Can the balance control compensate for a slightly defective tube?

[NightPhotographer]

Can somebody please explain how the balance control works in Crack atwoa? Since it's downstream of tubes, I'm not sure if it can compensate for imbalance caused by a slightly defective tube.

[Paul Birkeland]

It's upstream of the tubes, but that doesn't actually matter.  The balance control simultaneously increases the attenuation of one channel while reducing the attenuation of the other.  It will compensate for gain differences of a given tube, though most imbalance tends to come from low level imbalance in the volume pot itself.

[NightPhotographer]

Thanks for the information. In this regard it is superior to the stepped attenuator, isn't it?
I was about to place an order for Crack + Speedball but I found the a-two-a upgrades very nice and tempting. Is the sound signature identical to that of Crack + Speedball? I decided on Crack since every HD 800 owner seemed to be satisfied with that combination.
There is also another thing I'm concerned about. This would be my first DIY project. The website says a-two-a is not really suitable for inexperienced builders. Is it a big no or it just takes a newbie a little more time to build it?

[lthomash]

Not if one uses two mono stepped attenuator, IMHO.  I have never understood the reasoning for a balance control over two mono attenuators?

The Crack a-two-a is a much better amp especially with the HD 800.  The only reason that HD 800 owners seem satisfied with the Crack is that they have never listened to the HD 800/a-two-a combination.  It is more expensive and has a much larger footprint, things to consider.  With a lot of upgrades to the Crack the cost difference becomes smaller, the a-two-a really doesn't need upgrades, even though I can never resist trying to improve any Bottlehead build.

You will not have any problems with the a-two-a if you just pay attention to the directions and take your time.  Spend extra time in trying to be clean and orderly and always check all solder joints.

Lee Hankins

[NightPhotographer]

Does a-two-a include any SMT soldering?

[Paul Birkeland]

Does a-two-a include any SMT soldering?

Oh goodness no.  We like our customers too much to do that! 

The balance control and volume pot combo on the Crack-a-two-a is to remedy the tracking issues at very low volume levels with the stock volume pot.  The attenuator upgrade has perfect channel balance, so this isn't necessary. 

The most imbalance I would expect from the tubes themselves would be a few tenths of a dB, so I would personally be far happier with the better sounding controls that do not retain the balance function.

-PB