[HAM] What are the appropriate ohms values for the cathode bias

[Geoff Williamson]

Thanks for clearing that up Brad, I was just going by memory of  
various amps I've toyed with over the years that have different  
cathode resistor values with different plate & screen voltages - as I  
said to Kon, it's a good idea to get the *datasheet* or a good valve  
data book as your reference if you're going to mess around with  
different output circuit designs.  Otherwise you'd be stuck with doing  
maths with too many unknown variables!  When you suggest setting  
cathode resistance to what you are 'comfortable with', are you  
referring to choosing a bias current in keeping with the specs from  
the particular valve's datasheet, (eg. derived from a percentage of  
maximum plate dissipation, or picking a point on the curve)?  Or are  
you talking about subjective sound choices or something else again?

Geoff



On 09/01/2008, at 1:53 PM, Brad Baker wrote:

> On Jan 7, 2008 12:56 AM, Kon Zissis <kziss at ozemail.com.au> wrote:
>> Geoff Williamson wrote:
>>> the value of the cathode resistor is dependent on the plate voltage
>>> applied to the particular valve type
>
> (as far as I know)
>
> Pentodes wired as pentodes have a high plate resistance.  This means
> that changes in plate voltage (B+ for example) cause very little
> change in plate current for a fixed grid/cathode voltage.  Look at the
> curves.
>
> The cathode bias resistance is chosen to select the value of no-signal
> bias current you are comfortable with for that tube.  This current
> value should be reasonably insensitive to the plate voltage.