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Friends
I found a type 36 tetrode and built the above circuit to test it as a preamplifier. However, I observed that from 20Hz to 15KHz the circuit provides 35Vpp at the output, but this voltage goes down to 27Vpp at 20KHz.
What can cause this loss of gain? The capacitances on the tube look ok.
Thanks
Aldovan
Follow Ups:
With such high amplification you will observe capacitance plate > G1 that
is amplified aka miller effect.
Remove cathode cap , this will reduce amplification and create a local feedback and test again.
you are referring to miller capacitance which applies to triodes but the OP is using a tetrode.
dave
Miller effect refers to capacitive coupling between plate and grid. And
yes it's more significant for a triode, but it's there for tetrodes too.
Even if the tube was perfect there is some pf in the tube socket, and
cabling.
In this case with that high amplification it's an effect that exists.
That's why i suggested to reduce amplification AND introduce some
NFB.
Do the voltage measurements match your implementation?
This tube in tetrode has an amplification factor of 500. What kind of preamp are you trying to build?
What is your load?
As you increase the frequency beyond 20K does the voltage continue to drop?
What is the frequency response at 10 v pp?
You have checked that your signal generator is not dropping voltage as you go above 15K?
If it is not, there is some significant capacitance to ground internal to the tube or on the output. The tube may not be perfect.
Yes, the voltagens is ok.
The voltage output voltage is 35Vpp with a input of 1Vpp.
The load is the grid resistor of the next stage ans is 330K.
Yes, with frequency superior to 20k the voltage continue to stop.
The voltage drop and no stop.
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