Standard Crack Channel Imbalance + Noise

[3ph00n]

Hi everyone,
I built the standard crack a few days ago. The process was pretty straightforward.
However after some time I discovered 2 issues:
1. some cracking in the left channel when the tubes warmed up (no problem so far as it stopped after 1 minute, the cracking was also audible with no headphones on just by listening to the tube) but after some listening time and a few days later the cracking doesnt stop when the tubes are warm (cracking not audible outside the headphones).
    I can create some strange cracking sound in the  left channel by tapping the chassis next to the 6080 tube or the base of the 6080 tube itself but the cracking won't stop after tapping.
    From time to time the left channel also hums slightly (not the typical 50 Hz buzz, it's slightly higher)
    I already tried reflowing the solder on the 8 pin socket and the terminals near it... No effect (I don't think i got a cold spot, I had soldering experience before however you can never be sure)
2. the right channel is louder than the left one
    I already tried padding the pot (which looks a bit funky because i only got smd chip resistors around) but the imbalance is still there
    The left channel puts out .125 V while the right channel puts out .155 V with the pot turned halfway up

Can you help me with this?
Kind regards,
David Bacher

[Edit] My setup looks like this: PC -> Schiit Modi 2 -> Crack -> Sennheiser HD 6xx    I tried different sources and different headphones but the problems were still there

 

[Paul Birkeland]

How are your voltages?

If you tap on the chassis and get pops and crackles, there is a solder joint that isn't doing its job.  (I would look beyond the octal socket)

 

[3ph00n]

thats the strange thing... it's not really a popping sound but more like some kind of metallic sound as if the filament inside the tube is expanding and scratching somewhere (I know that because of the heat this is the case but you shouldnt hear it... Right?)

I turned the chassis around and resoldered some spots but the problem still persists.
After visual inspection of the solder spots I tapped every single spot but couldn't recreate the noise... however i could do so by tapping on the 8pin socket and the tube directly.

I rechecked the resistances and everything is correct, my voltages are as follows:

1 74.7 V
2 183.2 V
3 0
4 183.2 V
5 80.1 V

6 0
7 106 V
8 0
9 113.8 V
10 0

 

[Paul Birkeland]

You can also try removing and inserting the 6080 a few times, in case the tube pins are a little oxidized and not making good contact.

 

[3ph00n]

I tried that (even cleaned all the pins + socket with isopropyl alcohol) and it had little to no effect.
The crackling seems to fade away after some time leaving the crack on (maybe due to burn in of the tubes, a few days ago this didn't happen even after listening for a few hours).
The channel imbalance however is still there (I fed the crack a 50 Hz sine signal and measured the output voltage with an AC voltmeter)

Thanks for your help anyway! Is there anything else I could try to do?

 

[Paul Birkeland]

What voltages did you get at the output with your 50Hz tone?

 

[3ph00n]

I measured the following (after padding the pot):

Pot    Left      Right
1/4    .08 V    .10 V
1/2    .23 V    .28 V
3/4    1.11 V  1.25 V
1      1.83 V  2.01 V

1/4 of the pot means I turned it to nine o clock, 1/2 is 12 o'clock etc.
1 is as far as it goes

I don't know the exact input voltages as I used my smartphone with volume turned to maximum and a signal generator app to feed the crack.

At first (right after assembly) I thought the issue is the pot so I padded it...
Resistance before padding:
Left: 98 kOhm
Right: 88 kOhm

After padding:
Both: around 98 kOhm

 

[Paul Birkeland]

It is odd that everything is about 1dB off, especially if the pot is turned all the way up.  I would repeat this test but measure the AC voltages available at the RCA jacks themselves.

 

[3ph00n]

The Input voltage is 0.98 V (Left) and 0.99 V (Right).

I just realised that I made a slight mistake at measuring the output voltages the first time... I measured them without load (I just plugged an adapter in so the plug isn't shorted and then measured the voltage directly on the pins of the plug body).
After this I re-measured the voltages and they are slightly lower (about 10 percent) but still not in balance.

 

[Paul Birkeland]

When the pot is all the way up, the input and output are shorted.

I would redo the measurements with the pot all the way up, as the outputs of the pots should be as balanced as what comes into the RCA jacks.

 

[3ph00n]

Input Voltages: 0.98/0.99 V
Output Voltages: 1.35/1.53 V

Measured with load (headphones plugged in) and volume pot at max.

 

[Paul Birkeland]

The output of the pot is the middle lug on each level of the pot, you'll want to check there to measure channel balance of the pot itself.  You do not need headphones connected for this.

 

[3ph00n]

Oh sorry I misread that, I thought you meant the output of the amp.
The voltage at the output of the pot is 0.233 V (Left) and 0.236 V (Right) with the input voltages I mentioned above...
(The pot is padded so I think these measurements are reasonable?)

 

[Paul Birkeland]

OK, turn the pot down half way and repeat the measurements, then down 1/4 of the way and repeat. 

 

[3ph00n]

Pot    Left          Right
1/4    0.01 V      0.011 V
1/2    0.03 V      0.032 V
3/4    0.144 V    0.146 V

Please notice that the voltmeter I use isn't very reliable with voltage differences as low as a thousandth of a Volt. Sometimes I got measurement errors as big as 0.025 V (guessed average) in between measurements.
But once I connected all the leads the measured voltage was stable at the values you can see above.

 

[3ph00n]

I will try to follow the signal path from input to output and try to get some measurements using the 50 Hz tone. Maybe this could help?

 

[Paul Birkeland]

Now you can repeat this test with the amp running and no headphones connected.  See if your meter can read AC volts between ground and terminal 1, as well as ground and terminal 2.

 

[3ph00n]

Both readings are around .007 V

 

[Paul Birkeland]

Yeah, some meters won't measure AC in the presence of DC. 

If the pot isn't the balance issue, then it could either be a flaky solder joint or a tube that isn't well balanced.  Often times a tube that has a channel imbalance will remedy itself if you leave it running for a few days. 

 

[3ph00n]

Is it possible to check if the problem is the tube beside using it and waiting for the problem to fade away?
If it's the tube, does the warranty cover a replacement of the tubes?