LTPI little help, please.

[catalin gramada]

Hello,
I did a ccs to force in balance my 12au7 PI. I used what I have on hand...another 12au7. The circuit is not finally tuned but it seems it is nothing to tune...The PI works as well imbalanced with ccs as a fixed 22k resistor. Can You take a look to see what I did it wrong ,please? There are not power mosfets here lying around so I ask for a suggestion how to get the best ltpi balance with this 12au7 tube, Thanks.

[pdf64]

Have you missed decoupling off the 'other' grid?

[catalin gramada]

Have you missed decoupling off the 'other' grid?

I did..sorry, fixed.



It was just a drawing issue, the cap is there together with 1M dc reference

[Tony Bones]

By my estimation, the plate resistance of your 12AU7 CCS is only about 15K to 20K. Merlin talks about using the same tube to get 36K, but I don't see it. http://valvewizard.co.uk/ccs.html

[Helmholtz]

By my estimation, the plate resistance of your 12AU7 CCS is only about 15K to 20K. Merlin talks about using the same tube to get 36K, but I don't see it. http://valvewizard.co.uk/ccs.html

Excellent point!

I get a current source impedance of 15.5k (@Ip= 7ma), which means that the 12AU7 makes a rather poor CCS.

[catalin gramada]

By my estimation, the plate resistance of your 12AU7 CCS is only about 15K to 20K. Merlin talks about using the same tube to get 36K, but I don't see it. http://valvewizard.co.uk/ccs.html

Thanks
I did not any estimations, but it realy measured like 15-20k. 22k fixed tail did not help to much, nor 27k either. Over that I cannot pass. Want to keep hk potential under 180v

[Tony Bones]

I don't think you can get more resistance at 7mA with 1/2 of a 12AU7. Maybe both halves in cascode...

[Helmholtz]

Instead of trying to increase non-inverting gain you may simply decrease inverting gain by tapping/splitting the plate resistor at the inverting plate for better signal symmetry.

[catalin gramada]

Instead of trying to increase non-inverting gain you may simply decrease inverting gain by tapping/splitting the plate resistor at the inverting plate for better signal symmetry.

Thanks.
Sorry, but don't get it.... It is not the same result in effect, please?

[nickb]

Hello,
I did a ccs to force in balance my 12au7 PI. I used what I have on hand...another 12au7. The circuit is not finally tuned but it seems it is nothing to tune...The PI works as well imbalanced with ccs as a fixed 22k resistor. Can You take a look to see what I did it wrong ,please? There are not power mosfets here lying around so I ask for a suggestion how to get the best ltpi balance with this 12au7 tube, Thanks.

[ATTACH=CONFIG]54668[/ATTACH]

It might be rubbing you your inner need for vacuum tubes the wrong way but you'll much better balance at a fraction of the cost by using a couple of 10 cent transistors. The ratio of signal current thru the cathodes to that through current the source is about 54dB (500:1) with this arrangement. Near perfect balance.

[catalin gramada]

Thank You Nick.

[Helmholtz]

Thanks.
Sorry, but don't get it.... It is not the same result in effect, please?

My proposal means matching the signal level of the left PI output to the lower level right output.

How much is your signal asymmetry?

[catalin gramada]

My proposal means matching the signal level of the left PI output to the lower level right output.

How much is your signal asymmetry?

Thanks
Clearly understood. What wasn't is: what made a difference if I match inverted output in respect with non inverted one , or I match non inverted output in respect with inverted one (as already have a trim pot soldered there to adjust signal level) ?. Does it matter, cause for me seems the same thing
The difference between outputs is exactly 10 percent. And can be compensated replaced 47k non inverted plate resistor with 53 k one. The voltage gain of stage is 5,8 on 100k loads
No global feedback will be applied over the stage

[Helmholtz]

Thanks
Clearly understood. What wasn't is: what made a difference if I match inverted output in respect with non inverted one , or I match non inverted output in respect with inverted one (as already have a trim pot soldered there to adjust signal level) ?. Does it matter, cause for me seems the same thing
The difference between outputs is exactly 10 percent. And can be compensated replaced 47k non inverted plate resistor with 53 k one. The voltage gain of stage is 5,8 on 100k loads
No global feedback will be applied over the stage

But that means that you could achieve signal symmetry with your circuit. Now I don't understand your problem.

Also consider that the 2 triode systems may easily differ by 10% or more.


BTW, how did you measure current source impedance?

[catalin gramada]

What is surprising for me,with 12au7 clip relative hard . Why ? The outputs are quad of el34 triode strapped into 4 ohm load resistor.
Put some pics with output and PI output scooped together. No nfb was used.
Is something you don't want to use for guitar, I guess...