12V Deep Cycle battery with Pure Sinewave inverter for tube amp?

Be careful handling lead acid deep cycle batteries! Hydrogen gasses and fumes emitted during charging is both an irritant and potentially explosive so ventilate the battery and area well at all times.

@imhififan should I be worried during discharge(connected to inverter while powering amp)? I will be doing all charging of the battery outside in a ventilated area.

If there is a danger with fumes releasing, would a gel type battery solve this problem?

If there is a danger with fumes releasing, would a gel type battery solve this problem?

Yes, Sealed Lead Acid type of battery is the answer! However,  Gel cell batteries must be charged correctly or they will permanently reduces capacity.

I used a 100AH Odyssey PC2150 deep cycle battery to power my modified Sony SCD-1.  They have vanishingly low impedance, which is a function both of their design and high AH capacity.  It's good to add a large amount of capacitance between the terminals-- particularly using an array of parallel capacitors.  This filters noise and improves transient response, even into low current loads.  Keep the battery cables to the inverter thick and short.