Re: Does heater voltage affect grid current?

RS <RS@xxxxxxxxxxxxxxx> wrote in
news:ievd14pifojub4g0g17qfo25nj8eq7ic4n@xxxxxxx:

On Tue, 29 Apr 2008 00:17:13 GMT, Doggone <Running@xxxxxxxxxxx> wrote:

Refering to tube data, several have the following statement
pertaining to the grid bias voltage:

"With cathode bias, the grid circuit may have a resistance
not to exceed 0.5 megohm, provided the heater voltage is not
allowed to rise more than 10% above the rated value uinder
any condition of operation."

It's obvious that the idea is to keep grid current from
affecting bias, but how does the heater voltage come into play?
How would a 10%+ rise in heater voltage affect the grid circuit?

As an example, check out:

http://www.mif.pg.gda.pl/homepages/frank/sheets/084/6/6V6GT.pdf

Also found the same or similar statement for the older 6K6G,
6F6 and 6L6 metal.

You'll find some restriction on the "DC path to ground" parameter on
any power pentode. Maybe not published but the principles hold.
The parameter (a resistance in ohms), will be lower for fixed bias--
usually a fraction of the spec for cathode bias.

Although most guitar amp designs use grid leak resistors well
in excess of the tube manuf. recomended values.

The reason for this is not just short-term affect on bias voltage.
That could be trimmed. It is long-term ionization and slow destruction
of the tube. Also an obscure lockup condition can arise (covered in
Radiotron, if you're interested).

Would like to read about that if you can post a link. TIA.

Of course the rate of emission is affected by the filament, and that
in turn will affect the above. But that is the tie to the grid
resistance spec that you mentioned.

I'll have to setup a test bed and experiment with varying fil. voltages.
I have a few metal/microphonic/sacrificial 6L6's and 6F6's in my junk
box.




--
If it's not broken, fix it till it is.
.

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