Correct driver tube plate voltage for grid of output stage tube?

[theGopher]

My first post here. Before I start an upgrade job on my second hand Crack, I've read just about every relevant post I can but could someone much more knowledgeable than me please clarify a few questions I have:

1) Assuming the input stage cathode is biased using to 1.56V using the stock HLMP-6000 red LED, what is the correct voltage that needs to be on the driver tube plate/output tube grid for optimum operation of the output tube? The non-speedball version in the Crack manual has 90V, but the thread about changing the value of R1 in the input stage C4S circuit seems to favour a target voltage of 75V. Which is correct or doesn't it matter?

2) If I want to use a 5687/7119 driver tube instead of the stock 12AU7, do I need to change the cathode bias LED from the red HLMP-6000 to a blue one as per the C4S? What would the appropriate bias voltage be in this case?

Thanks in anticipation.....

theGopher

[Paul Birkeland]

75-90V is a nice range.  100V is the upper limit.

Do you want to use the 5687 with the Speedball or stock circuit? 

[theGopher]

Thanks for the reply Paul. I was veering towards with the C4S on both tubes. Why? does it make a difference to the appropriate cathode bias voltage?

[Paul Birkeland]

You can either change the bias voltage by using a different t led or substituting a resistor, or the C4S current set resistor can be changed.

Sometimes both is a good idea.