Valve preamp build... questions!

[Morocotopo]

Hi people, me again with some questions for you:
I´m going to build a valve preamp. It´s the M*rshall JCM800 preamp, I believe the 2204 model. The project for this was posted in this forum some time ago:
http://diyers.forogratis.es//viewtopic.php?t=23
Now, I tried to subscribe to that forum to ask questions there, but the admin never activated my account, despite several mails sent...and now even the .pdf for the project, that included the schem, layout and PCB is not available anymore. So, thats why I´m asking here, my alma mater in all things electron related. I will post here the schem from
the original file (that´s available in the net anyway), hope the author is OK with that (estas ahi, Gak???), not the layout or PCB.
Anyway, before I make this, my first valve project (I already made some power supplies, so it´s not my first hi voltage project) I want to be sure nothing blows up in my face or turns me into barbecue.

Schem:



The questions are:

1) Are the transformers Ampere ratings OK? The thread talks about 800 mA for transformer 1 and 300 mA for transformer 2, so I guess they will be OK at 1A and 500 mA respectively, wich are the ones I plan to use (availability reasons).
2) Diodes: Are 1N4007 OK for the rectifier bridges? That´s what the parts list says, but just to be sure...
3) Caps voltage ratings:
-->For C1, 2, 3, 4, 5 and 6 the parts list says 350V. I will use 450V, I believe that´s OK and leaves a little more safety margin than 350V ones. Am I right?
-->For C7 the parts list says 16V, isn´t that too close to limits since it´s after the rectifier bridge for transformer 1, that puts out 12V, so the cap will see (at least theoretically) 12V X 1.414:16,968V?
-->All the other caps don´t have a voltage rating in the parts list. Can you tell me what voltage ratings do I need for C8, 9, 10, 11, 12, 13, 14, 15, 16, 17? I know they are not power supply caps, but I´m not sure about what voltage I will have in the different parts of the schem...
4) Resistors: R1 and R2 are 1W ones, but what about R3, R4 and R5? do they need to be 1W ones too? And all the rest, are they OK at 1/4W?
5) Transformer shielding: I plan to put the PCB and transformers into one enclosure (BIG stompbox), but I worry about the transformers inducing hum into the audio. If I put the transformers into a metallic sub-enclosure inside the box (with ventilation of course) connected to ground, will that eliminate/isolate the transformers hum inducing(¿electromagnetic?) fields?
5) Power cable connection:
    I will use this diagram for power connections:



Now, the symbol attached to the safety wire is chassis, right? Also, the circuit ground has to be attached to the chassis also, right? Or not?
Also, why is there a different name for the AC lines? I mean, If they are AC, they alternate between positive and negative, right? What happens if I connect them the other way around (I won´t do it, just asking to know...)? would I fry myself? The transformer? Any suggested reading I could do about this? I think it´s a crucial safety subject and i would like to know more about it.
Another one: Is there a difference between using a power switch that cuts just the Line wire and one that cuts both Line and Neutral wires? (I plan to use those rocker ones with a neon lamp inside)
6) Fuse: What fuse should I use? Here (Argentina) we have 220V power. Also, It always goes in the Line wire, right? Could I leave the fuse out, or is it too dangerous?
7) 7812 regulator: Will it need a heatsink?

Sorry for the amount of questions, But this is a new area to me, and i want to get it right.

Thanks in advance for reading, for the patience, and for the help.

Morocotopo

[bancika]

1) yes, I'd say so
2) yes, that's what's generally used. If you want to go fancy grab some ultra-fast diodes (start with UF), they don't have much switching noise as the one you mentioned (but cost almost 50c each).
3)
-why not
-get 25v then
-generally you want 400 or 630V for coupling caps (the ones that go from plates), for cathode bypass you'll want 16V or better.
4) R3,4,5 should be 1/2w as well as all others, I wouldn't use 1/4w, they are generally ok but 1/2 should be less noisy
5) well, it wouldn't help with EM radiation from transformers unless it's ferrite. Try to keep  everything as far as possible from transformers, I'd use toroid transformers for that because they should be better as far as noise is concerned.
6) don't leave the fuse out, I used 1A slo blo for my similar build (I have 220V too)
7) not necessarily but I'd use it
cheers

[Morocotopo]

Thanks bancika, getting a clearer picture now.
Still, some doubts remain. Anyone?

Morocotopo

[ambulancevoice]

i thought when chassic grounding the mains, you couldnt connect the circuit ground to the chassis too??

[bancika]

it doesn't ground mains, it's just connected to the safety ground. And yes, both safety and circuit grounds should be connected to the chassis at one point.

[Morocotopo]

Thanks again bancika, I´ll reread your tube preamp project web page (reactor?) to see if any info is pertinent to my build.
So, enclosing the transformer(s) in a grounded metal box won´t do any good regarding hum pickup? Hmmmm... So, if , for example, I build a power supply for a pedalboard it´s the same to build it in a plastic case or in a grounded metal one? How do you avoid hum pickup in pedalboards? Questions, questions...

Morocotopo

[bancika]

yeah, my reactor is very similar to what you're trying to build. I used back to back transformers but I fried one of them so I got custom toroid.