The 5U4 and 5AR4/GZ34 effectively have the same pin out.� Big difference is how much current each can deliver to the amp, and their internal resistances which effect the voltage drop across them (and
thereby the overall B+ level). The 5AR4 has a smooth turn-on characteristic which is good for your
filter caps and tubes since the high voltage 'ramps up' rather than coming on all at once. The 5AR4 has a low internal resistance and your B+ (plate) voltage will likely go up a bit, so you'll want to re-bias the
amp to use it. I should say you should re-bias the amp whichever rectifier tube you switch to. The 5U4 has a higher internal resistance and will drop more voltage. If you have been using a 5AR4 and want to drop your B+ voltage a little, a 5U4 is a good start. I would say that the design of the power supply plays a far more important role than what rectifier tubes you use.� If you spec your power supply to run well within it's limit then the 5U4 is probably fine. If you are more conservative and have more $$$, then 5AR4 is probably a better choice.
I have amps that uses both rectifiers.� I think that alot of people say that the 5AR4 sounds the best especially Mullard 5AR4s. I tend to agree.� I'm using a Sherwood S5500II integrated amp that uses 5AR4 with 7591 outputs.� Have to say that it sounds really good.� Nice tight, focused bass, and airy mids.� The Mullard 5AR4 are pretty expensive. They run around $50 each (metal base Mullard +$75) but I also have amps that uses 5U4 and am happy with it.
Good luck.
Swasdee krup.
ABMOGGY
Governments have a tendency not to solve problems, only to rearrange them.
RONALD REAGAN
"The point of living and of being an optimist, is to be foolish enough to believe the best is yet to come." - Peter Ustinov
