Calc Plate Load for Cascode/SRPP?

OK... I'm lost here...I want to build a Slo Blo2 using 12SN7's for each stage... as per the metas. The final stage has the plate of one triode conected to the grid of the other.... with the output between cathode and earth.

My dilemma is this.....the 12SN7's I get hold of will determine the voltage I run at... older ones seem to prefer less voltage. This will change the plate load....OK.... Now I'm building this for a friend who wants it to go into an EQ with a 50k input impedance. I want to roll my own output tranny.... but I'm unsure how to work out an accurate load line relative to the voltage.

The RCA manual give a plate resistance of 7700ohms at 250 volt. OK that gives me a 1:2.6 ratio for output into 50k..... but I don't know if this is correct for this topology????????

Later 6/12SN7's have a max voltage of 450v. I like to live on the edge..... what if I run 'em at 400v ......chew the valve a bit.... but like the sound..... :roll:

I dunno what load to wind to.... Any direction appreciated...... and yeah.... I should stick one hand down the front of my pants and keep it there while working with this sort of voltage. :shock:

Your main voltage problem will be cathode-to-heater voltage, as the cathode of the upper tube will sit at ½VB.

The Tubecad.com journal had a very good issue about SRPP, math and all. It is among their back issues, available for free download from the site.

Jakob E.

A big thank you for the pointer Jakob! I've added that one to my favourites.....


http://www.tubecad.com/articles_2002/SRPP_Deconstructed/index.html

(Edit: A little knowledge ........ beats no knowledge at all.......just :? )