The more gain an amplifier offers up the more likely it is to oscillate and hum. That's why many high gain amps have "extra" high frequency filtering plate load bypass caps, DC preamp heater voltage, shielded signal cable and "stability" caps across the phase inverter plates. Component placement, lead dress (wire length and placement) and power filtering all become more critical. Higher gain amps can take what would be an acceptable level of noise and amplify it to the point the amp is unusable. If you build a high gain kit amp you can expect to spend some time troubleshooting hum and noise issues until you get the kinks out--you've really got to pay attention to lead dress, especially around the first couple of gain stages.
The 5F1's V1 and V2 use "common cathode biasing", also referred to as "self biasing" or just "cathode biased." V1A's bias voltage is set by cathode resistor R4 which is connected to V1A's cathode (pin 3). V1B's bias is set by R6. V2's bias is set by R8. Capacitor C6 is a cathode bypass cap that helps decrease local feedback and increase V2's gain. Although not shown on the original 5F1 Champ Fender schematic and layout, most Champs came from the factory with the C7 cathode bypass capacitor shown at extreme right. The bypass cap boosted the amp's gain. For Champs without the C7 bypass cap adding one is a common and recommended modification.
For the tube's control grid to control the flow of electrons between the cathode and plate there must be a voltage difference between the cathode and control grid. The voltage difference is what repels the electrons to control their flow. The cathode is 'boiling off' negatively charged electrons and a more negatively charged control grid can keep them in place because like charges repel. This voltage difference between the cathode and control grid is called tube 'bias.' Common cathode tubes use the voltage drop across a cathode resistor placed between the cathode and ground to generate the bias voltage.
The much more powerful 5E3P amplifier shown below uses an "adjustable fixed bias" system to supply the bias voltage. It is called "fixed bias" because a steady bias voltage is applied to the tube control grid. A cathode biased amp's bias voltage will fluctuate with the input signal (it's not fixed). A fixed bias amp applies a negative voltage (usually between -35 to -50 volts DC) to the power tubes' control grids and the cathodes are connected directly to ground at zero volts (there is no cathode resistor). Power tubes have a maximum heat dissipation rating given in watts. Exceed this limit and you can melt the tube. Setting a tube's bias for a higher % of maximum dissipation is considered a hotter bias. The power tube grid voltage is always negative on fixed bias amps and a hotter bias will have the grid voltage closer to zero. A hot bias for the 5F6A Bassman would be around -44V DC. Setting a lower % of max dissipation is considered a cooler bias and the grid voltage will be a larger negative number like -50V. See this for info on How to Measure and Adjust Bias.
Power Transformer on left supplies 50 volts AC to the Rectifier Diode. The AC power flows into the diode's negative terminal (cathode) so 50 volts of pulsing negative DC flows out. The 1K resistor and large 50uF Filter Capacitor form an RC (resistance capacitance) low pass filter to smooth out the DC pulses. The Bias Adjustment Pot adjusts the amount of negative DC voltage that flows to the Power Tubes' Control Grids. The 27K resistor sets the maximum hot bias, reduce it for hotter max bias, increase it for cooler.
A 1kHz 37 millivolt sine wave (AC) audio signal is injected at a 65 Deluxe Reverb Normal and Vibrato channels' Hi input jack (upper left) with all the volume and tone pots set to a half turn. The 1kHz audio signal path through the amp is highlighted and each stage's gain factor is shown in red with an "x". Yellow ovals list the audio signal voltage.
The 1kHz AC sine wave test signal measures 37 millivolts AC RMS (root-mean-square average) at the V1A (Normal channel) and V2A (Vibrato channel) grids.
V1A and V2A amplify the 37mv AC signal on their grids to 1.6 VAC (volts AC) at their plates. This is a voltage increase (voltage gain or gain factor) of 43 times (.037v x 43 = 1.6v).
The tone stack and volume control load the AC signal down from 1.6 VAC at the V1A and V2A plates to 47mv AC at the V1B and V2B grids. V1B and V2B amplify the 47mv signal 57 times to 2.7 VAC (gain factor of 57).
The Vibrato channel's signal off the V2B plate is attenuated by the reverb circuit from 2.7 VAC down to 115 millivolts AC at the V4B grid. V4B amplifies the Vibrato channel signal 33 times. One explanation for the lower gain factor of this stage is the load applied to the plate from the tremolo circuit. Disconnecting this load with a "tremolo off" mod will significantly boost the Vibrato channel's gain.
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