#26 pre amp

[merlin el mago]

Channel right plate 174V
Channel left plate 160V

 

[stajo]

Go for 20 k. Where we were yesterday. We'll get into the channel differences later

 

[merlin el mago]

Last filter R 20K channel right plate 156,6V
Last filter R 15K channel left plate 150,9V

 

[stajo]

Ok. Seems both channels circuits are ok. Now we found the problem, the grid was floating. Always a learning.

Now, the difference might be tube differences, lets measure your current again. Measure B+, over OPTs, 100 R if you want and over 20/15k resistors.

After that, you can plug in scope and measure ripple before and after 20/15k resistors. That can give guidance on how to best drop that voltage without rising the impedance.

Try also what you started with. Some squarewaves thru the circuit.

Another lession learned: Dont ask whats wrong when your circuit doesnt follow posted schematic. We could have found that much quicker. Right?

 

[merlin el mago]

The grid floating, the cathode resistor don't grounded, etc I'm a complet disaster, thank you so much for your kind support and patient with me.

 

[stajo]

My pleasure

 

[massimo]

The grid floating, the cathode resistor don't grounded, etc I'm a complet disaster, thank you so much for your kind support and patient with me.

Sursum corda Pope Francis would say....
(Lift up your hearts)

 

[merlin el mago]

Both tubes measurements

 

[merlin el mago]

Now is time to do some scope measurements with calm.

 

[iko]

Your right channel idle current is 1.8mA? Either something is wrong with the circuit, or the tube is old. Exchange tubes between and measure again. Also, show the measured voltage between grid and ground.

 

[stajo]

Cool. Your circuit looks like its working, at least the left. Right channel takes way to less current if measures are correct. Check grid leak there again, viltage between grid pin and kathode pin. How much does 15/20k drop?

 

[merlin el mago]

Your right channel idle current is 1.8mA? Either something is wrong with the circuit, or the tube is old. Exchange tubes between and measure again. Also, show the measured voltage between grid and ground.

How do you calculate 10R:9,79V? voltage gri to ground 0,001V

 

[merlin el mago]

Sursum corda Pope Francis would say....
(Lift up your hearts)

Habemus 26

 

[merlin el mago]

Cool. Your circuit looks like its working, at least the left. Right channel takes way to less current if measures are correct. Check grid leak there again, viltage between grid pin and kathode pin. How much does 15/20k drop?

DVM plus lead grid pin to DVM minus lead to cathode pin -9,6V

 

[massimo]

Habemus XXVI

 

[merlin el mago]

Oooops I forgot roman numbers.

 

[stajo]

DVM plus lead grid pin to DVM minus lead to cathode pin -9,6V

Then cathode is positive to ground (and now grid also), as it should be. Slow down now. Look, measure and maybe listen to your circuit.

 

[merlin el mago]

So can't change the right channel tube as Iko suggested?

 

[merlin el mago]

Cool. Your circuit looks like its working, at least the left. Right channel takes way to less current if measures are correct. Check grid leak there again, viltage between grid pin and kathode pin. How much does 15/20k drop?

20K drops 92,5V
15K drops 96,1V

 

[iko]

If you take the tube from the left channel and put it in the right channel, and if you still measure 0.18V across the 100R resistor, then the problem is not an old tube in the right channel.