, the left is 4% louder at the 9:00 setting. I did the rest and got left 7% louder at 12:00, and 7% quieter at 2:00. That kind of variation is typical for ordinary pots.

[afrank]

I am measuring between where the RCA left and right wires go into the pot (red and white), and where they come out on the way to the tube. I have not been measuring the ground (black wires). Sorry for any confusion on this. I have limited experience working with electronics, and took one physics 101 class about 10 years ago now... we spent about a week working on basic circuits.

I should have clarified that, I did not measure the volume at zero before, but here is the measurement:

                                        Left     Right
0% (or 6:00 position)   89.8K    99.8K

If it is the 6080 tube, is there any way to test for an issue without just ordering a new one and plugging it in to see if this fixes it?

[Paul Joppa]

That made a difference! Using those numbers, I calculate the gain difference (left to right) to be:

max    0%
2:00    -6.2%
12:00  +7.5%
9:00   +5.8%
7:30   +7.8%

This suggests that most of the gain difference is in the pot, at least in the range between 7:30 and 12:00. This forum has many posts about upgrading the pot.

[afrank]

I did do some research in the forms and saw the ALPS volume control come up several times. Ideally, I wanted to get the stock kit working correctly before modifying but it sounds like the imbalance I am getting is essentially within spec for the stock pot, is this correct?

If I where to go for the ALPS, is this a good way to go (see below link)? Should I be getting a different model?

https://www.ebay.com/itm/133558196511?_trkparms=amclksrc%3DITM%26aid%3D1110018%26algo%3DHOMESPLICE.COMPLISTINGS%26ao%3D1%26asc%3D264959%26meid%3D632f24009ad94ae89c32d6f456d23159%26pid%3D101196%26rk%3D1%26rkt%3D12%26sd%3D333392845354%26itm%3D133558196511%26pmt%3D1%26noa%3D0%26pg%3D2332490%26algv%3DCompVIDesktopATF2V4WithDefaultIMAFeature%26brand%3DUnbranded&_trksid=p2332490.c101196.m2219&itmprp=cksum%3A133558196511632f24009ad94ae89c32d6f456d23159%7Cenc%3AAQAJAAABELUK373GZYwoSkhvUXwZwr2SLMQzpCH9tKsUlXR5IUOV%252Bm7SAr6RG3b6Z%252BxE1GpFDaz1%252B84c7scSmyMJ0od73n%252FOdjcmNdjwMNAKtVMwBXTRhRYVe4kGknkoaVuLv3nlxDR6p2IVIwvfLoKiA90Ljd6hfmmSA8DIES9%252BcyMTSg32gaYG8EZc7%252Fnp%252Bf%252FVYzQ5tkDexNCf3qU6b6KjW9kfvKslt%252FpaKzUFfRXnU2wG2F%252B3f6BK%252B%252BHc0TWtBg6ds1CbaDxl8PrLhl9MPqyVzULqck6ubqMwkDDGIvq291NhCH%252Fm3nOmWSCX9xq3B9p72mWoBfAUUKqrZa3N8JbDaKlLZ0ggWOoYrEGQSB74tHsYHM6r%7Campid%3APL_CLK%7Cclp%3A2332490&itmmeta=01HWKXAW3ZF6Q5WZJV1M0MTH5M

[Paul Joppa]

Yes, the pot seems to be in spec.

Hopefully, PB will weigh in - he knows more than I do about the parts market. The ALPS control is indeed widely used, enough so that it is frequently faked. The price on that Ebay page seems too low to me, but I don't know enough to give good advice. I do know that you need the PC board.

[afrank]

Is there a specific online retailer that you would recommend outside of Ebay? I am happy to pay more to avoid the hassle of potentially getting a fake part that does not work as well. 

[afrank]

Just a follow up to this, I was able to find a pot for sale from Mouser:

https://www.mouser.com/ProductDetail/688-RK27112A00CC

It looks like all the specs check out, but just as a sanity check, does this look correct?

[2wo]

Parts Connection has the Alps pot as well as a few other's

/alps-100k-blue-velvet-stereo-potentiometer?_pos=2&_sid=da425443e

[Paul Joppa]

Parts Express and Mouser are both reputable dealers. Be sure to get a PC board, the pot is made for board mounting, not for direct soldering. There are man posts about it, see for example

 https://forum.bottlehead.com/index.php?topic=13725.msg125297#msg125297

[Paul Birkeland]

If you use the Alps pot, Parts Express seems to have the best price.  You'll also want to snag a PC board off eBay for mounting.  Do not solder any wires directly to those pins!  Audio Note and TKD make pots with solder lugs if you'd prefer to go in that direction.