5U4 in place of a 5AR4?

[Sean]

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.

[brad347]

probably OK. Those transformers are generally over spec'ed a lil bit. Don't take my word for it though, wait for the real experts to weigh in.

[ES350]

Run it awhile and see it the PT is running hot---you are drawing 1 amp more than the PT is designed for (a non-trivial amount). While it's true that older Fenders were somewhat tolerant of larger current draws, who knows about the new stuff...? If you want more voltage drop than a 5AR4 with no increase in current draw, try a 5V4 (1 amp) instead of the 5U4 (2 amps).

[brad347]

Run it awhile and see it the PT is running hot---you are drawing 1 amp more than the PT is designed for (a non-trivial amount). While it's true that older Fenders were somewhat tolerant of larger current draws, who knows about the new stuff...? If you want more voltage drop than a 5AR4 with no increase in current draw, try a 5V4 (1 amp) instead of the 5U4 (2 amps).

good point! Listen to him!

[Blue Strat]

Best case, don't let incorrect specs (1 and 2 amps per above) or your "hand thermometer" influence your judgement.

Get a 5R4GB for under $20 and get 5U4 performance with only 2 amps (like a 5AR4) heater current.

[Sean]

Thanks guys. I think I have a 5R4 in my stash- have to look.

[ES350]

Sorry for the misinfo---I stand corrected. But if you do need one, see Mike...

[billyguitar]

A 5V4 can give a bit more sag in some amps. I swapped rectifiers around in my Maz 38 one time. The stock tube is a 5AR4. The 5u4G felt slower but didn't seem to sag. The 5V4 reduced the output but really didn't seem to sag but I only had it 1/2 up. I am using the 5V4 in my Stang Ray for the reduced volume. 5Y3s are not recommended in the larger amps.