12at7 clipping

[Helmholtz]

I recommend to look up what Vk means.

It is not a regulated or limiting voltage, rather it defines a threshold above which the diode gets into constant current mode.
The voltage across the device can get as high as 100V or even higher.

[J M Fahey]

Yes it is and I wonder why ?

No, I am asking why do you want it to clip at 2 Vpk?

The reason why itīs clipping at 1.4Vpk is already in my answer.

[Helmholtz]

Was installed 470k resistor over 500k pot for a total 230k grid leak value. Not convinced I can drive it till 0V but close. I have to proper wire to do the tests. The osscilation from picture is due to my messy wiring board, no worries about..
Now I have to replace the pot with something lower...250k or so...

If I understand the above correctly, a 250k pot will result in about the same condiion (i.e. the same load to the 12AX7) as the 500k pot with a 470k resistor in parallel (i.e. with pot fully on).

[Helmholtz]

The GK diode clearly starts passing current at 1.4V Pk

This means that the GK diode starts conducting around Vgk = 1.44V - 2.1V = - 0.66V, which is a bit surprizing (to me).

[catalin gramada]

No, sorry. I.ve measured wrong things. The input signal was from generator. Doing with 12ax7 didn.t drive. Sorry.

[vintagekiki]

One picture speaks more than 1000 words.
https://schematicheaven.net/boogieamps/boogie_d180.pdf

http://www.tubecad.com/2011/08/blog0210.htm
Cathode Bias with a Constant Current Source

https://www.talkbass.com/threads/mesa-boogie-d-180.1412194/#post-22857910
Constant current source bias

[catalin gramada]

This is how it looks drived from 12ax7 . PI biased at 2.56V. The input was clipped at 1.9v around
Yes I can drive it till 0.6V when it start to conduct so changing grid leak did not do any difference.
I don't intend to install a buffer so think solution is to bias a little bit high and that's it...as time I want to use a 12at7...

[vintagekiki]

Question
Vmax(2) = 1.96V is input voltage on g1 PI
What does represent and where was measured Vmax(1) = 72.0V ?

[catalin gramada]

I will put the pic drived from generator just for comparison

[catalin gramada]

Question
Vmax(2) = 1.96V is input voltage on g1 PI
What does represent and where was measured Vmax(1) = 72.0V ?

Yes 1.96 Vpk in g1 PI, 72Vpk output of g1, cathode biased at 2.56v
The output from first PI stage. The output tubes are at -77v

[vintagekiki]

Yes 1.96 Vpk in g1 PI, 72Vpk output of g1
The output from first PI stage. The output tubes are at -77v

#24

2VAC on g1 PI is sufficient drive for the full power on output stage. The output tubes is biased with -70VDC, how much is quiescent current (measured in cathode).

[catalin gramada]

At -70V is 50mA for the tubes installed now

[catalin gramada]

No, I am asking why do you want it to clip at 2 Vpk?

The reason why itīs clipping at 1.4Vpk is already in my answer.

I want lo use a led to keep internal resistance as low as I can. My stock are all in 2V range.

[vintagekiki]

It's already hot biasing. Very brave.

[vintagekiki]

Vmax (2) = 2.04V. Input voltage is clean
Vmax (1) = 74.0V has some distortion on lower part of the sine wave.
Maybe problem is in relative no matched power tubes, or in bias adjustment.
Are there distortions only at max power or in example at half power.
Question
If Vmax (1) = 74.0V how much is output power measured on load (speaker)

Useful link
http://web.archive.org/web/20110621210909/http://diyguitarist.com/GuitarAmps/PowerTubeBias.htm
Common Power Tube Bias Charts

https://www.tungsol.com/specs/kt120-tung-sol-specs-curves.pdf
KT120 - Tung-Sol tubes