DC voltage at output

[Bolivar]

Hi all,

I've just completed building the Crack. After some initial trouble(the included 12au7 tube was defective), the amp seems to be working well for the most part.

What bothers me a bit is that the turn on dc voltage spike goes up to about 24V. I goes back down to about 20mV-40mV right after the initial jump.This is with no headphones connected. Then I tried it with a 160ohm load at the output and the spike is only about 1.6V and only a couple mV after that. I've changed the output caps to 220uf F&T electrolytics, which should be quality caps. Before this "upgrade" the spike was about 14V. Still higher than the 9V suggested in the manual. Should I be worried, or is it just a symptom of no load at the output?

Thanks for any and all input.

 

[Laudanum]

Mine was about 9.5V at start-up stock.  24V at start-up after speedball and 100uf film output caps.   I asked a related question, another member measured 14V with speedball and film caps.
As you found, it's much lower with a load.  The variable mv measured after the spike is leakage from the caps.  It should/would be lower with film caps.  I imagine that your higher start up voltage is related to the different caps, Im assuming a result of the larger 220uf capacitance.   I fire Crack up for a little while before plugging in the phones but it isnt necessary.  I hang my phones up after listening but if I kept them plugged in like a lot of folks, I would just fire it up with them still plugged.  I havent read any reports of problems so ...

 

[Grainger49]

That 160 ohm resistor discharges the DC much faster than the 270k ohm resistor.  That is what the 270 ohm resistor is there for, to kill the DC that leaks through the output capacitor.

 

[Bolivar]

Mine was about 9.5V at start-up stock.  24V at start-up after speedball and 100uf film output caps.  I asked a related question, another member measured 14V with speedball and film caps.

Forgot to mention that my crack has the speedball upgrade. Seems my voltages aren't that unusual then.

 

[Laudanum]

Sounds like you are a-ok.