A Simple Clamp circuit for a Classic Tetrode Transmitter

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AB2EZ:
Attached is the schematic for a simple, solid state clamp circuit... for grounding the screen of a tetrode (e.g. a 6146) r.f. output tube if the grid drive (and therefore grid bias) is lost. It is the solid state equivalent of the clamp tube protection circuit used in rigs like the Johnson Ranger.

The FQA11N90 is the high voltage FET that many of us use to build Class E transmitters. They cost just a few dollars. They need to be mounted to the chassis using an insulating pad, because the drain of the FET is in contact with the metal on the back of the FET package. They can handle 900 volts from drain to source, and 11 amps of drain current. For this application, the drain current will be, at most, a few 10s of milliamps... so heating will not be a problem.

The purpose of the Zener diode at the input of the FET is to protect it from having an excessive voltage (more than 15 volts) applied to its gate. Excessive voltage on the gate will kill the FET instantly.

The positive voltage shown at the lower left of the schematic can be any voltage that is large enough to turn on the FET when the grid drive to the r.f. tube is not present. Assuming that the grid voltage, in that situation, will be essentially zero, the gate will rise to 1/2 of the positive voltage... which, in this case, will be 6 volts. The turn-on voltage for the FET is 4 volts (or less).

The current flowing from the positive voltage supply is just 10s of microamps, so a simple half wave rectifier working off of the 12 volt filament supply, with a 1N4007 rectifier and 1 uF capacitor across its output should do the job of providing this voltage.

K1DEU:
Very Nice Stu; I hope you add this to your home page along with your FET direct coupled audio driver circuit that eliminates any thoughts for using audio driver transformers. Perhaps the Dirkatron will use this in his Biking 2.    73  John

WA1GFZ:
Stu,
The top 330 K will load the bias circuit when it is negative due to the reverse diode through the zener. It may be ok at the 330K value. The two values can be adjusted for the right clamp action. maybe one should be a pot???.

AB2EZ:
Frank

Typically, the existing grid self biasing resistor (grid-to-ground) is less than 33k ohms (it's 27k on the Ranger, 22k on the Ranger II, and 27.5k on the DX-20)... so I selected 330k as a compromise between having a small effect on the accuracy of the grid current reading vs. worrying a lot about the value of the reverse leakage current of the Zener diode at voltages below the turn-on voltage of the FET.

Stu

k4kyv:
I have a mid-50's vintage amateur radio slopbucket handbook published by CQ magazine.  It contains an article about "class-C"  linears.  It is basically a class-C amplifier with clamp tube, with careful attention to component values, connected in such a way that the clamp tube controls the PA plate current well enough to make it function as a linear.  The author claimed he accidentally discovered it while converting a DX-100 to slopbucket, then found some articles by a ZL using the same basic idea.  There are a couple of circuits using 813's in the final.  According to the article, the amplifier is just as linear as a class-B amplifier.

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