Is rectifier tube arcing a problem?

A 5U4 should handle that load just fine. I think you've had bad luck with tubes... Is this happening in both channels? Do both channels sound the same?

If this is "only" happening when you power down one of the amps and then restart it within a very short period of time, just wait longer (5-10 minutes).

If it happening @ other times, then it needs to be looked into.

I would not reuse a rectifier that I observed arcing, but I've always had plenty of spares.

I found a schematic of the power supply section @ the Steve Hoffman Forums, but could not link it.

I Googled your amp + rectifier and it came up on the first page of hits.

DeKay

Add a negative temperature coefficient (NTC) thermistor on the primary side of the power transformer and bypass it with a time delay relay may help.

Short answer is Yes, the arcing happens in both channel.  
 

for the 5C3S, at the beginning, I don’t think they arc.  Then after a month, both tubes arc.  For the RCA, they both arc right the way.  For the TungSol, so far, only one arc.  And when I switch the tube, same behaviour.  
 

maybe as you say I have bad luck, but the percentage seems too high, 50% or 3 out of 6 pairs arc.  That’s why I think could it be a me doing something wrong which cause this.  But there is only one button, I can’t think of anything except I didn’t wait long enough when restarting the amp.  
 

is it very common for 5U4G to arc?  I only have two amps that use 5U4G, and I didn’t roll the tube in my previous amp.  

@atmasphere and others

BTW, is there other rectifier can be used for this amp?  Such as a soft start 5AR4?