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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?

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

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I can explain it to you, but I can’t understand it for you.

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?

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

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I can explain it to you, but I can’t understand it for you.

Gotcha! Thanks.

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

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

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Walt

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

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.