Re: Noise gate between amp and speakers?
If it is as you suspected - lacking ground - you can just find a source for earth/ground like radiator in a room or similar.
Even the full grid 115VAC or 230VAC(europe) rely on actually sticking a pole into the ground. You are not send a ground cable back to a power plant, I mean.
The problem as already described by many - is that if tube amp, the very end from amp is a transformer winding expecting a certain load. If 8 ohm you have one winding in output transformer for that - if 16 ohm you have another winding. And tubes are really sensitive not getting a proper load. Tubes feed the primary winding of transformer and speaker on secondary winding is the load.
Common to both power amps, solid state or tube - is that the load need to take the power as the speaker does.
So question arise - which electronics would be transparent as speaker load and yet work taking away hiss or hum at a certain level alternating current like sound.
If you have 50W r.m.s into 8 ohm that is 20V AC and 6.25 A current coming that way over to speaker. This must be unmodified not to alter sound. So you would need some fast switching device, like a tyristor or something connecting a circuit like a filter at high rate that can withstand 50W and/or ensure that load 8 ohm is maintained.
This is easy to do at signal level, just drop signal to ground when below a threshold, but with output circuit rather advanced to maintain all properties that a speaker is.
I remember from my youth getting to a concert - like Deep Purple or Zeppelin - and anticipation for concert to start. You just heard the noise from power amps on Marshall stacks waiting to burst into sound later.
If that is what you get - see it as adding to the listening experience, maybe.
Having had really bad power grid 230 VAC here a many years I looked for solution to stabilize this, since some tube preamp pedals just shut down when a carpenter at this farm started a drilling machine or something simple as that. Really bad. You immediately got below 170V or so.
I found a double converter that made dead current from mains, and an inverter creating a nice 230 VAC 50 Hz again from that - really nice. But what I also found that is that specs on these are very different - some have restrictions on what kind of load they can take. Some clearly stated no electric motors etc. Usuall electronics were normally fine, since they usually have a transformer that do low voltages for inside units. Tube amps is a bit of different as I see it - since you often in real life take mains and just rectify as is as anod voltage for tubes since so high. 300-400V or so. No stabilizer or anything usually, straight from mains pretty much.
I watched a rig tour for Brian May and he used a frequency converter in the price range of $2000 that gave him exactly 234 VAC 50 Hz for his Vox AC30 amps, if he had 4 on stage, two regular use, and two spare ones if anything breaks down. So he got them sounding exactly the same wherever he went in the world on tour.
The one I found was $250 and worked fine for my 20W amps at least. No noise or anything. So the inverter part creating AC from DC battery would be the same issue - making AC from DC. Total 1000 VA(or W if to simplify). Entire studio worked fine on that. Amps, computers, synths etc.
So I think it's about how simple circuit you went for. I have seen plenty for 12V in cars making mains level AC etc to run a laptop or similar electronics and cost $50 or so and we can imagine how good they are.
If you just rectify mains out to anod voltage for tube amp - this noise would come through as is over to audio. Normally filtered for the purpose it's designed for. Any flutter on input AC is making it's way to audio.
But as I understand you found a different inverter package that works better.
