I just want to share a mod of cathode bias, that I have good results on my Dynaco SCA-35. It will be equivalent to "fixed bias" without the need of negative power rail.
You need one this mod for each single tube, so that you can adjust bias point of each single tube.
Q1 needs to be medium power transistor such as DB140 to dissipate the heat.
You need one this mod for each single tube, so that you can adjust bias point of each single tube.
Q1 needs to be medium power transistor such as DB140 to dissipate the heat.
Attachments
Last edited:
I built an SE output stage test rig that enabled me to easily test many different types of tubes and transformers at different operating points. I used a TL783 in the cathode bias circuit.
The TL783 specs are 1.25 to 125 volts up to 700 ma.
With this test rig I tested 6V6, 6l6GC, KT88, 4D32 and others. I think the maximum values during my tests were 120ma and 40 volts. I used a small heat sink on the device.
Steve
The TL783 specs are 1.25 to 125 volts up to 700 ma.
With this test rig I tested 6V6, 6l6GC, KT88, 4D32 and others. I think the maximum values during my tests were 120ma and 40 volts. I used a small heat sink on the device.
Steve
Last edited:
Put up your Target, before you aim at it.
Before deciding on a circuit for biasing an output tube, think about what you want to accomplish:
1. Constant Current Bias
2. Constant Bias Voltage
3. Self Bias
4. Fixed Bias
A settable adjustment to any of the above (1-4).
Now you will know what circuits that are presented will work to accomplish your desired Bias function.
Before deciding on a circuit for biasing an output tube, think about what you want to accomplish:
1. Constant Current Bias
2. Constant Bias Voltage
3. Self Bias
4. Fixed Bias
A settable adjustment to any of the above (1-4).
Now you will know what circuits that are presented will work to accomplish your desired Bias function.
Did something similar in my 833C mono amps under the 833C, using a high-voltage regulators as auto-adjusting resistors. I needed to be VERY careful in selecting a high power, linear MOSFET for the pass element so it could handle continuously dropping 247V at 160mA, plus gobs of grid current when I got crazy with the volume knob.
Attachments
You just need a high voltage probe...this one is good to 10kV, or 50kV with the red attachment!
Attachments
It's been a learning experience. At first I was lured in by the impressive power ratings of MOSFETs that were optimized for switching, but it turns out they are often poor for continuous use in the linear region, and tend to develop hot spots and fail over time. so I went through:
IRFP450 - failed after 7 years
IXFR48N60P - killed one in one day by turning up the volume on a particularly bass heavy track
But now I'm settled on the IXYS IXTX22N100L, where the L stands for Linear. These are designed to be run in linear mode and should last pretty much forever in this application. They even have tiny current-limiting resistors designed in to each junction in the chip to ensure current is evenly distributed.