Because you already know at least the basics, and whatever question you ask is probably going to be specific and thoughtful.I built my first tube amp in 1967, but I would never claim to anything like EE knowledge, so I've had to learn everything the hard way, a piece at a time, by...blowing things up (like electrolytic capacitors, that is)
...or frying tubes (hmmm...maybe the data sheet was right!) 
....or the frequency response was only 2 KHz to 6 KHz (so THAT'S what qts means....) 
But for many of us...who have discovered tubes, the glooooooowwwww of tubes, the sssoooouuunnnddd of tubes....but who only have a smattering of understanding (or none at all)...there's a real chance of making a complete nuisance of yourself, and finding yourself ignored or even insulted on forums. It's no fun, and it doesn't get you what you want...which is your own tube amp, proudly built by you, which fills your house with glorious sound!!
So, here are some rules to follow that I've gleaned from watching various newbies self-destruct and remembering my own blunders over the years
1. Lurk!!!! Learn!!!
Just read, read, read, and stay quiet. Follow several forums (I have a list of great ones on the main page on the right side). Over time, you will discover some things:Some of the members really know stuff, and they will post projects (or have projects at their websites) worth building. Start thinking about which one you might want to build...but don't jump at it...you'll be amazed at how that first project you thought was soooo great doesn't really suit your needs. You don't really want to fill the center of your living room with an amp spread out on plywood you have to walk around to the other side of the room to work on - and which, if the cat ever wanders into the center of, is likely to cause domestic discontent as you try to explain to your loverly/handsomerly other how the cat turned into a smoking, blackened, greatly puffed-up carbon-filament fuzz-ball, and why it no longer moves or makes hungry noises.
I will be posting a category of projects worth building, culled from the net. Amps and speakers that look to me like really good work, something you could build and really enjoy.
2. Try to resist the impulse to join right up, and say Hi!! to everyone, and tell about your Uncle that was a Ham, etc., etc.
Sorry, tube amps are actually serious stuff, and the old members of the forum - most of whom are serious electronics geeks - don't give a rat's behind. If you want friends, join AOL (I guess, I wouldn't know) or maybe a band groupie-club. If you want to make a great tube amp, pay attention to the masters and leave Uncle out of it. Now, it's true that you may wish to join in order to view images and schematics that may not be available to you as a visitor. That's OK. Just don't announce it. Nobody cares. 
Sorry. I really am. Oh, the founder of the forum might care, it's ego-building to get a new member, and founders of forums don't usually get much thanks and certainly no payment (except for the forum of an amp builder who sold you an amp, but this is DIY-land we're talking), so you'll get a welcome from the founder and a few others, but....well, take it with a grain of salt, that's all I can say.3. Start to notice who's nasty and who's nice. And who's just a nut blatting away on a one-note trumpet. This will be good to know later when you finally decide to post; think about whether you really want to post in a topic dominated by a nasty (doesn't have to be the moderator) or a nut. Insults hurt. Flame wars really hurt; if the nasty is a board-member, you will lose the flame war (not that there is ever really a winner in a flame war). And then there are "pig-piles", too, which you don't want to find yourself at the bottom of.
And all of that is on the web forever. All the forums claim to forbid such nasty behavior, but good intentions and actually enforcing it are two different things. And even when a moderator steps in, I've noticed that the insults don't always get deleted or moved. 
So, they stay there...forever. 
4. Corollary to #3: I've noticed that, all in all, the better projects seem to belong to the nice guys. The nasties do stuff, yes, but their stuff tends to be a little quirky or....off... "Their way or the highway" kind of thing. Which doesn't mean that they're right. Just close-minded. And if you post something that doesn't agree with their view of the world, get ready to be insulted.
5. GOOGLE is your friend.
(unless you live in China....sorry) Try really hard to use google to answer most of your own questions. Don't barrage the forum members with dumb questions. Look it up! Figure it out! Forum members do NOT get paid to answer dumb questions...SORRY!! They do get tired of explaining, for the thousandth time, what size bypass capacitor is needed for the cathode resistor. Some forums DO have the motto: there's no such thing as a dumb question...and that's a really good thing...but after the twentieth dumb question you might find their responses a little...ummm...tepid, shall we say? Now, if I had a forum...which I probably never will, but who knows...I would create a way of easily directing simple questions to FAQ answers. If there is one already somewhere, I'll gladly link it. Someday maybe I'll make one up myself. But I know that using FAQ lists are a royal pain, nobody likes them, so a friendly directive would be nice: "Great question, everybody has to learn how to figure that out to design a tube amp; go to this link: cathoderesistorbypasscap/FAQ/HTML. If you need more info after reading it, please feel free to ask." Now, once you've done your research, and you're still stumped, or have popped up to a higher level and have a more sophisticated question, that's different - explain briefly what you've read - a few links are a good thing - and ask what's puzzling you. You'll get lots of friendly help, because you tried. 
