[ETO_Alpha] Soft Fault # 20

[Sante - IK0HBN]

On my 87a since some weeks I got a Soft Fault trip nr. 20: Filament 
current > 3,5 Amp.
 From where should I begin to resolve the problem?
Initially it occurred on higher bands, but now it affects even the topband.
The amplifier had been upgraded with Alpha-MAX and Alpha Remote 2 years ago 
and tubes seem to deliver the initial 1,5Kw.
In the case it would mean that one tube is going dead for short filament, 
how do you suggest discovering the bad one? May the 87a run with ONE tube 
only so to be able to understand which tube faults, or you suggest changing 
both?
At moment it runs the factory tubes, but if another brand more hard and 
cheap should be available on the market, please let me know.

IMPORTANT: the fault happens even when the amplifier is on STAND BY and 
radio is OFF.
Any hint??