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Sometimes when you're not sure what's happening with an amp, especially one that's blowing fuses, you need to bring the voltage up slowing with a current limiting device in series. Amps with tube rectifiers are problematic in the rectifier tube doesn't start working until about 50 volts AC are applied. Substituting a solid state rectifier in these cases lets the amp start working at lower voltages.
A solid state rectifier is also useful for testing to see if a rectifier tube is bad. The simplest test is to swap in a know good tube, but you may not have all the varieties on hand. This solid state rectifier can sub for:
5AR4
5U4
5Y3
GZ34
The B+ voltage will be higher with a solid state rectifier than with a tube, but for quick tests it's simpler to have one SS rectifier than 4 tubes.
I also keep this in my first aid kit when I'm playing out. If the rectifier tube fails, I can get through the night.
A solid state rectifier is easy to build. You'll need a burned out tube, and 4 diodes (1N4007, 1N5403, etc). Carefully break the glass bulb (I use vice grips set just smaller than the glass diameter and wrap the tube in a cloth rag) and unsolder the wires to the pins. Then solder in the 4 diodes to pins 4, 6, and 8 as shown.
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