Note - This discussion applies to circuit construction of vintage point-to-point wiring for printed circuit boards with through-hole leaded components. Equipment manufactured with SMDs (Surface Mount Devices) is a seperate topic.
RF Circuits require NOS vintage carbon composition resistors or modern non-inductive type OX or OY style resistors as indicated below.
807 AMPLIFIER EXAMPLE
R1 IS THE GRID LEAK RESISTOR.
IT IS THE LOAD FOR THE GRID-CATHODE DIODE, WHEN IT RECTIFIES RF FROM THE DRIVER STAGE FOR CLASS C OPERATION.
IT GENERATES THE NEGATIVE OPERATING BIAS FOR THE 807 TUBE.
THE BYPASS CAPACITOR SHUNTS ANY RF ENERGY TO GROUND.
THE RFC BLOCKS ANY RF FROM ENTERING THE NEGATIVE BIAS POWER SUPPLY, WHICH TYPICALLY KEEPS THE TUBE FROM EXCEEDING ITS PLATE DISSIPATION.
RF IS ON THE UPPER END, SO IT NEEDS TO BE NON INDUCTIVE.
R2 IS PART OF THE PARASITIC SUPPRESSOR.
IT IS USUALLY ABOUT 50 OHMS.
OBVIOUSLY AT THE PLATE, SUSBSTANTIAL RF EXISTS.
THE PURPOSE OF R2 IS TO REDUCE THE Q OF L1.
L1 LOWERS THE FREQUENCY OF SELF RESONANCE OF THE TUBE INTERNAL CAPACITANCES AND INDUCTANCES FROM THE VHF REGION DOWN TO THE HF REGION WHERE THE TUBE IS STABLE AND WILL NOT OSCILLATE.
A MODERN RESISTOR WILL DEFINITELY INTRODUCE VHF PARASITICS IN THIS CIRCUIT.
USE ONLY AN OX OR OY STYLE MODERN RESISTOR OR A VINTAGE NEW OLD STOCK CARBON COMPOSITION RESISTOR.
R3 IS THE OSCILLATION STOPPER RESISTOR, TYPICALLY 68 OHMS FOR TETRODES IN THE 6L6/807/FAMILY
LIKE R1, THE BYPASS CAPACITOR ON THE SIDE AWAY FROM THE TUBE, SO A NON INDUCTIVE RESISTOR IS REQUIRED.
R5 IS ON THE OTHER SIDE OF THE BYPASS CAPACITOR, SO IT CAN BE ANY STYLE RESISTOR, EVEN A WIRE WOUND POWER RESISTOR.
R4 HAS A BYPASS CAPACITOR BETWEEN IT AND THE TUBE CATHODE.
THEREFORE NO RF IS ON R4 AND IT CAN BE ANY STYLE, EVEN A WIRE WOUND.
6BA6 IF/RF AMPLIFIER
R1 IS THE GRID ISOLATION RESISTOR THAT COUPLES THE AVC TO THE TUBE GRID FOR BIAS TO CONTROL THE STAGE GAIN.
TYPICALLY ITS VALUE IS IN THE RANGE OF 100K TO 1 MEGOHM.
BECAUSE SIGNAL IS ON THE GRID OF THE TUBE, A NON INDUCTIVE VINTAGE CARBON RESISTOR IS REQUIRED.
THE BYPASS CAPACITOR IS ON THE BOTTOM OF R1 TO KEEP THE RF OUT OF THE AVC SYSTEM.
R2 IS A VHF OSCILLATION STOPPER RESISTOR, TYPICALLY 100 OHMS OR BELOW
IT MUST BE A NON INDUCTIVE RESISTOR.
IT IS IN THE SIGNAL PATH FROM THE TUBE TO THE IF TRANSFORMER.
R3 HAS NO RF ON IT BECAUSE OF THE CATHODE BYPASS CAPACITOR.
R4 HAS NO RF ON IT FOR THE SAME REASON.
R5 HAS NO RF ON IT BECAUSE OF THE BYPASS CAPACITOR.
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