Solid State 6BF5


By: Don Kang


REDUCE HEAT from your Collins S-Line Receiver

Designed to replace 75S- series 6BF5 audio amplifier tube directly. No additional parts to install. Just plug in. Total power consumption is 2.5Watts. It measures 2.1 inches high , 1.0 inch wide and 3/4 inch thick It weighs about 1 oz. (see picture). Delivery is 1-2 weeks.

The AF amplifier tube 6BF5 in the Collins S-Line gets so hot, it turns dust particles into carbon and deposits a coating on the cabinet cover. Today, a power MOS-FET can substitute for the 6BF5. The MOS-FET amplifier shown here is designed to consume 2.5W. The audio power output is acceptable for a normal receiver operation.


Most N-channel Power MOS -FET made for at least a 200V switching application can be used for this project. An IRF730 was used because I had many of them. An IRF620 is also a very good candidate and available from Mouser Electronics. The common properties are: 1) they are normally “OFF”, 2) start conducting around 3V, and 3) go into the “ON” state near 8V. The “ON”state is simply a saturation condition. Somewhere between 3 to 8V there is a linear region where this amplifier will operate.


The main objective of this project is “low power consumption”. The power consumption should be no more than 6BA6 or 6DC6 to make this device attractive. These amplifier tubes consume about 2W. The maximum power to this device is set at 2.5W. The voltages available from the 6BF5 socket of my 75S-3 are 145V at the plate terminal and 134V at the screen grid terminal. The negative bias of control grid is not compatible with the MOS-FET. A positive bias is needed to operate in the linear region.

The drain current is 2.5W/145V = 17.2mA. It is a very low value for a transistor, which is capable of handling many amperes. This means the MOS-FET is barely turned on. The operating voltage is very near the turn-on threshold voltage, which is around 3V. Regarding the drain voltage requirement, let us look at the load. It is 2.5Kohm output transformer and the linearity of the MOS-FET near the drain current of 17mA may not be very good. A useful range of drain current swing can be no more than +/-50%. That is 17.2mA x 100% = 17.2mA. The voltage swing at the drain becomes 17.2mA x 2.5Kohm = 43V peak to peak. I do not think we need 145V drain voltage. Any excess drain voltage is wasted as drain dissipation. I feel comfortable setting the drain at 60V. To get the drain voltage of 60V, the source should be at 145V-60V = 85V. This requires a resistor (R4) at the source terminal. This resistor, like a cathode bias resistor in vacuum tube circuit, will give a negative bias voltage to the gate. That is OK, because a large positive 134V is available from the screen grid terminal.


The circuit is shown in the picture. The sample picture has a red LED in series with R4. Source resistor R4 will produce 85V at the source end. The R4 value is 85V/ 17.2mA = 4.94Kohm and the power rating is 85V x 17.2mA = 1.46W. For R4, use three 15Kohm1/2W in parallel. The MOS-FET has shifted 85V of excess drain voltage to this resistor, thus reduced its own power consumption by 1.46W. The gate voltage is 88V, 3V higher than the source. When the gate is forced to assume 88V, the source has no choice but to settle at 85V by passing 17.2mA through R4. This is an auto bias scheme. We don’t have to know the exact gate bias voltage, as long as it is near 3V. The current though the R4 will automatically adjust to get the right voltage to the gate. The input capacitor C1 blocks DC voltage and passes an AF signal. The R1 and R2 provide gate bias voltage. R3 is there to ensure stable operation. Any value near 1Kohm is OK for R3. Since there is no DC loading, a simple divider will provide the gate 88V from 134V. R2/R1 = 88V/(134V-88V) = 1.91/1 = — 180Kohm/ 91Kohm R2: 180Kohm and R1: 91Kohm Higher values of R1 and R2 will give less loading to the AF voltage amplifier 6TA6. C1: 0.01uF / 200V AF coupling C2: 1 to 5uF / 200V AF bypass A 7pin miniature tube was used for the base. When you break the tube, you should be very careful not to damage the base part of the tube.


Finished device met my original goal without any modifications. At higher volume levels the sound distortion becomes obvious. It probably is due to poor linearity of the region where this MOS-FET is operating. No attempts were made to find better MOS-FET for this application.

May 25, 2002
Don Kang

Price: $20 ea for USA address shipping and all others S25. Above prices include shipping cost.

Send payment to: Don Kang
15520 On Orbit Drive
Saratoga, Ca 95070 USA

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