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Old 3rd January 2010, 04:28 PM   #1
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Default Tubelab SE Driver Bias Help

I posted this on the other Tubelab Forum as well. It hasn't been getting the traffic that this forum gets, so forgive me for posting it here as well.

I am in the process of firing up my Tubelab SE. 5842 drive tubes with 300b output.
B+ is running at 440V with no output tubes and 400 with output tubes.

When setting the driver voltage, I find that I am only able to obtain a maximum of 130V on the coupling cap (plate voltage) on one channel and 160V on the other.

I also found that the CCS's are passing 7.6mA when measured across the sensing resistor of R16/R27. That is for both channels. I thought it was suppose to be 12mA?

I set both channels to 130V and set the 300b bias to 65mA and tried to listen to it. One channel is playing ok but the other is distorted.

Any ideas to trouble shoot? What I know is that the cathode resistor(s) are of the right value and function as designed. (R= 330 to 0 ohm based on pot). Swapping tubes will swap voltages but distortion remains on same channel. I have replaced CCS's but same problem.

Any thoughts?
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Old 3rd January 2010, 08:36 PM   #2
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A little more data.
B+ = 415V Plate is a little hot at 412V

Per my osc scope. I have good signal up to the grid of the 300b. I don't have leads to read on the opt primary however. The output signal at the speaker terminal is distorted. Both channels are distorted. I switched xfmrs to a 5k primary and same distortion. My original OPT was 3.75k.

It appears that I am losing it on the output stage.

FYI. I raised the drive current to 12mA and was able to get 175V on the plate of the 5842

Filament is at 4.94V Bias voltage is running approx -104V
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Old 3rd January 2010, 10:21 PM   #3
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The last post gave clues to the answer. I am afraid to admit it but 65mA across 10 ohms is NOT 0.065V but in reallity 0.65V. I just was excited when raising the bias and saw 0.065 and stopped.

Now my B+ is 360V and plate around 345V. No distortion, just good music.....
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