Tuesday, 24 February 2015

A Better HT / B+ Reduction Scheme. Or my problem with Zeners.

 WARNING Please do NOT attempt unless fully competent in working with lethal voltages in a safe and orderly manner! Always PULL THE MAINS supply out of the socket if you want to turn off.

In the UK there are various voltages on the mains, the limits are  ~253V to 217V RMS, though I have seen both limits exceeded. This variance is a pain for valve / tube amps especially when the HT / B+ is already on the high side. Most remedial solutions revolve around  a big Zener diode to the centre tap of the transformer to take a chunk of voltage out.   However a variable mains supply with a constant Zener drop extends the HT variation still further.

A much smarter way would be to put in an opposing A/C into the A/C HT with an additional small mains split bobbin transformer. In addition to dropping the overall HT the line regulation will be better than the Zener option and other windings (6.3V etc.) remain untouched. The transformer I chose is a 250mA 15-0-15 (7.5VA) output from Maplin which whilst cheap and small, is more than enough for the job. The trick is to get the winding the right way round. Using a Mulit-Tap output extends the possibilities.

From measurements, with a 252V mains I have 520V HT. Using the opposing transformer this drops the HT by 55V DC to 465V, much more manageable. If I used the Zener method I could also achieve the 55V drop.  However If I then plug into a 220V mains supply then the Zener will still drop 55V where the opposing transformer would now only drop 48V leading to better overall regulation. Something to watch if you generate the output valve bias from the HT line.

Addition of a 230V - 30V 250mA transformer.

Overall the cost is similar but I get the satisfaction of better regulation.  The transformer should last the life of the amplifier. To locate the correct polarity, wire the opposing transformer to the mains input and connect one end to the low side of the transformer. Then measure the AC between the unconnected output and the HT high side. The voltage will either be higher or lower than the HT depending on the phase being additive or 180 degrees out. When you measure a lower voltage then this is the end to connect to the transformer and the other lead will connect to the rectifier.

Adding the Red in opposition to the Blue results in the Green output.
So where does this leave you if you have a CT (centre tap) full wave arrangement. You cannot just add the winding between the common ground (marked "Point A" below) as the phase means that one cycle will be opposed and the other will be in addition. This configuration requires an opposing transformer with a pair of separate output windings, so a single mains input and say a pair of 0-30 & 0-30 output windings. A 30-0-30 with a single common tap is no good . You then apply the secondary windings as before but in each arm.

A transformer with separate output secondariness for CT Power Supply.