Design Help (load line) on a SE Mono Block Amp build

[myDogFred]

Hello,

I was hoping to get some advice here on selecting the correct values on a triode (12AX7), I will only be using one stage, and for the power tube I will be using a Pentode tube (5881 I think) cathode biased and maybe running in triode mode. My rectifier will be either a GZ34 or 5Y3. The power transformer is putting out about 410VDC at the first capacitor (no choke).

My problem is that I am having some trouble understanding how to plot a load line on the 12AX7 data sheet in order to select the proper values for Rg, Rp, Rk & bypass cap, Co, ect......

How do you guys determin these values or how do you plot the load line?

Thanks!

[Ian444]

You can go here: http://www.valvewizard.co.uk/ and download the "triode gain stage" PDF. I found that link in a sticky at the beginning of the Tubes section here under "On Line Tube Learning for newbies". The Valvewizard was the best place for me to get some basics happening...

[myDogFred]

Thats a great web site!

Thanks

[Miles Prower]

Originally posted by myDogFred
My problem is that I am having some trouble understanding how to plot a load line on the 12AX7 data sheet in order to select the proper values for Rg, Rp, Rk & bypass cap, Co, ect......

How do you guys determin these values or how do you plot the load line?

Go here: Steve's Tube Pages, scroll down to the "Technical Reports" links (extreme right-hand column in the table) and look for the five part series on loadlines. That'll answer just about any question on using loadlines.

As for values for Rg, it's not too critical. For capacitor values, see the "Poles and Zeros" link from the same technical reports column from the above web site.

Although I have my doubts as to just how suitable a 12AX7 will be driving a trioded pentode final. 12AX7s have very limited current capabilities and like to see a Hi-Z, Lo-C load, and that doesn't describe the grid circuit of very many finals.