David McGown wrote:
One thing about a high voltage amp using GM70s rather than, say a lower voltage 300B amp using a similar bias voltage, is that less total power is wasted across the cathode resistor. At my GM70 operating point with a plate voltage of 1025V and a voltage drop across the cathode resistor of around 85V, I lose only 85V/1025V or 8.3% of total power (voltage) available to the output tube across the cathode resistor. On a 300B amp with a 450V supply and similar bias voltage of 85v, the loss is 85V/450V or 18.9% power (voltage) loss. Therefore there is less penalty associated with using cathode bias on the GM70.
I am not in a rush to implement a mixed bias scheme, since I can currently adjust the GM70 bias with the adjustable (inductive) cathode resistor. I just have to flip a heavy amp upside down to adjust the resistor tap, and because the power supply caps are so tall (which the amp rests on while upside down), I can operate the amp with the GM70 installed. Though it would nice to just turn a pot accessible from the top of the amp to bias.
David
[color=#0000FF][/David you might want to make a jig out of scrap wood to hold the amp upside down. I did that for the Cherry Bomb amp that David and I built because I did not want all the weight of the transformer and chokes to bear down on the heat sinks when it was inverted. Here is a picture of that. It is make of junk wood with some felt hot glued on to prevent scratches.
Tomcolor]