hi i recently found an old amp with 3 12ax7 and decided to build a guitar preamp, to operate with the 250v i am using a dc boost so i dont have a transformer that gives me that voltage along with the 6.3v ac for the filament, currently the filament is working on 12v dc, but the tube gets very hot and in some posts they say this shortens the life of the filament, my question is, would there be some problem if I control the intensity of the filament with PWM like they do to control the intensity of a led in other projects?
To which pins did you connect your 12 Vdc? Pins 4 and 5 need to be used for 12 V (12.6 V, to be precise) heater supply, pin 9 remains open.
Best regards!
Best regards!
yes, exactly connect the 12v dc, to pins 4 and 5
I use 12v dc since they go through a regulator because I use 24v dc for the dc booster
Then everything is in good order. Tubes do get hot, that's the priciple of their operation (check "thermionic emission").
Are you creating the needed voltages from the mains without using a mains transformer ? That is
DANGEROUS and might kill someone.
my question was if I can lower the filament temperature a bit through pwm since in some publications they say that this lengthens the life of the tube
You can lower the filament voltage by 5% or so. This may inject noise into the audio.
Most just use a series resistor instead.
Most just use a series resistor instead.
Good!, then my warning was unneeded.
If the 12AX7/ECC83 are genuine they will consume 150mA filament if feeded via 4 and 5
Adding 80 ohm resistor in series with the tube makes it possible to power from 24V.
( each tube should have 80 ohm in series )
12A_7 heaters do not burn out before something else goes wrong. I had a 12AU7 with 10,000 hours on it working great.
Yes, but not with PWM as that will inject noise spikes. Series resistance is the easiest way, but since you are using 12V for a 12.6V heater anyway its already running cooler.