Thank you, Mona.
In the first schematic, I calculated the cathode to grid voltage wrong (2.2V instead of the correct 5V).
Still I'm a bit puzzled. Doesn't the part of the 'swing' from -1.3V to -0.5V at the control grid of the pentode section cause grid current? I thought that in many tube types (among them both the sections of the PCF80) this will start from about -1.3V going.
In the first schematic, I calculated the cathode to grid voltage wrong (2.2V instead of the correct 5V).
Still I'm a bit puzzled. Doesn't the part of the 'swing' from -1.3V to -0.5V at the control grid of the pentode section cause grid current? I thought that in many tube types (among them both the sections of the PCF80) this will start from about -1.3V going.
At -1,3V grid current is very small, from -0,5V up it can give problems.
On of the reasons to use the pentode side for the feedback, usualy low resistances, not very sensitive for grid current.
Another is, the triode side has -4,7V on the grid, you need a coupling capacitor.Since normaly connected to the anode previous stage, there is the need for a capacitor anyway.
And third, the triode needs a far bigger swing on te grid compaired to the pentode(triode), much less acticity at the cathode, making live for the current source easyer.
Mona
On of the reasons to use the pentode side for the feedback, usualy low resistances, not very sensitive for grid current.
Another is, the triode side has -4,7V on the grid, you need a coupling capacitor.Since normaly connected to the anode previous stage, there is the need for a capacitor anyway.
And third, the triode needs a far bigger swing on te grid compaired to the pentode(triode), much less acticity at the cathode, making live for the current source easyer.
Mona
So there is control grid current flowing? And it is flowing about half of the time, increasing and decreasing in value? But it is not a problem for audio purposes?
That would open a door for me, that until now I thought was not even a door. Like using the PFL200 (with the F section being vari-mu, but at low values for Vg1 you could stay away from operating in the tail) and the PCL200 (Aikido topology, cathode-follower).
That would open a door for me, that until now I thought was not even a door. Like using the PFL200 (with the F section being vari-mu, but at low values for Vg1 you could stay away from operating in the tail) and the PCL200 (Aikido topology, cathode-follower).
There is not much data on this subject.
Indirectely I find the following.
According to Philips data on the ECC83, Vb=250V, Ra=100k, Ia=1mA and biased with a 10M from grid to cathode(ground).The tube is working with Va=250V-(100kx1mA) = 150V and 1mA means -Vg=1,15V.
The grid current gives 1,15V over 10M or grid current 0,115µA.
Somewhere else they give a grid current of 0,2µA without the corresponding Vg.
I would say we can safely asume the grid current is well below the µA region at grid voltages ~-0,5V.
Mona
An article about the subject: Grid Current and Grid Blocking
Re-reading post #22: Doesn't the tail of the LTP also represent resistance over which the grid current causes voltage changes?
Re-reading post #22: Doesn't the tail of the LTP also represent resistance over which the grid current causes voltage changes?
The grid current has to come from the current source tail.
Gives an inbalance between the anode currents. But a sub µA current compaired to allmost 8mA source, no big deal.
If there is a coupling capacitor at the grid with current, the capacitor will be charged resulting in extra negative voltage on the grid.Is what also happens with an overdriven final stage resulting in a shift of bias.
Mona
I don't find it convincing.
We agree on rising and falling grid current flowing in half a cycle (caused by only one of the two systems in the LTP).
If this grid current is meaningless or not, I still don't know. You stated that up till -0.5V there are no problems. But what is the basis for this statement?
I can't help but repeating my answer to the question of TS: I would not use the pentode section of the PCF86 for audio, looking at the Vg1-Ia and Vg1-Ig2 graphs. I add: This is because you will have to operate it either with grid current to stay in the reasonable linear part of the curves, or in the 'tail part' of the curve, where it is not so linear.
We agree on rising and falling grid current flowing in half a cycle (caused by only one of the two systems in the LTP).
If this grid current is meaningless or not, I still don't know. You stated that up till -0.5V there are no problems. But what is the basis for this statement?
I can't help but repeating my answer to the question of TS: I would not use the pentode section of the PCF86 for audio, looking at the Vg1-Ia and Vg1-Ig2 graphs. I add: This is because you will have to operate it either with grid current to stay in the reasonable linear part of the curves, or in the 'tail part' of the curve, where it is not so linear.
We are talking here about less then 1µA on a current more then 3mA, that's 1 in 3k.Anode resistors with 1% precision make far more difference.
If you look at the Ia-Va graphics, the -Vg have the same curve form up to -0,5V.The Vg=0 has a bump to the left.So I suppose, since the tube behaves uniform with -0,5V and lower, everything is ok up to that -0,5V.
Yes shure, but the TS asked what to do with those tubes he has.Dump them
or play with it 
Mona
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