Oli, the SS rectifier sub actually has more switching noise than the tube rectifier does, but perhaps there might be less total system noise with the solid state rectifier....I've never really thought about it that way.
Paul, since the rectifier converts AC to DC inside your amp, any change at that point with a device that does that function more or less efficiently will affect the DC on the other end, and this includes the plate voltage, bias voltage, etc. Essentially every DC voltage in the amp is controlled by the function of the rectifier and how well it does it's job. It isn't too hard to learn to set the bias yourself if you really need to hear what it sounds/feels like to use the tube rectifier and you can't wait for a tech to do it for you. It would require something like a Weber Bias Rite meter to do quickly and easily however. There are others out there too like the Bias King, but I think Weber's product is superior myself. You could sub in a 5U4 or other rectifier tube, but there are other considerations there. Each rectifier tube type draws a given amount of filament current from the 5v winding on the power transformer, with the 5u4 drawing around 3 amps. The GZ34 draws closer to 2 amps of filament current. If the winding is specced for a GZ34 (which it is) then they may not have included the extra filament current capability necessary to run a 5u4, and doing so could cause the power transformer to run hot and burn itself out. Further, something like a 5U4 is much less efficient and will drop the plate votlage quite a bit. (around 50v compared to around 17v for the GZ34.) This will lower the headroom and give much more sag. It will also lower the volume on a very marginal basis. If you don't want to deal with the filament current considerations, then you could sub in a Weber Copper Cap that emulates the 5U4 and see what the affect is and if you like it, but this would still require a rebias.
Greg
