Hi everyone, hope you're all OK.
I was wondering what is the max safe value for supply voltage of a resistor loaded triode in a common cathode gain stage? Take parallel 3A5 for example; the datasheet mentions max anode voltage value of 130V, and max current value of 10ma (5mA each triode), but how high can the supply voltage go? Can I use for example a 280V supply and a 22K anode load resistor for around 100V and 7 mA at anode?
I know that 100V-7mA is safe for the anode, but how about the warm-up time, when cathode is still cold and internal resistance of triode does not form a voltage divider with the load resistor and in fact the anode voltage is equal to 280V B+; Can this be dangerous for the tube? I mean something like internal arcing?
I was wondering what is the max safe value for supply voltage of a resistor loaded triode in a common cathode gain stage? Take parallel 3A5 for example; the datasheet mentions max anode voltage value of 130V, and max current value of 10ma (5mA each triode), but how high can the supply voltage go? Can I use for example a 280V supply and a 22K anode load resistor for around 100V and 7 mA at anode?
I know that 100V-7mA is safe for the anode, but how about the warm-up time, when cathode is still cold and internal resistance of triode does not form a voltage divider with the load resistor and in fact the anode voltage is equal to 280V B+; Can this be dangerous for the tube? I mean something like internal arcing?
The RCA data sheet for the 3A5 has curves plotted to 240 volts, so you are safe at 240 volts. I generally don't go much higher than what the curves show, especially on really old tubes because of the possibility of residual gas ionization. My guess is that you can get by with 280, but it depends a lot on the vacuum quality in the tube. Even small amounts of ionization will raise the distortion in the stage.
You can often use a series string of NE-2 style neon lamps to make a voltage limiter. These are a 1 pF capacitor until their strike voltage is reached. It may take a hand full of them to find a set that will limit the voltage to 240 or less and extinguish when the plate current comes up.
You can often use a series string of NE-2 style neon lamps to make a voltage limiter. These are a 1 pF capacitor until their strike voltage is reached. It may take a hand full of them to find a set that will limit the voltage to 240 or less and extinguish when the plate current comes up.
Thanks for the response,
It's funny how linear 3A5 is; even with a 240V supply and 18K anode load, I calculated the THD to be around 0.8% max at 23Vpk output, pretty enough for driving 4P1L in PSE. I know I can lower the supply voltage for preamp, and then use a plate choke or CCS for 3A5, but I wanted to try resistance load for this tube to see how it sounds.
That's an interesting idea, I will try it for sure.
Thanks again.
It's funny how linear 3A5 is; even with a 240V supply and 18K anode load, I calculated the THD to be around 0.8% max at 23Vpk output, pretty enough for driving 4P1L in PSE. I know I can lower the supply voltage for preamp, and then use a plate choke or CCS for 3A5, but I wanted to try resistance load for this tube to see how it sounds.
That's an interesting idea, I will try it for sure.
Thanks again.