EL81/6CJ6 operating point

[Giulio Maiocco]

Hello,

Sorry to stress you guys again with my strange questions, bud does anyone knows a suitable operating point for a g2 driven EL81 push-pull? I am thinking to build the preamp section following the Berning EA230 schematic.... The problem, as usual, is the required plate to plate load for the 2 EL81, the negative g1 voltage and the b+ voltage to the plates

Thanks again

Giulio Maiocco

[ilimzn]

This is not a simple problem as it is possible to find many operating points given the variety of possible combinations of -Vg1 and Vg2. In theory, an extensive drawing of curves for variuos -Vg1 could give you a sweet-spot.

Another approach would be to make an educated guess.
Have a look at the datasheet at:
http://www.mif.pg.gda.pl/homepages/f...030/p/PL81.pdf
Speciffically page 6 (class B PP operating point example) and page 12 (G2 curves for Vg1 = -1V). As you will see, the Vg2 curves are similar to pentode curves so the pentode class B operating point could be used, perhaps with a higher V plate and higher P-P load. For lower Vg1 the required Vg2 will be lower, ditto Vg1, but the linearity might be worse.

[Giulio Maiocco]

Hello,

This probably will show that I'm completely ignorant when it comes the need to use the tube curves, but could what I "drawn" possibly work without melting the E/PL81?

I have quite a lot of them, but I'm always disappointed when I have to junk tubes! Just telling you a "funny" thing, last week I had to trash 3 Sylvania 6L6GC

Also, plese could some kind soul explain me how to calculate the optimum plate to plate load starting from the curves of the tube? Please, no difficult math, as I have a hard time figuring how it works!


Regards

Giulio Maiocco

[gingertube]

Look on Patrick Turner's webpages.

http://www.turneraudio.com.au/

Click on the Educational and DIY page. All you ever wanted to know about load matching.

Cheers,
Ian

[rdf]

Originally posted by Giulio Maiocco
This probably will show that I'm completely ignorant when it comes the need to use the tube curves, but could what I "drawn" possibly work without melting the E/PL81?

Not without a liquid nitrogen bath, the datasheet specifies an anode dissipation limit of 8 watts. Start by plotting points on the screen characteristics where Va*Ia=8. I think you'll find almost the entire usable area falls into uncharted territory beneath the Vg2 = 100 trace.
And what gingertube says.

[Giulio Maiocco]

Hello,

Thanks a lot for the link about the plate to plate load calculations, I have to read it again and "experiment", as for now it is still a bit fuzzy.

Anyway, I tried to do the "plot" for the EL81 and it seems it is not one of the best tubes to be used in audio applications, mainly because of the very low anode dissipation! I traced the plate dissipation on the graph, it is hand- drawn, so not much precision!

Is this time the "plot" right or should I correct something else?

Cheers

Giulio Maiocco