#16 Re: Pentode PP Amplifier Complete Set of Drawings
Posted: Wed Aug 05, 2015 2:45 pm
Noise from zeners is way below that of a VR tube, and due to the uncorrelated nature of the spectrum, falls off as a function of the square root of the number of zeners.
You really do need to use true zeners, not avalanche diodes for a statistical regulator - the other significant way to lower the noise figure is to increase the current though the zeners - the noise is inversely proportional to the square root of the current, so higher currents, within reason, are better,
Using 5.6V zeners, e.g. BZX55C5V6s, you get about 3.5uV noise @ 20mA, i.e. -107dBu (20*log(3.5e-6/.775)) - at 10mA there is 5uV of noise, i.e. -103.8dBu or 3dB worse...
The noise advantage (improvement) of 38 x 5.6V zeners over 4 x 47V ones for a given current will be further 9.8dB - significant.
So, using a CCS (2 x DN2540N5) set to 20mA and a string of smaller, true, zeners could be a very worthwhile change to the screen supply...
You really do need to use true zeners, not avalanche diodes for a statistical regulator - the other significant way to lower the noise figure is to increase the current though the zeners - the noise is inversely proportional to the square root of the current, so higher currents, within reason, are better,
Using 5.6V zeners, e.g. BZX55C5V6s, you get about 3.5uV noise @ 20mA, i.e. -107dBu (20*log(3.5e-6/.775)) - at 10mA there is 5uV of noise, i.e. -103.8dBu or 3dB worse...
The noise advantage (improvement) of 38 x 5.6V zeners over 4 x 47V ones for a given current will be further 9.8dB - significant.
So, using a CCS (2 x DN2540N5) set to 20mA and a string of smaller, true, zeners could be a very worthwhile change to the screen supply...