Rectifier comparison and interchangeability chart
Rectifier Interchangeability and it's relation to Clean Headroom
Here's the list of the most common guitar amp rectifiers in order of most to least clean headroom (highest to lowest voltage output) and least to most sag:
1) Solid State Rectifier (NOT Weber Copper caps)
3) GZ37 (3 amp filament current)4) 5V4G/A (Possibly lower if plate current of the power tubes is high enough)
5) 5U4/G/A/B (3 amp filament current)
They're all interchangeable within limits. Every amp and each type of power tube has different limits so there are no general rules that apply universally.
All of these will "work" in the sockets intended for any of the other ones on this list.
Cautions: Most of these require 2 amps of filament current, the 2 exceptions are listed. Using a rectifier which requires higher filament current than the rectifier your amp was designed for, pose a potential risk of power transformer overheating. Often, there's no problem using them but caution should be exercised.
When changing from a lower output to a higher output rectifier, the new higher B+ voltage may pose a danger for the filter caps in your amp. Rebiasing of your power tubes is absolutely required. NEVER make this type of swap in an amp with 15+ year old power supply capacitors (in reality, you should never operate an amp with 15+ year old power supply caps anyway). Always check the new B+ voltage against the maximum ratings of the capacitors.
In addition, when changing to a higher output rectifier in a "self biasing" (aka "cathode biased" amp, eg, Champ and others) never assume that the power tube(s) is/are safe. ALWAYs check the bias and idle power of the tube with the new rectifier to be certain that it is running within spec. Increasing the cathode bias resistor is often necessary.
Be careful when going to a higher rated rectifier in 6V6 amps. You should always avoid having more than 430 volts across a 6V6 (plate voltage minus cathode voltage).