The HV Delay

Linear Audio High-Voltage delay unit

Update 16 Nov 2019: The MOSFET specified for Q3 in the kit (FDP12N60NZ) has an internal zener diode between gate and source. It has been pointed out to me that these zeners are primarily meant for ESD discharge protection. In my design that zener does double duty to limit the gate voltage during operation. I have not had any problems with that during several years of use and multiple units, but it may be prudent to add explicit protection. This figure shows how to add it; a 12V 400mW zener  is OK. Note that this view is from the top of the board, and the zener should be mounted on the bottom. Take care not to short the zener wires to other connections.

You can now read the original AudioXpress article online.

Many years ago I designed a delay unit for tube power amplifiers. The idea was that it would delay the application of high voltage to the tubes until their heaters had been warmed up sufficiently. Tube heaters need some time to heat up, before a tube starts conducting. Depending on the tube type this can range anything from a dozen seconds to a minute. In itself this is no problem – you just wait a little bit before your music starts playing.

But normally the high-voltage supply for that tube comes on at the same time as the heater voltage comes on, often supplied from the same transformer. That means that the tube sits there for up to a minute with high-voltage applied and no conduction. This may shorten tube life; it depends on the tube type and the actual high voltage, and the jury is still out on how significant it is, but the effect is known. Another issue is that the high voltage initially, when the tubes do not yet draw current, can be quite a bit higher than the voltage under load, putting additional stress on the tubes and possibly the high-voltage capacitors. So we should not take chances with limiting the life of our precious tubes, and switch on the high voltage only after the cathodes have been properly heated up.

In 2014 (Feb. issue) Audio Xpress published my design for this little unit, which worked fine, but it had two disadvantages: it needed a separate 6.3V heater winding for power, and was limited to a selection of just two delay periods. The additional heater winding had to be separate from the other heater windings used in the amp because it floats on the high voltage. That often meant getting an additional transformer for it, which meant extra cost and extra space required in the chassis. The dual delay selection meant sometimes it was thought that one was too short and the other too long.

So I set about to upgrade the design, making it self-sufficient; there's no additional outside power required; and you can yourself set the delay between 20 seconds and 254 seconds in two second steps, so this should cover all requirements.

Again, the good people at AX have agreed to publish the new design, and it is slated to appear early August in the September issue. Jason at diyaudio.com has kindly agreed to include a half-kit for the delay in the diyaudio.com store; it will be available by the time the article appears. If you want to be notified, just send me an email (see 'Contact').

... but how do I connect it?? 

Ahh, thought you'd never ask! Very simple, there are just three connection and it leaves all your precious high-voltage and ground wiring intact. It just connects between ground and the transformer center tap (or negative side of the bridge for a full wave rectifier). Then a third wire connects to the top of your first rectifier capacitor. Simple and quick. See the picture - beats a 1000 words!