13-12-2018, 01:55 PM
Thanks Bill, they do get a bit toasty so I won't use them as everyday amps.
Got a puzzler that's perplexing me, these amps I've just built use a minus 78v rail to feed the cathodes of a LTP, both grids are grounded, one directly, one through a 680k R , how can the cathode swing positive with reference to ground/0v? It doesn't make sense, surely any voltage drop across a resistor or CCS should still be minus whatever volts, or am I being dim? The maths using ohm's law doesn't jive either.
If for instance we have a 5k6 tail R to set both cathodes at 7mA each (-5v ish Vgk - it's never bang on as per loadline), total 14mA, R x I = 78v , therefore with all the voltage dropped across the resistor, the cathodes should be at 0v not the 5-7v I'm getting. This is really bending my mind, I must be missing something obvious.
Andy.
Got a puzzler that's perplexing me, these amps I've just built use a minus 78v rail to feed the cathodes of a LTP, both grids are grounded, one directly, one through a 680k R , how can the cathode swing positive with reference to ground/0v? It doesn't make sense, surely any voltage drop across a resistor or CCS should still be minus whatever volts, or am I being dim? The maths using ohm's law doesn't jive either.
If for instance we have a 5k6 tail R to set both cathodes at 7mA each (-5v ish Vgk - it's never bang on as per loadline), total 14mA, R x I = 78v , therefore with all the voltage dropped across the resistor, the cathodes should be at 0v not the 5-7v I'm getting. This is really bending my mind, I must be missing something obvious.
Andy.
