Purpose of the 10000uf cap?

[mcandmar]

I am curious to understand the role of the large capacitor across the two LEDs?

Is it something to do with being inside the AC current loop, or just to stabilise the voltage reference set by the LEDs?

[Paul Birkeland]

In the stock Crack, when the 6080 cathodes warm up and start conducting, they do it pretty quickly and the 100uF coupling cap lags a little bit and takes some time to charge up.  That is the origin of the voltage spike at the output.  In the C2A, the 10,000uF cap slows that down and the startup spike is negligible.

[mcandmar]

Ahh.  So does the capacitor have to charge up first before two diodes have enough current through them to start working?

[Paul Birkeland]

When you first turn the amp on, there is 0V across the cap, and with that 0V means that the transistors in that part of the C4S see 0V between base and emitter, which means they are turned off.  Once the cap charges up, the LEDs are free to do their thing and the C4S functions normally.  The 33K resistor across the cap discharges it when you turn the amp off. 

[mcandmar]

Makes perfect sense now, thanks Paul.