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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: All fixed?Posted by anthony on: 2/2/2019
So, my two problems have been fixed, I think.
First was the issue with the 6e6p for the HF Channel not being able to draw enough current. I checked the bias and it was fine and the potentiometer allowed plenty of adjustment so I changed nothing in the circuit. A new tube was swapped in and bingo! the current is back and all is good. All that I can think is that all the time that I ran that tube without pin 8 grounded has worn it out and it may be near failure. When I first left the amp on for several hours a couple of weeks ago now the current from that rube was fine and it slowly dropped over time. However, the three 6c33c channels were varying as well so I did not think too much of it. The new tube pulls the required current with the pot somewhere near the middle of its range and I will leave the amp going all day today with the hope that the current will not fall as it did with the previous tube.
My second problem was not being able to set the DC Offset to zero. It sat at about a minimum of 230mV plus or minus 15mV regardless of the gas tubes I randomly swapped in there. I tried half a dozen all up and decided that the issue was elsewhere. What I did in the end was replace a 1k resistor that leads into the 100k potentiometer for trimming DC on the positive side with a 100k resistor, so 100 times larger. The offset now trims to zero with the pot close to halfway through its travel. Romy, I don't know if that is an error on the schematic or if I have some other issue, but the offset seems stable.Rerurn to Romy the Cat's Site