12at7 clipping

[catalin gramada]

No,no Vk. the picture with 2.04 Vpk clean is the input voltage from my low impedance generator directly in the PI input. I was posted just for comparison. From signal generator I can drive the 12at7 PI to 0V without problem. The problem is to drive the PI from another tube.plate. I tried 12ax7,12at7 and 12au7 meant a 6k output impedance. The 12at7 PI cannot be simply drived to 0V. It needs as Helm stated a very low impedance source as my generator is. Or to bias the PI as high and drive it to 0.6V when it start to clip

[vintagekiki]

If schematic is not top secret, please upload complete schematic together with power supply.
Publishing schematic parts leads nowhere. We spin unnecessarily in circle without results, and problem may not be in PI

[vintagekiki]

What happened?
Do you need any help?

[Dave H]

[B]
If required voltage is 2V, choice is an S-152T diode with a typical 1.5 mA current.

I don't think it works like that. Vk=2V is the voltage above which it operates as a current regulator, below 2V it won't regulate.

I think it goes like this (I may be wrong) You chose the diode for the required current and the voltage across it will be the Vgk required by the valve to pass that current with a plate load of 50k and B+ 500V
The B+ voltage is 500V, say you want to bias it at 300V to be closer to centre bias. Plate resistor is 50k so each triode must pass 4mA for 50k to drop 200V from 500V B+ to arrive at 300 plate voltage, therefore we need an 8mA CRD. If you measure the voltage across the CRD it will be in the 5V range with an AT7 not 2V

Catalin wants to bias at 2V because he only has 2V diodes but that biases the plate voltage way low possibly limiting output voltage swing. Why not use two diodes in series for 4V to bring it closer to centre bias?

[vintagekiki]

About Constant Current Source

https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/45282-phase-inverters-with-ccs
Phase Inverters with CCS

post #36

post #30

http://web.archive.org/web/2016*/http://www.thetubestore.com/lib/thetubestore/schematics/Mesa%20Boogie/Mesa-Boogie-Simul-Class-295-Schematic.pdf
Mesa Boogie Simul Class 295

https://ampgarage.com/forum/viewtopic.php?t=30079
Mesa Boogie 295 - Retroengineering
You must log in or sign up

[nickb]

Late to the party so forgive me if I go over the same ground.

You seem to be saying

1) I want to clip at +2V
2) I want to bias at 2.1V
3) I want to drive from a 12AX7

Short answer - you can't have all three.

The best info I can find is this plot of grid current for at 12AT7.



Take the grid currents at Va= 200V and plotting the grid current (y axis) we get the red line. I extrapolated this to add the blue line.



This suggests you should expect significant grid current above Vgk > -2V and explains the problem you have. And indeed is what you are seeing. You need a low impedance source to supply the grid current and would need to consider the effects that will have on coupling capacitors charging and so shifting the bias point.





Now here is the DC load line. The chosen Vg of -2.1V gives you a very asymmetrical output swing. It would be better IMHO to go for about 300V on the plate which means a Vg of -5.5V. So, you have plenty if room to move your bias from -2.1 to something a bit more negative, -2.7V or so to get the 2Vpk clipping you want. You seem to be wedded to the idea of using LED. I think a resistor is a better choice as you gain flexibility in your selection of bias point and you get better DC stability to compensate for different 12AT7 specimens. Not of consequence here but LEDs can be a source of flicker noise too.

[vintagekiki]

Is it theme extinguish or 12AT7 stopped clipping?

[Helmholtz]

....Short answer - you can't have all three...

Excellent contribution.

I think your extrapolation is a bit pessimistic (I read Ig = 2mA@Vgk = +1V and Ig = 5mA@ Vgk = +2V from the tube chart, so grid current might start somewhat above -2V).
The principle is clear, though.

(Generally I'm not in favor of vastly exceeding the 12AT7 plate voltage limit of 300V, which applies to plate supply in class A.)