by Roy Mottram Sun Feb 21, 2016 2:02 pm
Howlin' . . . (and everyone else that wants to understand how the bias circuit works) . . .
looks like Steven who posted the original question followed the suggestion and the fix worked.
Now think about this logically, and the answer should come to you.
The bias circuit has a diode, a 10K resistor, a 10K pot, and another 10K resistor to ground.
The problem is there isn't enough negative bias voltage to bring the tube current down.
So Peter suggested replacing the 10K resistor connected to ground to 5.6K.
To make this really simple, assume there was about -60vdc after the diode, then divided down by the two resistors and pot.
As was, the center position of the pot would have around -30vdc. By changing the resistor to ground,
the center position of the pot would now have around -35vdc (more negative, which brings the tube current down).
Or he could have changed the 10K connected to the diode to about 15K, leaving the other 10K to ground the same.
Same effect. The center of the 10K would now be more negative.
On my MK3 amplifier upgrade (which is the same as the MK4 circuit) what I do is change all 3 resistors to give a wider range of adjustment.
I connect a diode to the negative bias lead of the transformer, add 100uF to ground (was 50uF originally, but don't go larger than 100uF or the bias will come up way too slow).
Then a 1K resistor (instead of 10K), then another 100uF to ground (this removes more of the ripple voltage, making cleaner DC), then a 50K pot so you have a very WIDE adjustment range, then finally a 27K to ground. This makes the total resistance to ground 78K (instead of the original 30K), but the center of the pot is now around -40vdc, with a range of about -30 to -50vdc instead of the original narrow range of -25v to -35v. Seems newer production tubes want a more negative bias than the older tubes, plus line voltage out of the wall is closer to 120vac now than the 110vac that was 50-60 years ago.
