Selecting fuse value for DIY amp?

[TubeDriver]

So what is the general rule of thumb for selecting current value on a fuse for a DIY tube amp? Do I simply take the max current values for all PT secondaries that are used or should I calculate the actual current used by tubes in my amp and add 15% (?) for startup surges (ie I have a single stage 6BX7 driving a 300B with a GZ37 rectifier etc etc)? Thanks?

[Roscoe Primrose]

If you have an amp that has any significant startup surge, 15% margin is going to be too small unless you use a slow-blow fuse. I generally use slow-blow fuses at about 1.5x expected current draw, although I've built a few amps that had small enough margins that they'd blow slow-blow fuses if you ran them for several hours non-stop Something like this: https://www.amazon.com/P3-P4400-Electri ... B00009MDBU is a great way to determine normal current draw.

Roscoe

[TubeDriver]

Thanks, that thing looks handy.

But just to clarify, you are suggesting basing fuse value on the overall draw (including start up surge) and not just the theoretical max value of the PT secondaries that are in use?

If you have an amp that has any significant startup surge, 15% margin is going to be too small unless you use a slow-blow fuse. I generally use slow-blow fuses at about 1.5x expected current draw, although I've built a few amps that had small enough margins that they'd blow slow-blow fuses if you ran them for several hours non-stop Something like this: https://www.amazon.com/P3-P4400-Electri ... B00009MDBU is a great way to determine normal current draw.

Roscoe

[Roscoe Primrose]

Yes. Especially if you're using an oversized transformer. If you're using a PT big enough for a PP parallel 6550 amp do you really want to use a 6A fuse if you put that transformer in a line stage?

Roscoe

[TubeDriver]

Gotcha! Thanks.

Yes. Especially if you're using an oversized transformer. If you're using a PT big enough for a PP parallel 6550 amp do you really want to use a 6A fuse if you put that transformer in a line stage?

Roscoe

[tomp]

Another approach is to use an inrush current limiter. It will not only be easier on the fuse but also the other components as well. They are inexpensive and widely used. The fuse rating can then be closer to nominal current demand. Here is a link to give you some info.

https://www.ametherm.com

[Pelliott321]

Tom those are in series are they not. How do they affect noise. Do they add or subtract

[SoundMods]

Another approach is to use an inrush current limiter. It will not only be easier on the fuse but also the other components as well. They are inexpensive and widely used. The fuse rating can then be closer to nominal current demand. Here is a link to give you some info.

https://www.ametherm.com

I'm not a fan because inrush current limiters, even after the initial rush, raises the power supply impedance too much.

[Pelliott321]

Thanks Walt, I knew there was a negative with current limiters

[HAL]

You can always put a relay switch across the inrush current limiter and turn it on after warm-up with a simple timer. As long as the contacts are rated for max current draw should be fine.

Use a flyback diode across the relay coil to prevent inductive kickback to the switching device like a power MOSFET or BJT driver driven by the timer.