• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

LED wiring for power indicator

Like most of us, I usually prefer to let tubes do the talking and show that an amplifier is ON.

However I am currently finishing off a project in which the tubes are enclosed in the chassis, so will need to use a resistor + LED combination and 'steal' the power supply from the filament power supply.

Apologies for the crude drawing, can anyone confirm that wiring it like this is the best way to ensure no noise is injected into the system? I have seen some folks wire the LED in series with a heaters, but that doesn't seem smart in the event of the LED failing.

blue LED.jpg
 
Administrator
Joined 2007
Paid Member
You will get no noise and the resistor value might need to be higher than you expect as modern LED's are very efficient. You show 6.3 volts as DC which means there is zero flicker from the LED. Perfect. 6.3 volts AC would give noticeable flicker and also require protection for the LED from reverse polarity.

There is no way an LED can be placed in series with the heaters as the current flow is massive and the volt drop across the LED significant. The LED would fail in milliseconds.
 
  • Like
Reactions: 1 users
Perfect. 6.3 volts AC would give noticeable flicker and also require protection for the LED from reverse polarity.
Reverse protection is essential, since blue/white LEDS are very fragile. A series or antiparallel diode could work, but the flicker will be quite noticeable.
This could be fixed by using a small bridge: 4x1N4148 is perfectly sufficient, and the 100Hz flicker will be barely noticeable. The series resistor should be in the 10's of K range
 
  • Like
Reactions: 1 user
Like most of us, I usually prefer to let tubes do the talking and show that an amplifier is ON.
Just challenging this statement. I think it is essential to have an independent indicator that the chassis is live. Most tube amps have fuses in various places, and also it is not always that obvious that a filament is orange. I thought that the rules are that that there must be an indicator, and I think most people add one.

I suppose strictly speaking it should be from B+ to ground with a suitable resistor in series, so that it provides an accurate warning about high voltages. Putting it on the primary side is greater protection, but then the LED needs a serial diode or bridge as well, as mentioned.
 
  • Like
Reactions: 1 users