by dcgillespie » Fri Dec 03, 2010 12:41 pm
Hi mm --
The main question I have is: Is the bias is adjustable at all? If not, then there is a basic wiring/component problem. If it is (as I suspect) but you are just not able to bring the bias into range, then it is likely just the bias needs your particular tubes have, versus the general range of bias the circuit offers.
The reason I believe that is the issue is this: If the regulator was open, the amplifier would not work at all. If it was shorted, there would be no bias at all. By the way, I subjected my amplifier to all manner of endless (but safe) abuse to try and destroy the regulator, but could not. So this experience, coupled with the fact that these are not the conditions you are describing, is the reason I think a simple tweak is all that's needed.
Looking at the circuit, as the bias control is rotated towards the 360K B+ dropping resistor, the voltage drop across the regulator is increased, which increases the bias (cathode voltage) for the tubes, which in turn reduces current draw through them. As the slider is rotated in the opposite direction towards the 15K resistor, the voltage drop across the regulator is reduced, reducing the bias for the tubes, increasing current draw through them.
In my amplifier, with the bias properly set for the required current draw, the voltage at the Output terminal of the regulator was 14 vdc, while the voltage at the Adjust terminal was 15.25 vdc. Of course the In terminal is grounded. These voltages were obtained with the bias control at about 1/3 rotation from the low current draw end of rotation (the end with the 360K resistor). If your particular tubes need less bias in order to obtain the proper current draw, first confirm your unit will produce this general bias voltage at the regulator output at some point in the rotation of the bias control, and that the bias is in fact adjustable. Then, if all is OK there but more current draw is needed, then reduce the 15K resistor to a 12K or even 10K value. This will then allow less bias to be developed across the regulator, and increase current flow through the tubes.
If you are still having problems after this, it will be helpful if you can supply some basic voltage reading to help in the trouble shooting process. I have no doubt that this will be easy to correct. Let me know if I can help any further!
Dave