Pentode PP Amplifier Complete Set of Drawings

[jack]

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...

[Cressy Snr]

Cheers Jack,
You know what I might do; I might just put in a nice big order for the diodes and CCS you suggest and apply it to the input stage screen supply, in the first instance. Noise performance on the input stage will be more critical than the output stage, and the benefits will probably be more significant there.

It'll probably be next week before I get it sorted but I'll post as soon as it's done.

[Cressy Snr]

Well I've found 10 off DN2540N5 in a box upstairs, so I only have the zeners to get hold of. No excuses now then

[jack]

Well I've found 10 off DN2540N5 in a box upstairs, so I only have the zeners to get hold of. No excuses now then

Them Zeners cost a fortune

1.6p each at Farnell atm...

You'll need a 10uF kelvin-connected cap across the zeners - the CCS should be set for 20mA plus the screen current, i.e. need to measure 20mA through the string.

[Cressy Snr]

Yikes!
I've ordered 100 of the little divils

£1.63. with free next day delivery.
As a Yorkshireman, that's music to my wallet