Question about the voltages on 12AX7 pre amp tubes

[V_amp]

Hello all

Recently I finhished an 18W TMB (dockery) and noted that for the 12AX7 pre amp tubes, the differece of the voltages at the pins 1 - 3 and 6 - 8 is around 100-120V.

But depending of the tube, this voltage are different:

First Tube:
at pin 1: +/- 128V and at Pin 3: +/- 1V
grid 0V

Third Tube (phase inverter)
at pin 1: +/-165 and at Pin 3: +/-55V
grid : 26V

Second Tube
at pin 6: +/-230V and at pin 8: 130V
grid 126V

Third Tube
at pin 6: +/-174V and at pin 8: 55V
grid 27V

Couldn't I use 120V for each triode of every tubes?
It will not produce similar effects?
Why change the voltage of the screens?

In summary why are there these differences?

I image that the answer is a course of tube amps, but it can be explained step by step I think.

Thanks all
FAB

[snoof]

There is no screen on a triode. You might have your terms mixed up.

[V_amp]

It means Grid ( I changed in the topic)

http://www.tpub.com/content/armyordnanc ... 5b0021.htm

[zaphod_phil]

Each triode is biased differently to get different tonal variations. One triode is a cathode follower (in front of the tone stack). The last two triodes are the phase invertor. The CF and PI voltages are very different to the other preamp triodes'. And yes, they have to be that way.

[CurtissRobin]

Couldn't I use 120V for each triode of every tubes?
It will not produce similar effects?
Why change the voltage of the screens?

In summary why are there these differences?

I image that the answer is a course of tube amps, but it can be explained step by step I think.

The short answer to your first question above is "No."

Each tube and circuit has a purpose and changing the circuit to put the same voltage everywhere means that the tubes will no longer function the way they need to do to yield an 18 watt amp. One answer is certainly an electronics course or two or five but you can learn enough online to be conversant with if not proficient in tube electronics. I suggest starting with the front hundred pages of the RCA Receiving Tube Manual which gives a concise introduction to tubes and their applications. You'll still have a long way to go but at least you'll be on your way at a very reasonable price.

KennyO