Re: [Harp-L] Harp king question

[THOMAS FIACCO <mx714@xxxxxxxxxxx>]

 1. "self bias" that the tube biases itself via a cathode resistor and a grid at 0 volts. Considerable power supply watts are wasted across this resistor and it gets HOT. Europeans like to call this "auto bias" which suggests some additional circuit monitoring cathode current but they are referring to the cathode resistor.

2. Fixed bias is actually variable via a bias pot. The cathode is usually returned to ground via a 1 ohm resistor where the current can be measured. In this example a 1 ohm cathode resistor produces 1 millivolt for every milliamp. If the reading is 60 mV it would be better to say "my cathode current is 60 mA". In early amplifiers there was generally no bias pot because the tubes were very consistent and had low transconductance (Gm). The 2A3 is an early high transconductance tube and the first to have mention about circuit design changes (though not till there was some trouble in the field) that should be considered when using this tube. The RCA manual has notes about fixed bias to educate circuit designers about these tubes. Low Gm tubes like the 71 and 45 were not that fussy.

It is important to note that the large cathode resistor is usually bypassed by a capacitor. This combination degenerates the DC gain (bias) but preserves the AC (signal) gain. It also limits power output and pre-maturely clips on sustained loud passages.

With fixed bias the AC and DC gains are the same and equal to the MU of the tube.