High Quality MOSFET Audio Driver for Class B Tube Modulators

By Steve Cloutier, WA1QIX (formerly, KA1SI)

Copyright 1995 S. R. Cloutier

This document outlines the design and implementation of an output transformerless audio driver for class B modulator grids. The exact circuit shown is used to drive a pair of 811-A triodes, however the same circuit can be used to drive class B modulator tubes with drive requirements of up to 500 V. or more peak to peak such as a pair of 833-A triodes by simply increasing the power supply voltage.

Design Considerations

The major problem in driving class B grids, where the grid is driven positive with respect to the cathode, is the widely varying and nonlinear load which the grids present to the driving circuit. During the periods of the audio cycle where the grid is negative with respect to the cathode, the grid impedance approaches infinity. However, as the audio cycle continues, the grid voltage rises and eventually goes positive with respect to the cathode causing a sudden change in the grid impedance as the grid begins to draw current. Furthermore, the current change is non-linear, rising rapidly as the voltage is increased.

A driver circuit can be thought of as a perfect voltage source with a series output resistor. The output resistor represents the driver resistance (or impedance). Due to the non-linear nature of the grid impedance, this resistance which exists in the driving circuit will cause distortion of the grid waveform since the voltage drop across the driver resistance varies with the grid current, which is non-linear.

The best way to eliminate this problem is to reduce the driver's internal resistance to as low a value as possible.

The circuit shown accomplishes this by using a Source Follower as the output stage to drive the grid load. The output resistance of a source follower is very low, and the source follower used in this circuit can deliver over 1 Ampere of peak current to the grid load. The output resistance is in the order of a few ohms.

Circuit Description

The implementation shown uses a plus and minus 160 volt supply, and will deliver almost 300 V peak-to-peak. The voltage can be increased to plus and minus 300 volts, which will allow the driver to deliver more than 500 V peak-to-peak. Changing the supply voltage is discussed later.

The input transformer, T1 is used simply to isolate the minus 160 V supply voltage from the input. A UTC A-series transformer will work quite well in this application, however any good quality low level transformer will suffice. An input level of 2 V peak to peak should be sufficient.

Starting at T1, the signal flows to Q1 which acts as a phase splitter. From there, the signal is amplified by Q2 and Q3, which deliver around 15 V peak-to-peak to the gates of Q4 and Q5. Q4 and Q5 further amplify the signal to a voltage approaching the plus and minus power supply voltage. The 12 volt zener diode connected from the gate circuit of Q4 (and Q5) back to the source protects the MOSFETs from the possibility of damage due to high voltage at the gate, since the MOSFET gate may be destroyed if the voltage from gate to source exceeds 20 v.


From Q4 and Q5, the signal flows to source follower Q6 and Q7, which provide the high current necessary to drive the grid load.

The grid bias of the modulator tubes is derrived by establishing a DC offset at the gates of Q6 and Q7, controlled by the bias pot R3.

The voltage at the drain of Q4 (and Q5) is controlled by the 250K resistor, R1 (and R2). This voltage should be set to be equal to the output voltage. The two 12 V zener diodes in series, shunted by the .15 uF capacitor allow some variation (approximately 12 V) between the voltage at the drain of Q4 and Q5, and the voltage at the gates of Q6 and Q7. This will prevent small circuit drift caused by temperature or minor power supply output changes from changing the bias applied to the modulator grids. If the difference between the voltage at the drains of Q4 and Q5 and the gates of Q6 and Q7 is greater than 12 V, the R1 and/or R2 should re-adjusted.

The 12 V zener diode connected from the gate of Q6 (and Q7) to the output protects Q6 (and Q7) from the possibility of damage due to excessive voltage from gate to source. The 1/2 Ohm resistor between the source of Q6 (and Q7) and the output, and transistor Q8 (and Q9) form a current limiting circuit. If the current across the 1/2 Ohm resistor exceeds 1.5 amperes or thereabouts, the voltage drop across the resistor will be sufficient (more than .65 volts) to turn on transistor Q8 (or Q9). At this point, the transistor will clamp the input voltage, preventing any further increase in output current. If the overload or short circuit is sustained, the output fuses will blow, protecting the MOSFETs from overheating. Without this circuit, a failure in a tube, or short circuit could destroy the output MOSFET Q6 or Q7 when the current rating of the device is exceeded.

The negative feedback circuit (outlined in broken lines in the circuit diagram) is optional. The value of the 2 meg Ohm resistor should be selected to suit the particular modulator circuit which you are using. In the circuit shown, the current flowing through the 2 meg Ohm feedback resistor is around .5 mA. The amount of negative feedback is controlled by adjusting R4. If you do not choose to use a negative feedback circuit, do not include any of the components shown within the broken lines in the circuit diagram.

There are a number of reference voltages shown in the circuit diagram. These reference voltages are useful for verifying that the low level circuitry is assembled properly and operating correctly. All of the voltages shown which are enclosed in rectangular boxes are with respect to the "0V Ref." point shown on the diagram. This reference point is connected to the negative power supply voltage.

