Both my valve (tube) amps have a power switch and a standby switch. When I first got a valve amp, I didn’t know why it had two power switches and I kind of assumed that the standby function was just a convenience to keep the amp warmed up without having it fully on.
Recently, however I was asked to fix a 1960’s vintage Vox AC30 and I noticed that it only had one power switch (i.e. no standby switch). It got me thinking and I decided to do a little research and it turns out that there is a good technical reason t0 have a standby switch on some valve amps.
If your amp has a valve rectifier (a GZ34 in the case of the Vox in question), the HT (high-tension) supply doesn’t come-up until the heater warms up and thus the HT voltage rises quite slowly. This means that the amp naturally comes on gradually over the course of a few seconds.
Conversely, if your amp has a semiconductor (solid-state) rectifier (and no standby switch), then the HT supply comes up immediately. This means that the anodes (plates) of your valves are seeing lots of high voltage way before their heaters have warmed up – which is bad for the valves and reduces their life.
So, the point of a standby-switch is that you start with both switches off, turn on the power switch, wait for a few seconds until the heaters warm up and then switch the standby on which connects the HT supply. When you turn the amp off, it doesn’t matter which order you turn off the switches.
And this leads us to the problem – forgetting to turn the standby off at the end of a session or gig – particularly if some kind soul pulled the plug elsewhere and your amp is already dead when you come to pack it away. If you care at all about your valves, it is at the least annoying when you next plug your amp in and it comes on straight away.
It turns out that this problem is the same as the no-volt-release problem with power tools. If the machine is running and the supply fails (for whatever reason) then the motor will stop. When power is restored, you don’t want the motor to start unexpectedly because that’s dangerous. Hence the no-volt-release which ensures that – in the event of a power loss – the motor will not restart until you press the start button. Here is the classic schematic for no-volt-release:
The RUN and STOP switches are momentary (spring-return) action. When you press the RUN button, the relay coil is energised and the relay contacts close. Contact 1 latches the action of the RUN button, so when you take your finger off the run button, Contact 1 effectively keeps it pushed. If you then press the stop button or lose Vsupply (for whatever reason) the relay coil de-energises, the contacts open and the circuit stays switched off until you press the RUN button again. Meanwhile, relay Contact 2 is used to do something useful – like switch the motor on.
We can use this arrangement to effect a no-volt-release on the standby circuit of our valve amp, so that when the amp loses power (by switching the power switch or unplugging) the standby (and thus the HT) goes off automagically and won’t come back on until you manually press the button.
But – even better – we can use a baby microprocessor to achieve the same end which allows us to reduce the number of buttons required to one – and then we can simply replace the existing standby switch with a momentary version. We could even write the software so that we could use the existing (latching) standby switch.
Here’s the schematic (assuming a momentary switch):
Note that I have used the word GND in the following schematic, but it may not be (and doesn’t need to be) the same as chassis GND. In fact, it’s best if the circuit below is left floating.
Basically, there’s nothing to it (it’s all in the software). The only (slightly) clever part is the bridge rectifier, which allows the use of whatever power supply is available on your particular amp.