random questions

[Ingber]

Just for the sake of asking:

1) What are the unused screw holes left and right of the front tube for?

2) Would it make a difference if you connect the front tube central pin to 3, instead of to the potentiometer upper ground lug? That seems shorter.

3) Isn't it nicer to use white wire instead of red for the connections between:
 A1 and 5,
 B6 and 9,
since this is the Left channel ?

4) The potentiometer varies the input signal. Could you alternatively use a potentiometer to vary the output signal directly?

Regards,

[Doc B.]

1) What are the unused screw holes left and right of the front tube for?

The Speedball upgrade

2) Would it make a difference if you connect the front tube central pin to 3, instead of to the potentiometer upper ground lug? That seems shorter.

There's already a lot of wires connected at T3. It gets more difficult to assure they are all well connected as that number goes up. The distance difference is trivial in this case.

3) Isn't it nicer to use white wire instead of red for the connections between:
 A1 and 5,
 B6 and 9,
since this is the Left channel ?

Those wires are carrying B+ voltage and the usual convention is to color the red. In general The red vs. white coloring convention is more about signal carrying wires.

4) The potentiometer varies the input signal. Could you alternatively use a potentiometer to vary the output signal directly?

You have to consider the load the device sees. A pot at the output is reasonable with low level signals and we have pots at the output of our preamps. That advantage is that self noise generated by the preamp at the output tracks up and down with the volume control. But once the requirement for more power to drive the output load gets higher (like with an amp) a pot at the output becomes more problematic. One could use an L-Pad with some power handling capability (we did this with the Tode guitar amp) but that is not really the most effective means for this application.

[Paul Birkeland]

4) The potentiometer varies the input signal. Could you alternatively use a potentiometer to vary the output signal directly?

Along with Doc's valid points about why we don't do this, there are more reasons.  The Crack has 1.5V of bias on the first stage, so if you have a DAC putting out 2V RMS, that is 2.8V peak, and you will drive the input stage into grid current territory, which is problematic for many reasons and would lead to a marked decrease in performance.  Putting the level control at the output will also lead to a variable output impedance depending on the position of the level control, and this will also result in far poorer amplifier performance than it would otherwise have.

[Ingber]

Thanks!