Newbie needs clarification

[MlinarS]

Hi to all,
As you know from my previous post I'm new to this hoby of ours. This is my first try on design. I choose to start with SE. The 300B as output and mu topology in a driver stage.

I have read few books and some articles on the WEB about SE design with 300B. I have also seen so many designs with plate voltage of 425V. That's vhat make me confused.
I do understand that with increase in plate voltage one can get more PO, but operating 300B at this voltage is very close to the cutoff region that practicaly there is very small voltage sving on the negativa side of cycle. Also, in those same designs people are using OPT with primary impedance of 2K5 to 3k0. This makes a load line very steep. This all makes me wander if the goal is to operate walve in most linear region with biger voltage sving (les power at output) or is the power more important than voltage sving.
One more question, what determines impedance of the primary of the OPT (how do you choose impedance). Does any parametar of the walve dictates choice of impedance and if thet's the case, how? I think this have something to do with output impedance of the stage but I'm not certain about this.

Please help me start on the right track. My goal is to build this amp but more importantly to learn from ground up.

Thank you.

P.S. English is not my mother tonque so please bear with me.

[SY]

Best of luck on this project- it's a little like starting to do carpentry by building an apartment house.

Understanding load lines is important here. Morgan Jones gives a fine illustration in his Scrap Box Challenge design ("Building Valve Amplifiers"), one well worth studying before tackling a design.

[Sheldon]

Steve Bench also has a nice tutorial on his site:

http://members.aol.com/sbench102/po-dis.html

[audiousername]

300B anode characteristics from Audiomatica for reference:

Originally posted by MlinarS
The 300B as output and mu topology in a driver stage.

I don't like the 300B but that's just my own prejudice

Originally posted by MlinarS
I do understand that with increase in plate voltage one can get more PO, but operating 300B at this voltage is very close to the cutoff region that practicaly there is very small voltage sving on the negativa side of cycle.

What makes you think that? If you look at the curves above, 425Vak, -75Vgk is in the middle of the linear part of the curve - far away from cutoff. Of course operating at 425Vak and -125Vgk is a different matter....

Originally posted by MlinarS
Also, in those same designs people are using OPT with primary impedance of 2K5 to 3k0. This makes a load line very steep. This all makes me wander if the goal is to operate walve in most linear region with biger voltage sving (les power at output) or is the power more important than voltage sving.

With triodes, generally the higher load Z and higher supply voltage, the lower distortion. The circuit designer has to trade off between power output and distoriton.

Originally posted by MlinarS
One more question, what determines impedance of the primary of the OPT (how do you choose impedance). Does any parametar of the walve dictates choice of impedance and if thet's the case, how? I think this have something to do with output impedance of the stage but I'm not certain about this.

Sometimes it's just what you have on hand... There is a rule of thumb to say it should be 2-3 times the ra of the valve, though.

A thorough read of the linked page from Steve Bench's site above will help greatly here.

Originally posted by MlinarS
P.S. English is not my mother tonque so please bear with me.

Your English is just fine

[Sheldon]

Originally posted by audiousername
A thorough read of the linked page from Steve Bench's site above will help greatly here.

I'd go one step further. Make a few copies of the 300B plate curves, grab a pencil and straight edge and follow Steve's instructions step by step, including the calculations. Make sure you try few variations on the load lines.

I'm a real newcomer to this tube stuff and electronics in general. I read all I can, but using my hands, doing the calculations, and seeing the relationship between lines, power, and distortion, really gets the concepts down for me.

Sheldon

[MlinarS]

Thanks everibody for response. Your sugestions are very well taken. I need to do some more stadying.

Cheers