Using the LM317LM in a Tube Amp Build

[WilhemJ89]

Hey gang,

I’m in the middle of building a tube guitar amp with a regulated supply just for the preamp section, and found this little gem: the LM317LM. It’s an adjustable 3-pin regulator that can output from about 1.2 V to ~37 V depending on how you wire it. Since I’m still learning and this is my first semi-serious amp build, I wanted to run a few things by you seasoned folks:

I’m planning to use the LM317LM for the +12 V filament supply (instead of raw DC from the power transformer) to keep hum and voltage drift down. Does that sound like a solid idea for tube filaments? For the resistor divider to set the output, I read you need just two resistors. Any good rule-of-thumb values you use for filter caps or layout when using this regulator in audio gear? Since this is part of a guitar amp and we care about noise, did you ever run into hiss/hum or regulator-related noise with a LM317 (or similar) in audio builds? Would an LM317LM be “quiet enough” for a preamp rail?

If any of you have used the LM317LM (or LM317 series) inside guitar or audio builds, especially tube amps or DIY pedal-amplifiers. I’d love to hear your experience: what worked, what you wished you did differently, or what you’d avoid next time.

Thanks in advance for any wisdom!


Cheers!

 

[Paul Joppa]

Download a data sheet - and read it. They have tons of characteristics and implementation advice.

LM-317 is a very old device and has a large minimum dropout voltage; LM-1085 is not as old and has half the dropout.

If you don't have PSUD, get it to model the power supply voltage and ripple to make sure the dropout spec is met.

Check power dissipated and use an appropriate heatsink. - try to limit the temperature rise to no more than 40 degrees C.

Make the above checks at powerline voltages +/-10%

These chips have no sense of humor about exceeding specs, so don't exceed half the rated current without checking ALL the specs..

 

[Paul Birkeland]

To go with what PJ said, there's no requirement for regulated DC on tube heaters. If you can make 12V DC with rectifier diodes, resistors, and capacitors, that's often good enough as a starting point and it won't require any proto board or custom PC boards. PSUD is a good tool for dialing something like this in.

It's also not that big of a deal to put a full wave doubler on a 6.3V winding and then use that with an LM317 to make a 6V supply to heat your tube. There will be a lot of voltage drop across the 317, but if you are just DC heating the first 12AX7 in the amp, then you're in the realm of what a 317 would be comfortable doing with a small heatsink.