Question about Crack FAQ (Items 3-4)

[shujinko]

Hi there,

I saw the Crack FAQ and I want to apply the modification outlined in items 3 and 4, regarding the addition of resistors to the pot wiring.  I just want to make sure I understand it properly, specifically this part:

"Attach and solder one 33K resistor between each outer pair of lugs on each level of the volume pot."

Does this mean using a total of four 33K resistors (two on the top, two on the bottom) to go from 1-2 then 2-3, on both top and bottom?  I attempted an absurdly crude drawing in MS paint (attached).  Please let me know if this is correct.  Orientation of the resistors is not important, right?

The 75K resistors sound straightforward, any additional techniques to apply here or just trim them nice and close?

Thanks!

 

[Paul Joppa]

Two resistors, 1 to 3 top, and 1-3 bottom.

 

[adeep42]

I did the pot mod. Made a substantial difference in the volume adjustment and its really easy to do. If you live in the USA and you need the resisters, I have extras. Just PM me and I'd be happy to mail them to you.

Alan

 

[shujinko]

Interestingly enough, I just finished my second Crack build last night and this one doesn't seem to have nearly the same volume sensitivity as my older one from about 2010.  I don't have the older one in front of me to compare but is this an expected difference?

 

[Paul Birkeland]

Interestingly enough, I just finished my second Crack build last night and this one doesn't seem to have nearly the same volume sensitivity as my older one from about 2010.  I don't have the older one in front of me to compare but is this an expected difference?

Are you still using the same source and headphones that you were six years ago?

 

[shujinko]

You're right, I was at home and could have sworn I used this setup on the old Crack.  Brought it to work and did an in-place switch and volume was the same.

One other thing I've noticed with the new build is some slight channel imbalance, there is definitely a bias to the right.  Where should I be looking for fixing this?

EDIT:  This is happening at both home and work with different phones so I'm pretty sure it's the amp.  Happening at all volume levels.

Thanks,

 

[Paul Birkeland]

One other thing I've noticed with the new build is some slight channel imbalance, there is definitely a bias to the right.  Where should I be looking for fixing this?

Download a 60Hz tone to your phone, then play it through the amp (if you have the shorting mod, you'll need headphones or a 1/4" to 1/8" adapter plugged in.  If you have headphones plugged in, turn the volume on your phone down to protect your headphones).  Measure the AC voltage from tip to ground and ring to ground on the headphone jack with the volume pot 1/4 way up, 1/2 way up, and all the way up.

-PB

 

[shujinko]

1/4 Volume
Ring: 0.493 VAC
Tip:  0.507 VAC

1/2 Volume
Ring:  1.007 VAC
Tip:    1.078 VAC

Max Volume
Ring:  6.28 VAC
Tip:    6.39 VAC


EDIT:  The above measurements were with a J/J ECC 82 tube I had from my old Crack.  I can still notice some bias here, but it's nowhere near as apparent as with the RCA 12AU7 that came with the new Crack.  These are the measurements with the RCA.

1/4 Volume
Ring: 0.415 VAC
Tip:  0.505 VAC

1/2 Volume
Ring:  0.907 VAC
Tip:    1.152 VAC

Max Volume
Ring:  5.63 VAC
Tip:    6.76 VAC

 

[Paul Birkeland]

1/4 Volume
Ring: 0.493 VAC
Tip:  0.507 VAC

0.24dB

1/2 Volume
Ring:  1.007 VAC
Tip:    1.078 VAC

0.6dB

Max Volume
Ring:  6.28 VAC
Tip:    6.39 VAC

0.15dB

1/4 Volume
Ring: 0.415 VAC
Tip:  0.505 VAC

1.7dB

1/2 Volume
Ring:  0.907 VAC
Tip:    1.152 VAC

2.07dB

Max Volume
Ring:  5.63 VAC
Tip:    6.76 VAC

1.43dB

You have a little bit of a channel imbalance from your pot at low volumes that favors the left channel.  That RCA tube also happens to have a slight imbalance that favors the left channel, but with the other tube installed, things look to be solid.

Another option you have is to switch the left and right channels on the pot if you like that RCA tube.  You just have to switch the levels of the pot that each white and red wire is connected to.

-PB

 

[shujinko]

Thanks.  Okay, so this is more of a tube issue than a wiring/soldering/component issue?

 

[Paul Birkeland]

Yeah, and the second set of data points you gave was really helpful in figuring out exactly where things were getting funky, that was a great idea!

 

[shujinko]

Great!  Yeah, I'm trying out some other combinations and things sound normal.  Phew.   Thanks for your help, Paul.

 

[jjvornov]

I'm doing this volume mod and just wanted to check something. The pot's number 3 on top and bottom are tied together to ground, correct? So I can just run both resisters to the top lug which is accessable and has room for the black ground wire and the two 33 kOhm resistors? Or am I missing something that led to the suggestion that they are separate?

James

 

[Paul Birkeland]

Yes, both 33K resistors need one leg connected to ground, so your plan will work out OK.

-PB