Slightly Unusual Tube Power Supply

I use a somewhat uncommon power supply configuration for fixed bias output tubes. Here is an example: http://users.adelphia.net/~thomasholley/Bass Power Supply.gif . I used this in a Bass Guitar amp with four 6550 output tubes. The power transformer was a Hammond 278CX with additional filiment supplies.

The uncommon part is the way the negative voltage is derived from the center tap. Admittedly, I overdid the wattage here, but you do generate some significant heat. It also takes some generous capacitance to adequately smooth the bias supply.

The benefit I see in this configuration is the way the power supply sag interacts between the B+ and bias. It renders a very good result in both musical instrument amplifiers and Hi-Fi amplifiers using fixed bias. When using this configuration I always get a very pleasing sonic quality to the amp so I use it exclusively on fixed bias arrangements.

That's not fixed bias. It is self-bias but with the entire chassis current flowing in the cathode resistor. This was once very common. For radios with many tubes in addition to the output tube, it gives semi-fixed bias. For a typical 6550 instrument amp, you have ~10mA of other tubes and 100mA-200mA of 6550, it is 90% the same as self-bias.

Here is an article on that bias method:
http://www.aikenamps.com/BackBiasing.html

HTH

Nice article on back bias. Aiken has lots of good information at his site.

PRR, I know this method has been used in the past, but I have seldom run across it. I have never seen it used in classic Hi-Fi or in musical instruments' circuits. But I am certainly not as widely read as you either. I am not clear on how this could be considered cathode bias though. I understand that the entire circuit current flows into the bias resistor to derive the negative voltage, but once that is established, it goes to the grid to set bias like normal fixed biasing. I can see calling this semi-fixed as the voltage varies with current draw to some degree.

Anyway, it is not a common method of biasing today, and I thought I would share it with all of you. I certainly like it on many applications where stiff supply is not necessary.