DC coupled C.F. question

[catalin gramada]

Hello, how to avoid grid current into a 12ax7 C.F. DC coupled ,please ?
The tube is supplied at 300V around and coupled at 75V through a 1M/1M voltage divider from the 150V plate of previous stage.Should be biased at -2V arround but it is not. I instaled 1K grid resistor. I don't have a precise milivoltmeter but on my fluke175 still recorded 0.1mV over it which is pretty evident as time I cannot proper bias my C.F. stage. The actual cathode resistor is 100k and draw almost same amount of current as recorded over 220k plate resistor of previous stage. With 47k seems the derive is better somehow...Please let me know which is the best way to limit the current to none through the CF grid. Thanks.

[Dave H]

The actual cathode resistor is 100k and draw almost same amount of current as recorded over 220k plate resistor of previous stage.

That sounds about right to me. (300-150)/220k = 0.68mA. 75/100k = 0.75mA.

The plate of the stage driving the CF is at 150V so the grid of the CF should be at 75V and its cathode at 77V. Is it not like that?

[catalin gramada]

That sounds about right to me. (300-150)/220k = 0.68mA. 75/100k = 0.75mA.

The plate of the stage driving the CF is at 150V so the grid of the CF should be at 75V and its cathode at 77V. Is it not like that?

Thanks for answering. No, the difference is 6V around. I wonder why?

[g1]

You are measuring with one probe on grid and your other probe on cathode? (neither probe to ground)

[catalin gramada]

No, I measured voltage of divider and voltage over cathode resistor and I did the difference.

[Helmholtz]

Can you post a schematic of the stage with voltages?
Did you try different tubes?
At high cathode current grid bias of the DCCF may vary considerably with individual tubes.
It is not uncommon to find positive grid bias in the DCCF of e.g. an AC30.

[catalin gramada]

Is a straight design, changed tube, measured resistors and here we go...

[Helmholtz]

Is a straight design, changed tube, measured resistors and here we go...
[ATTACH=CONFIG]51913[/ATTACH]

So problem solved?

[catalin gramada]

...

[Helmholtz]

...

First of all your voltages indicate that the DCCF has negative grid bias.
Most DMMs have an input impedance of 10M, so voltages across an 1M resistor may measure up to 9% low depending on rest of circuit.
Do you still measure a positive grid-cathode voltage at the CF, attaching the red meter lead to grid and the black one to cathode with the changed tube?

[Dave H]

No, I measured voltage of divider and voltage over cathode resistor and I did the difference.

Your meter is loading down the voltage divider.
Measure the plate voltage of the driver stage and cathode voltage of the CF.
Do they measure as you would expect knowing the divider ratio and -2V CF bias voltage?

Edit: I see Helmholtz beat me to it

[g1]

Most DMMs have an input impedance of 10M, so voltages across an 1M resistor may measure up to 10% low depending on rest of circuit.

If I insert that parallel 10M into that voltage divider I calculate 71V at the grid.

[catalin gramada]

Yes, should be like that. The divider resistors are perfectly matched so the voltage should be half (except if grid C.F. do not start to conduct) In this circumstance the difference was -2.8V around with 100k in cathode. Changed to 47k and adjusted the supply as same as before (300v) get -2.25V

[catalin gramada]

It have no sense to me. If I want to do the grid less negative in respect with cathode I suppose to raise the cathode value for the same fixed bias in grid.....but is happen exactly opposite....

[Helmholtz]

It have no sense to me. If I want to do less negative I suppose to raise the cathode value for the same fixed bias in grid.....but is happen exactly opposite....

Why? A lower value cathode resistor requires more cathode current (as cathode voltage follows grid voltage within a couple of volts), thus more positive/less negative grid bias.

What is the actual problem?