Newb asks Plate Voltage question

[Nathan]

Hi all, I am getting close to knowing my way around tube amps but have only really copied circuits before without understanding just what is going on. I hope you can answer this simple theory question for me:

I built an amp from scratch and the triode plate voltages are low (2 are almost half where they should be). What do I do? 

a) Decrease plate resistor value to allow more B+
b) Increase plate resistor value
c) Something else altogether

Thanks!

[tubenit]

Nathan,

We need to know what tube. For example, in the same circuit a 12AX7 will have a different voltage than a 12AU7.

What is the "proper" voltage and what are you getting that you feel is "half" or what it's supposed to be?  Please share what tube and what your current voltage is.

To increase voltage .............. 

lower plate resistor
lower B+ dropping resistor to that node
increase cathode resistor

with respect, Tubenit

[PRR]

> almost half where they should be

Where "should" they be?

Does it play OK?

[tubeswell]

It would help to see what circuit you are talking about. Are you able to post a schematic (or a link)?

[jjasilli]

Ditto to the above posts.  Still, it helps to work backwards for design purposes.  For preamp tubes usually* the plate resistor-cathode resistor combination biases the tube @ the B+ supply voltage.  That's the B+ voltage fed to the plate resistor (@ its B+ side).  Working backards from there you eventually get to the desired plate voltage for the power tubes.  Then you figure what secondary voltage the PT needs to supply.  The resistors in the B+ rail serve (at least) 2 functions:  a) their presence isolates filter stages from one another; and b) their value drops B+, per Ohm's Law, to the supply voltage desired at ea stage of the amp.

The plate resistor is not used to set the supply voltage; that's done in the B+ rail.  The value of the plate resistor will, of course, affect the plate voltage.  But that's a biasing thing and should be distinguished from the supply voltage.

*there are other biasing alternatives.