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Old 1st November 2006, 12:57 PM   #11
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I offer the following suggestions:

The capacitor at the grid of the 6BH6 is not needed since both sides of it are at ground (DC) potential.

The 2 meg screen resistor on the 6BH6 will need to be changed. The 6BH6 will draw a different screen current than the 6SJ7. Experiment with the value to get a reasonable tube current. I am at work now and I don't have access to a data sheet. The 250 K plate resistor may need reducing also. I just tweak these for the best sound.

I would also reduce the 5 meg grid resistor to 1 meg. Very high value resistors tend to be noisy and drift prone. Any reasonable value resistor will work here.

Again, be prepared to adjust the values of the screen and cathode resistors in the output stage to get a good tube current and a good sound.

Your 6CU6 tube looks like it has seen a bit of use (worn getter). I would keep the plate dissipation down to 8 watts or so since it is the only tube that you have. Sweep tubes tend to be underrated since they run at full power every minute that the TV is on. I have been known to push them beyond the published ratings, but I do this with tubes that are plentiful and cheap.

Quote:
does it seem me that the first valve works in A2 calss ?
It was common in the old days to develop "grid leak bias" by using a large (5 meg) value resistor in the grid circuit. There is a small (1 or 2 volt) negative voltage developed on the grid because some of the electrons emitted from the cathode land on the grid. This works but the bias voltage is highly dependent on the quality of vacuum in the tube. Resistors and capacitors were not cheap then, so this was done to save a few parts. This design is no longer common since the results are unpredictable. This is why I recommend reducing the grid resistor. Keep the grid at zero volts, and use cathode bias.
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Old 4th November 2006, 01:44 PM   #12
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Thanks Tube_Dude & Thanks Tubelab.com
for your suggestions

I have modified some value...
Schematics V1.3 Click the image to open in full size.

Click the image to open in full size.

Now i need draw the load line static and dinamic of endstage tube...
for to determine the correct value of the components.

- Loadline static :
For Anode Current = 0 --> Vak=Vcc (on X axis)
For Voltage Anode = 0 --> Vcc/(Rk+Rdc) (on Y axis)
where Rk = cathode resistor and Rdc = dc resisrance of output trasformer.

Click the image to open in full size.

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