#26 pre amp

[neskor]

try to swap tube from left channel to right channel and measure again

 

[merlin el mago]

Thanks Neskor I will do tomorrow because I have to re-adjust the Rod Coleman heater regs.

 

[stajo]

Pics of left channel, 1st grid, 2nd anode & 3rd output.

If you didnt I suggest you calibrate your tools. Your measurments differ around 5000 percents.

 

[disco]

.. Also pic with schematic with measurements.

The cyfers in black represent a bias of (10+11.4)/2 = 10.7 volt.
The red ones give (9+10.4)/2 = 9.7 volt.
So, you're one volt of bias low with this tube.
As a result 16mA of current is standing, instead of 5mA.
Can you replace the 10 ohm resistor?

 

[merlin el mago]

Attached pics: DC measurements & current + resistors pic (it's a little bit complicated to replace😡)

 

[merlin el mago]

Heaters voltage are the maximum can reach with Rod Coleman reg.

 

[Rod Coleman]

Please measure input voltage to Regulators (Measure at the Regulator PCB terminals).

 

[merlin el mago]

15.3VDC

 

[merlin el mago]

Attached pics: DC measurements & current + resistors pic (it's a little bit complicated to replace😡)

With the new DC left channel still doesn't amplifies.

 

[Rod Coleman]

With 130V and about 10V of bias, you should get about 5mA.

If you really have 15mA, sounds like a bad 26.

Need to make sure dc voltage of grid = 0V

 

[merlin el mago]

0.192V

 

[Rod Coleman]

Try another 26

 

[merlin el mago]

OK

 

[merlin el mago]

Changed both tubes: left & right channel. Attached DC measurements left & right channel as you can check left channel still draws a lot of current. I measured the OPT both 1.290 ohms.

Tubes bought to Andyjevans so I guess both tubes are right.

 

[iko]

Check for oscillation. DN2540 could be oscillating too.

 

[merlin el mago]

I swaped the tubes (right for left) & the left heater reg the max voltage can reach is 1.365V, voltage at the input heater reg is 15.12V

 

[merlin el mago]

Check for oscillation. DN2540 could be oscillating too.

Thanks Iko, how can check? Is when I see more than one sinewave moving on the scope?

N.B. Are you sure DN2540 is sharing both channels.

 

[iko]

Explore the high frequencies with your scope.

 

[merlin el mago]

10KHz is enough?

 

[iko]

No, it could be oscillating in the mega Hz. With no input signal, go all the way to the end on the time scale. At each setting adjust the mV/V scale so you get a clear picture.