regarding the tube voltage issue I think that can be overcome by I can place a capacitor bank or a Voltage Stabilizer between alternator and battery when will drastically improve voltage smoothness.
I am fixing to build a battery Isolator probably diode based, which is going to insure my main system is safe from draining with the secondary.
I found a bunch of super capacitors that ranged from 100F-3400F and voltage ranged 2.7-5.5v for $6-$45 each. I can wire how every many in series to obtain a needed voltage in the rear along with either a XS Power battery or a sodium ion power bank that has plenty of reserve. I can probably find a DC - DC booster that can handle the voltage difference the tube will need.
I might be a gleam idea but I really would like to design one and make it for enjoyment and the learning aspect. never got to play with tubes and now I'm older I want to experiment while I can and stuff a knowledge deep in my sons brain. he is always building paper robots. im going teach him to build speakers then we are going to try to build a rotary speaker for my truck that will be infinte baffle so should be absolutely bonkers in there.
based on your comment with my intended goal truly what I'd be trying to accomplish is a Class A tube amplifier that used Speaker Line inputs, change speaker line impedance, signal through gain adjustable amplifier circuit, output signal to RCA's, RCA connects to amplifier connected to speaker
so based on that would it be possible to then connect a simple opamp circuit for the impedance change and feed that to a Class A Tube amp input for the HiFi signal and do the biasing and switching with a Class D output and just output the signal to RCA's that will be used for the inputs of the amplifiers which then in theory if all Class D amps are used to amplify the signal system should be Eff but also crisp clean and breath taking all done with the desire to obtain it through non-dsp means just good ol circuit designing
It's not smoothness that you're worried about here. It's 12.6VDC. Period.
Your car's electrical system is rated for 13.8VDC. Your tubes want 12.6 (or 6.3 or 3 or 5 or 1 or whatever tube you've chosen.) Going above that range by too much will burn out the filament or stress it to the point where your expensive tube is now a glass bottle with some metal in it. Yes, a 12.6VDC tube can handle 13.8VDC, but that's the absolute upper limit. You're still short the plate voltages which you'll need to invert to get to the higher voltage. You can get inverters to do that, but solid-state stuff can induce noise. Your car's electrical system can induce a lot of noise as well, especially stuff with a lot of modern electronics. You're going to need a lot of filtering and shielding to do this right.
A word about what line-level means, when you say that it refers to a very specific thing. 1V @ 600Ω is typical line output, 10k is typical line input. You don't need more than that - I would usually limit my stuff to 2V when I did some audio work. I'd also usually tie the output of the preamp to 20k - that seemed to work well. Get too much of a mismatch and you get noise.
Are you looking to use 9 monoblocks for your finals? I assume you want mono output, and not stereo?
the sub amps are mono but my current door amps I have one is a Timpano tpt500.4d and my rockford is a 300x4.ab and I plan to grab a 3@ class d 4 channels one for front/rear Mid-Bass, one front front/rear Mid-Range and one for front/rear Tweeters ill use 2 class d mono blocks for subs. im just pondering how to make a lineout converter that has a amplifier attached to the signal raising the voltage to 6v preferably but upto 7.5 cleanily as i am seeing alot more amplifers with much higher input headroom then ever before. im basically dreaming up a super nice ass LOC and rca voltage amplifier all in one.
now i think on it the lineout converter can be the high level input of a amplifer, use the tube for the preamp of the amplifier boosting the rca signal outputs it powers.