I have one of the new Vibroverbs with a rectifier switch (ss/tube). The tube chart calls for a 5AR4. I notice NO difference in sag between the 2. The only thing the 5AR4 does is mush it up at high volume a little bit. The 5AR4 is a NOS RCA. I put a 5U4 in- it sounds good and gives me a bit of sag at low volumes. I have done some reading that a 5U4 is a 3A filament while a 5AR4 is a 2A filament. I want to be sure that my transformer is going to be OK. The 5U4 that I have on hand is from Groove Tubes if that makes a difference. I took a look at what the bias was doing when switching the rectifier switch- with the amp at 35mv the bias jumps up about 5mv when I engage the switch and use the 5AR4. When I put the 5U4 in and switch it on I get a drop of 2-3mv. Is the 5U4 safe in this amp? I obviously don't want to wreck anything, but it seems that from everything that I have been able to read that it is a fairly safe swap if your transformers are healthy.