6. Resist the impulse to post a project one step at a time, as you go. There might be exceptions, like building someone's project, and showing your progress, and maybe posting when you get stuck and need something clarified. But posting as you go, complete with mistakes, backtracks, changes, detours...this is one I've been guilty of, so I know...if you take it too far, everybody just gives up on it. What people like: when you've done all the hard work, and have a nice result, and can explain why it works. There, in black and white. What nice guys will be glad to help you with: when you've done hard work, but get stumped, and need a little help. No problem. Geeks love to solve problems...that is, real problems. There's a big difference between the problem of blissfully ignorant - where you haven't done any homework at all, that is, real bench-work - and the problem of a technical challenge you've come up against after putting honest work in.
7. Be careful where you wander....make sure you're still on the DIY part of the forum, not an associated COMMERCIAL FORUM FOR A MANUFACTUROR'S PRODUCT....this is especially true at audio asylum (see list of forums). Once, enough years ago I'm not embarrassed about it anymore, I was trying to get the answer to a simple technical question - I forget, it was probably really a DUMB QUESTION - on Audio Asylum, before I knew better. Somebody made the innocent suggestion - maybe, in retrospect, it wasn't so innocent
- that I pop it over to one of the MANUFACTURERS FORUMS. Wow, was that a mistake! I got tarred, feathered, burnt up by a flame-thrower, and they were getting read to ride me out of town on a rail when I realized they had completely misunderstood...or maybe not ...and I got the heck out of there, never to return. So that's why I'll never link to that commercial site or it's forum. Now, the owner may be the nicest guy in the world, but I'll never know. All I know is I got a very rude reception, I guess from "his posse", and will never go back. Which could be unfair to him, but criminey, all I did was ask one simple question....but if you wander in there without first buying one of his amps, don't say I didn't warn you....
....does the same apply to other MANUFACTURERS FORUMS? Well, at the very least it is NAIVE (maybe even rude) to ask a newbie question in a commercial forum you didn't buy a product from, but on the other hand, it IS a chance for a manufacturer to get you as a customer...so I guess it just depends. My advice, unless you're seriously considering buying one of their products and have a specific question about IT, is DON'T. 8. Now, this is a toughie.
Try really hard not to think you're going to invent some new circuit that's going to knock everyone dead, right off the bat. Trust me, it's all been done before. Well, almost. The only recent patent I think is worth a fart is one of Nelson Pass's. Most recent amp patents are boooolsheeet. I love it when Broskie (tubecad) makes mincemeat out of some "new" patent. 
I think it's really important to keep in mind that those guys back in the 1920's and 30's and 40's and 50's were SMART. (which is a good thing!!) Just about every possible way a tube - and it's grids - can be used has been tried. Uh, decades ago. So, learn, learn, learn, (go to Steve Bench's site, which I pray never disappears from the web...OOPS, it's gone! Thank you very much, AOL.), build something good that's already been done, and listen. Don't even think about trying to do original designs without an oscilloscope and a distortion analyzer - because how are you going to figure out how to optomize the design, and how are you going to post the design and back up your claim that it sounds good? Tube amp understanding has come a long way, and freaky designs without good measuring really don't deserve much attention. Is there still room for improvement, for new ideas, for new tweaks? Yes, absolutely, I'm just giving you a little friendly advice that true improvements in amp design don't come easy - it takes a lot of hard work and time - it took me years of getting back into tube amps and really working hard at it before I started to have an understanding of where real improvements in design could even be attempted, much less figure out how to get that improvement.
For example, I'm getting close to having a finished PCB board design for an auto-grid-bias board for push-pull output tubes that keeps them in balance - almost perfect balance - that keeps them in Class A as long as possible but allows temporary excursions into AB - that prevents runaway tubes ("cherry" tubes) even if the output is dead-shorted - but, you know what?
(I'll tell you what....twice, actually):First, it took me a year to develop this circuit. Of course, part-time, mostly stolen hours on the weekend from chores I was supposed to be doing
but, second...It wasn't really a new idea. As John Broskie pointed out in a tubecad blog entry: http://www.tubecad.com/2005/May/blog0046.htm
Alfred Blumlein invented a push-pull balancing circuit in the early 1930's (the "garter" method) which was, using what was available to him at the time, pretty good....
And my circuit is actually based on a circuit Broskie posted in the same blog...I took one element from his circuit, using an op amp integrator with a 1-second RC constant, and then developed my own from that, complete with my own bells and whistles (such as the protection against runaway mode even with output shorts, etc.)...
But I wouldn't claim my circuit was at all an original idea (for Heaven's sake, I'm using op amps!! - "transistors for dummies"), it's just my implementation of an eighty year old idea from my own perspective. That's all. No biggie. And it took a ton of reading everything I could on the web and in print about tube electronics (and op amps) before I was able to put it together. I actually call it the Broskie / Knouse Auto-Grid-Bias Circuit, because without that one essential building-block idea from Broskie, I would never have thought it up.
When I get the new PCB's and build and test a bunch, and all turns out well (it works great on the prototype board, but converting to PCB is always a nail-biter
), then I'll send some finished ones to Broskie and ask his permission to include his name.So, thanks for reading, I hope this has either entertained you or helped you. If you have suggestions for additions to the list of ways to be a good forum member, I'd love to hear them - as soon as I figure out how to enable the contact part of this blog thingie, that is...
Best, Charlie