This circuit uses the Motorola MTM-4N85 850 volt, 4 ampere MOSFETs. Other MOSFETs may be used, provided they will handle the combined plus and minus power supply voltage. Other transistors may also be substituted for the 2N2219-A.

Changing the Supply Voltages to Drive Larger Tubes

The power supply voltages used in this circuit may be increased to allow the driver to deliver a greater peak-to-peak grid voltage and/or to supply a greater negative bias voltage. If the bias requirements are greater than minus 20 volts, you will need to change the zener diode D1 to a larger voltage, or put additional zener diodes in series with D1. You should also increase the value of the negative power supply voltage if the peak grid voltage plus the bias voltage exceeds the negative supply voltage.

As an example, if you wanted to drive the grids of class B 833A triodes, you would need a minimum of 400 Volts peak-to-peak, and a bias voltage of up to minus 100 volts.

The power supply voltage in this case should be increased to plus and minus 300 volts, and the zener diode D1 should be increased to a 100 Volt zener diode, or several lower voltage diodes should be used in series. This should allow the driver to deliver around 500 Volts peak-to-peak, while supplying a negative bias voltage of up to minus 100 Volts.

If the voltage is increased above plus and minus 200 V, you should increase the power rating and the resistance of the 20K and 10K power resistors used in the MOSFET output circuit. The zero-signal current flowing through through the 10K resistors connected to the drains of Q4 and Q5 should be no more than around 15 or 20 mA.

With a plus and minus 300 volt supply, the value of the 10K resistors should be increased to around 20K Ohms. The value of the 20K resistors connected between the sources of Q6 and Q7 and the negative power supply should be increased to around 35 or 40 K Ohms, keeping the current flowing through Q6 and Q7 at around 10 mA.

When using a much higher supply voltage, you may not be able to obtain the correct drain voltage at Q4 (and Q5). If this turns out to be the case, adjust the the size of either R1 and R2 to a larger value, or increase the size of the 330K resistors connected to R1 and R2.

Practical Considerations

The plus and minus power supply must be capable of supplying the idle current of the circuit, and the plus power supply must also supply the grid current to the modulator tubes. In this circuit, the plus and minus power supply is capable of supplying 80 mA continuous current.

The voltage stability of the positive side of the plus and minus power supply is very important. If the supply voltage "sags" during periods of high modulator grid current, the voltage change will produce distortion of the modulating voltage. If the power supply voltage sags, you should use a simple regulator in the positive power supply.

The circuit shown here is an example of a simple regulator which you can use to keep the positive power supply voltage from fluctuating under load.

The MOSFETs in this circuit, although not dissipating large amounts of power, will need a proper heat sink. The power dissipation can be calculated by multiplying the voltage drop across the device by the current flowing in the circuit. In this circuit shown, the power dissipated in Q4 and Q5 will be around 4 watts for each device. The power dissipated by Q6 and Q7 will be greater due to the grid current which must flow through Q6 and Q7. I used a 50 watt heat sink for each device Q6 and Q7, and put Q4 and Q5 together on a single 50 watt heat sink.

If the power supply voltages are increased, the device dissipation should be re-calculated and the appropriate heat sinks should be used to keep the MOSFETs from overheating.

Setup and Adjustment

Warning: This driver is capable of supplying a considerable amount of current to the grids of the modulator tubes. Improper adjustment of the driver when connected to the modulator grids may result in damage to the modulator tubes if the misadjustment results in the driver supplying a high continuous positive voltage to the tube grids.

When you operate the driver for the first time, do not connect the grids of the modulator tubes to the driver. It is also highly recomended that you bring up the voltages slowly, using an adjustable autotransformer (or Variac). As you bring up the line voltage, check some of the voltages within the driver circuit to make sure that there are no wiring errors or defective components in the driver.

The first thing to set is the drain voltage of Q4 and Q5. Using a volt meter connected between the drain of Q4 and chassis, adjust R1 so that the voltage at the drain of Q4 is equal to the steady state voltage you want to apply to the modulator grids (the output voltage). Do the same for Q5 by adjusting R2. Note that the bias control R3 will not have much effect on the output voltage until the voltage at Q4 and Q5 is adjusted properly.

Once the voltage at Q4 and Q5 is set up, you should be able to make fine adjustments to the output voltage using the bias control R3. After adjusting R3 to obtain the desired output voltage, go back and check the voltage at the drain of Q4 and Q5, and re-adjust R1 and R2 if necessary to set the drain voltage equal to the output voltage.

NOTE: if the voltage at the drain of Q4 or Q5 differs from the output voltage by more than 12 V, the bias control will have very little effect on the output voltage, and the output voltage regulation will degrade significantly.

Once set up, the driver should be quite stable, since MOSFETs have very low temperature drift.

After the DC adjustments have been completed, the driver is ready to be connected to the modulator grids. If you are using the optional negative feedback circuit, you should first operate the driver and modulator without the feedback connected, to ensure that the driver and modulator are working properly. After this, you should connect the feedback with R4 set to maximum resistance (minimum feedback). Adjust R4 to obtain the desired feedback level. To reverse the feedback polarity, simply reverse the output connections between the modulator grids, in case you obtain positive feedback, and the modulator oscillates!

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