811A tube internal resistance ?

[Zoran]

Does anybody know about the 811A internal resistance
I try to establish that datas via anode curves diagram
througt grapical method but the results are too much
differs each others that make me very suspicious...
Thanks a lot...

[Fuling]

Somewhere around 10k I think.
µ is extremly high so it behaves more like a pentode than a triode, sort of.
Planing on using them in an A2 amp?
Been there, done that.

[Zoran]

Yes I am planing to begin seting one amp
with a 811A...
I even found some cores by "Wasner"
for the output trans...
Actualy I have problem to aproach to
primar inductivity that is becoudse i
need to know about the Ri of the 811A
What is Your experiance you said that You build a amp with...

[Fuling]

Here´s the basic topology of my amp:

Triode wired E81L input stage, EL86 cathode follower direct coupled to 811A grid. I can post a more detailed schematic within a couple of days if anyone´s interested.

[Zoran]

Thank You Fuling...I will post the settings
of my topology...soon
But on the first site I foun that
You feed the 811 from the cathode...
My intension is to put the input transformer first...

[Zoran]

Hi
I set the 811A at :
Ug = +15 V
Io =60 mA
Ua = 900 V
RL = 10468 ohm (10.5 Kohm)
with estimated :
Pa = 54 W
Po =12.875 W
H2 = around 0%
-A = 34.33
Ig = aroud 5 mA
Cdyn = 204 pF
that requires 5960 ohm input R for -3db at 131KHz or 19541ohm for -3db at 40KHz...
The problem is that I could not calculate the Ri from the anode curves datas and
with that value of Ri calculate Req of the output stage stage...
And when I dont have the Req proper calculated i simply could not calculate
primar inductance of the Output transformer for desired low frequencu response...That is all...

[Zoran]

My results on Ri established by the graphical method,varys from :
28Kohm to 6.3Kohm...
???

[SY]

Ri is nonlinear with voltage, operating points, load, and frequency- modeling it as a pure resistance is not terribly accurate.

[Zoran]

Of course SY...
But I have to start with something...
I am planing to try a litle simulations with a PSpice (I found a subcircuit for the 811A
tube )...
What I heard about the 811A tube from people who done somewhat with is :
That 811 in presence of DC signal behaves like decreasig the Ri rapidly...
and that is Ig increasing until the 20-40 mA ???

[SY]

The best thing to start with is the minimum resistance that the grid will present. I'd do some measurements (pulse excitation, please!) to determine, for the tubes you're actually using, what the Ri is at the maximum expected drive voltage. That can be done with a pulse generator, an amplifier, a series resistor, and a scope.