Q.: What is the difference betwen revision "A" and the newer revisions ?

Electricallly nothing, newer revisions are smaller , the layout was optimized.
Revision "A" boards are no longer manufactured.

Revision C has a newer layout, some optimization on the cooper paths.
There are two versions of this revision, one for 600 volts of HV, and another for 1200 volts.

The 600 volts version, which is now the standard version I manufacture starting sometime in fall 2012 features only one high voltage optocoupler (VO3052, instead of two MOC3011 or MOC3022), and no R4 and R5 resistors (1 meg for equalizing volatge between optocouplers).
Using only one optocoupler ensures in case of a lower HV installation, a succesfull firing of the SCR. 1200 volts version of this revision C board is assembled on demand.

see This link

Yes, indeed.

Not likely, please read the next question.

Disccussions of what is better, tube or solid state rectification is still unclear. My point of view is that for a guitar amp, you will find a noticeable difference between tube and Solid state, just beacuse of the fact that guitar amps are frequently set into distortion, on purpose, many times, and here, the tube rectifier will somehow soften this distorion, even to the extent of making it enjoyable (imagine a hard rock guitar).

In hifi equipment, it's another story, usually these kind of amps don't get into clipping regions, where a rectifier tube might soften the clipping (or distortion), so seldom is this softening even desirable.
But hey, everybody has it's point of view.

Q.: If I bring the amplifier to clipping, the HV-delay circuit disconnects, and begin its startup delay again (led is on).

This is not a bug, it's a feature !

If the ripple voltage on the main HV circuit is too high, the SCR resets itself.
More likely this happens only when the amp gets into serious clipping distortion, or when the volume is fully up, and you connect something to the input. This is why I do not recomend to install it in a bass or guitar amplifier.

Q.: The pilot lamp does not turn on, when I switch on the amplifier, but it turns on after the HV-delay circuit turns on

This is normal if the pilot indicator is of Neon type, it takes the voltage from the HV circuit and not from a 6.3 ac volts source.

Q.: I have an amplifier which uses 850 Volts in its Hv supply.

Following modificatons should be taken: D2 : 2 zeners in series, 100 volts and 48 volts
D3 : 2 zeners in series, 100 volts each
D4-D7 : Zener 100 Volts.
The SCR must withstand 800 volts. and you must change the optocoupler to MOC3022, instead of MOC3011

Q.: How much current does the circuit whitstand?

The SCR is rated 8 Amps, at that value, you would really need a heatsink. but at the current of a tube amplifier, you will see seldom currents above 800 mA, which is 10% of the max, rating of the SCR.
I never noticed a a temperature at the SCR tab above room temperature. though I never measured it.

Q.: I have an amplifier with separate power supply for the screen grids, how do I connect the HTR-214 ?

In this particular case, you will need two separate boards, one for switching the HV that feed the OP transformers, and another one for delaying the voltage for the screen grids. It is very important to make a modification in this latter board in order to assure that the plate voltage gets before the screen voltage.
This modification consists in changing the 330K resistor to, let's say 470K.

I haven't tested it thoroughly. You will need two boards and modify each to handle half the voltage, which in case of a 170 bipolar power supply, you will have to bypass zener diodes 3,4,5 maybe even Z2 . In case of the OTL, one board will be insterted in the circuit the other way around in the case of the negative supply. It will depend on the design and characteristic of your certain amp to see how it handles the curent.

Maybe a pop will be heard through the speakers. It is unlikely to find a vintage home amplifier to have such topologies.

I haven't tested this. The difficulty remains in the fact that a preamp or tuner will draw much less current than a power amp.

I will update this discussion after I make some tests.