Speedball LEDs not lighting on one side

[Paul Birkeland]

The plate current is fixed by the C4S board.  It is set by the R1 resistors.

[tintin]

Ok, I am just trying to learn some more about how to read and interpret tube traces. I am looking at the E80CC data sheet, and I understand the cathode bias by the LEDs is 1.5V so does that mean I should be trying to stay between the 1V and 2V curves? If I'm reading 80V plate voltage that would correspond to about 3mA, is that correct? And if it is fixed, is there a way to find the circuit's plate current first to find the appropriate voltage range, or do I have my order around the wrong way 

[Paul Birkeland]

Yes, you have correctly determined the plate current by looking at the curves.

Since the cathode bias voltage is fixed and the plate current is fixed, that means the plate voltage will wander around quite a bit between different tubes and as a tube ages.  With plate loading resistors and cathode biasing resistors, you can lock down the plate voltage a bit better, but the amp won't sound as good.

[tintin]

Ok thanks! Is there a way to directly calculate the plate current set by the C4S board, or is it correct to always assume a good operating for any tube is 55-90V? Just not sure now on which target number or parameter (current/voltage) I should be aiming to measure/get right  first. 

[Paul Birkeland]

Yes, the plate current is 0.855V/R1.  No, it's not correct to assume that 55-90V is a proper plate voltage for any tube.  This would be far too high for a few tubes, and far too low for many others. 

In the Crack circuit specifically, we would like to see the driver plate voltage to be roughly 70-90V.  This leaves enough of the B+ for the 6080 to be able to do its job and keeps the dissipation across the big Speedball board reasonable (or the 3K resistors in the stock circuit).

[tintin]

Ok that makes a lot more sense, thanks a lot Paul for your patient explanations!