Jensen Cascode Stage

Hi,

I don't know much about tubes, but have been trying to read up ready for a project I am designing. I have been reading the ValveWizard page on Cascodes (http://www.freewebs.com/valvewizard/cascode.html) and have found it very useful, and was then comparing it back to the Jensen schematic (ignoring the SRPP stage).

My calcs using the ValveWizard formulas say the Jensen cascode stage has an output gain of:
0.0016 x 100k = 160 -> 44.08dB
And an output impedance of 100kΩ, are these correct?

Also, what is the purpose of R3 & R4?

I have attached a cut up version of the Jensen schemo that I've been referencing.

Cheers

ej_whyte said:

My calcs using the ValveWizard formulas say the Jensen cascode stage has an output gain of:
0.0016 x 100k = 160 -> 44.08dB
And an output impedance of 100kΩ, are these correct?

Click to expand...

This is more or less correct, since the formulae are approximate.
As a consequence, you should discard the decimals after the comma in the numeric values: 160 => 44dB

Also, what is the purpose of R3 & R4?

Click to expand...

These are often referred as grid stoppers; they provide some decoupling between the grids and the relevant connections. These resistors are meant to be mounted as close as possible to the tube socket.

OK thanks, nice and easy to reply 

The inaccuracy will be ok since i already plan on having a trimmer to match the gain to another amp, but just out of curiosity how would it be done more accurately?

Is there a reason or formula to set the values of the grid stopper resistors?

Thanks

HI,
http://www.aikenamps.com/InputRes.htm
Laurent.