Amp primer

Power Tube Bias

The power tubes are the big ones at the end of the circuit that do the bulk of the amplification. If you look at the "Tweed" Bandmaster 5E7 schematic that was used in the power supply example, the 2 tubes labled "6L6G" are the power tubes. In order for them to work properly, they have to be "biased". Biasing controls the amount of current that the tubes draw. There are 2 ways that power tubes can be biased: fixed bias and cathode bias.

Fixed bias

In a fixed bias system, a negative DC voltage is applied to the control grid of the tube(s) and the cathode of the tube(s) is (are) connected to ground. The negative voltage comes from the bias supply.

The 5E7, like most Fender amps, has a tap on the secondary of the PT called, you guessed it, "the bias tap", which I believe is about 60V AC. After the current goes through the 6800 ohm resistor, it is rectified into negative DC by the doide.

Notice the polarity of the diode. Placed in a circuit this way, the diode blocks all positive voltage from passing and only allows the negative voltage through. The resultant negative DC is called "half-wave rectified" and is pretty rough, but gets the job done fine in a bias supply.

Notice also the polarity of the bias supply filter cap - the positive side of the cap is connected to ground, because it's filtering negative voltage

The cap isn't labled on this schematic, but is probably not that large 10 - 15 microfarads is typical.

After it is smoothed out a bit, the negative bias voltage goes through the 220K bias resistors and the 1500 Ohm grid resistors and winds up at it's final destination - the power tube control grids.

Stealing the bias : Instead of using a bias tap, some amps "steal" the bias from the high voltage winding of the PT secondary and the first resistor in the circuit is much larger. Marshalls for example, often use 220K for this purpose.

Cathode Bias

In a cathode bias system no bias supply is necessary. The cathode(s) of the power tube(s) is (are) not connected to ground, but rather to a resistor that is connected to ground. It is often connected in parallel with an electrolytic capacitor, but that is not required for it to work. The bias resistors are connected to ground.

This is the bias section of a popular 1950's amp - The Fender 5E3 "tweed" deluxe. It shows a 250 Ohm resistor rated at 5 Watts to ground from the power tube cathodes. In parallel with this resistor is a 25 microfarad electrolytic capacitor rated at 25 volts.

Why are some amps biased one way and some another? They do have a somewhat different sound. It is said that cathode bias amps are "chimy" and that fixed bias ones have a "tighter" bottom end. In vintage amps, the answer has a bit to do with economics. By the mid 50's most of Fender's amps that were cathode biased were the ones on the low-end of the price spectrum. A resistor and a capacitor is cheaper than having a bias tap on the PT and a selenium diode. Many good sounding amps use a cathode bias setup though, the most notworthy of them being the Vox AC30.

Tube amps contain lethal voltages and should not be worked on by those unfamiliar with the proper safety precautions that should be taken when working on them.