Hi peeps. Been here years and that's the first thread I start... I need your help here, cause I seem to have no idea what I'm doing.
I built this circuit based on the SOHA II input stage:
The 15 K resistors are 0.25 W so they can't dissipate more than 0.5 W together. I am using the 25 V rail of the output stage.
To help myself figuring an operation point, I drew a load line. The line I ended with goes from (0 V, 11 mA) to (160 V, 0 mA). Is this correct, or have I done the math wrong?
So anyways, based on this load line and the maximum power the place resistors can dissipate, I chose to set the tail CCS at 7 mA. This drops about 52 V across the plate resistors, leaving the plates at around 108 V.
I powered the amp and adjusted the trimmers until the plates sit at 108 V. The CCS seems to be working fine, the adjustments react as expected. Verifying the cathode voltage, they sit at 3.2 V, which seems a bit low but still close to the expected voltage.
But it's not working. I have no audio output. Only noise.
Debugging the circuit, I noticed that I get audio for a few seconds at startup before it goes silent. I also raised the current... until I got audio. I begin to have an audio output when the CCS is around 11 mA, the plates at 77 V and the cathodes at 0.7 V. I honestly have no idea what is going on here. Anyone sees something I don't? Anyone can explain to me why I only get an output at this point, and not before?
( and I can't leave the circuit working at this operating point as it drops almost a watt across the plate resistors, twice their rating. I also doubt the PSU could supply this much current for long. )
Any help would be appreciated. I'm a noob, and apparently I got no idea what I'm doing, so I won't mind being sent back to tube biasing 101.