Hello everyone! I'm new to working with vacuum tubes and could use some expert advice. I'm designing a power supply for a JJ Electronics 12AU7 triode and need clarification on the proper power-up sequence to maximize tube lifespan.
Power Supply Details:
Plate Voltage (B+):
- Transformer: 45-0-45 VAC (270VA).
- Regulated to 90 VDC via a MOSFET.
- A tap from this rail also feeds a phantom power (+48V) circuit using a TL783.
Filament Supply:
- Transformer: 0-10 VAC (5VA).
- Regulated to 6.3 VDC with an LM317 + slow-start circuit.
Key Question:
What is the recommended practice for applying power to the tube?
Option 1: Heat the filament first (6.3V), wait ~1 minute (via a switch), then apply 90V to the plate.
Option 2: Apply both filament and plate voltages simultaneously.
I’ve heard that sequential power-up (filament first) extends tube life, but is this strictly necessary for a 12AU7 in a preamp circuit? What technical factors should I consider (e.g., cathode stripping, thermal stress)?
The diagram doesn't show the tube's bias circuit since I'm solely focused on proper voltage application to prevent tube damage. Additionally, I'd like to know how to design an automatic delay circuit to apply the plate voltage (eliminating the need for a manual switch) if this sequencing proves necessary. Thank you.
Power Supply Details:
Plate Voltage (B+):
- Transformer: 45-0-45 VAC (270VA).
- Regulated to 90 VDC via a MOSFET.
- A tap from this rail also feeds a phantom power (+48V) circuit using a TL783.
Filament Supply:
- Transformer: 0-10 VAC (5VA).
- Regulated to 6.3 VDC with an LM317 + slow-start circuit.
Key Question:
What is the recommended practice for applying power to the tube?
Option 1: Heat the filament first (6.3V), wait ~1 minute (via a switch), then apply 90V to the plate.
Option 2: Apply both filament and plate voltages simultaneously.
I’ve heard that sequential power-up (filament first) extends tube life, but is this strictly necessary for a 12AU7 in a preamp circuit? What technical factors should I consider (e.g., cathode stripping, thermal stress)?
The diagram doesn't show the tube's bias circuit since I'm solely focused on proper voltage application to prevent tube damage. Additionally, I'd like to know how to design an automatic delay circuit to apply the plate voltage (eliminating the need for a manual switch) if this sequencing proves necessary. Thank you.
The 12AU7 or ECC82 doesn't have a filament. It is indirectly heated and has a heater.
Being indirectly heated there is no advantage waiting for the valve to heat up before applying anode voltage.
Being indirectly heated there is no advantage waiting for the valve to heat up before applying anode voltage.
Does this mean I can omit the 90V plate voltage switch and apply it directly without waiting for the 6.3V indirect heater to warm up?
That is how they worked in times gone by. The only provisos I am aware of if is there is direct coupling between tubes, or if a rapid switch off/switch on can stress some components with unloaded voltages, e.g. an electrolytic capacitor.
Tubes are quite rugged.
Tubes are quite rugged.
