Tuesday, 10 December 2013

HT / B+ Zener Voltage Drop in a Valve / Tube Amp

Note, Updated, please see also  Feb 2015 post.

I am looking to drop the EZ81 out of my amplifier to gain another ECC83 which will result in the HT or B+ voltage taking a hike as the silicon rectifiers drop a lot less voltage in operation than the valve rectifier. The mains transformer I will use has two windings and a centre tap, designed for two diodes to give full wave rectification. The easy and well-trodden path  is to drop ~ 40V of Zener into the centre tap to ground connection of the transformer. It all looks a very simple solution, so how hard can it be?

The choice I have initially taken is to use a string of 6.8V, 5 Watt devices where 150mA of  HT current will equal 1watt of thermal dissipation. The device I have chosen is the 1N5342B from ON Semiconductor.

Below is a schematic of the centre tapped transformer and two silicon rectifier diodes to give full wave rectification.
B+ / HT Zener voltage drop.

Zener choice. Well this has to be more than just dissipation, but that is a good place to start. The 1N5342B  is rated at 5 Watts  but this is at 25C (77 Deg F.). This diode will not be at 25C at the predicted ~1Watt maximum let alone at the rated 5 Watts. The device de-rating means at a 55C (133 Deg F.) body temperature it should only be rated 4 Watts. By using a relatively low voltage drop per diode I can keep the individual device dissipation down.   Leaving the leads long is also a mistake. It is better to get this to a turret tag than leave long wires. Leaving long leads just adds thermal impedance.

There will be a mV/DegC figure and a breakdown voltage vs current attached to the data sheet for whatever device you have, you will need to note these to ensure you stay within your limits. If you have a bias generator from the HT AC rail then this will proportionally track changes in the HT supply and change the bias to the output stage which will  subsequently change the Zener voltage drop as the output stage standing current changes.

Dynamic impedance or what happens when the current through the Zener changes and is also a factor that needs to be looked at. Zener diodes below about 6 volts use a field emission breakdown and at higher voltages avalanche breakdown. Field emission has a negative temperature coefficient and avalanche a positive. The impedance of avalanche is generally the lower but at around the 7V point the diode has a combination of both which gives a dip in the dynamic impedance at this point. So the 6.8V choice is looking good at this point.

Temperature stability is what it is on the data sheet but you can look at compensation if you know what your stability specification is. For example the 1N5342B has a mean temperature coefficient of  +3.6mV/DegC . A silicon rectifier diode (say an 1N4001) has a -2mV/DegC so two 1N4001 diodes forward biased will drop about 1.45V but should leave a residual 0.4mV/DegC temperature drift. Using silicon forward biased in series with the Zener diodes can  reduce the effects of thermal drift. I have already decided that a huge mixed string of diodes is not the solution I will take, but if you go for a single 20 Watt stud mounting device then this is an avenue you may wish to look further at. There are second order effects regarding impedance etc. but by now you can see the principal.