[HAM] Plate resistor values and bass

[David Anderson]

Kon,

Unless there is something terribly wrong with the power supply, the  
amount of coupling between tube stages through the power supply  
should be insignificant.

Also, depending on how the circuit is set up, every other section  
will be out of phase, canceling any coupling. In fact, some tube  
stages are cleverly designed such that there is little change in  
current draw from the power supply with applied signal. The input  
12AU7 of a 122, for example, should show little to no change in total  
plate current draw for both sections combined whether there is a  
signal or not.

I would note once more that by swapping a 1 Meg resistor for a 100k  
resistor, you are changing the operating point of the tube by  
reducing bias. Tubes are linear only within a certain operating points.

The current DIY trend towards loading triodes with Constant-Current  
Sources rather than plate resistors is a good example of why the idea  
that a larger plate resistor = more bass is not valid. The CCS  
appears to the plate as an almost infinite resistance, but by setting  
the proper bias point, small signal triodes can be made extremely  
linear via this circuit.

David

On Feb 18, 2008, at 7:13 PM, Kon Zissis wrote:

> Hi David , Christoph and Keith.
>
> David Anderson wrote:
>> but  to claim simply that increasing the value of a plate resistor
>> increases bass is cutting far more electrical engineering corners
>> than I'm comfortable with.
>
> Although I am not sure about the correct electronic  explanation  of
> what is happening , I suspect  that what is happening here is that
> because there is a much higher resistance between the plates of the
> valve stages with the 1 Mega ohms plate resistors , the negative
> feedback effect  is lessened between the various valve stages thus
> allowing a causing bass response.
>