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RCA 6K3 Preliminary Capacitor/Resistor Research
#83

Rod,

Here's where I'm at:
Pin 4 on both 6A8 and 6K7 are, according to the tube voltage diagram, supposed to test at 100V.  They are connected so naturally they will test the same.
Actual results for these pins are from a low of 61ish to a high of 66ish with tubes swapped for others from the other set I've worked on recently.

With tubes in, I show 172-176V everywhere from the junction of R14/R15, to C16, to pin 6 of the 6A8.  On the back side of R4 (side opposite the connection to the tubes) likewise.
Across R4 however, toward the tubes, we're seeing the 61-66 referenced above, depending on the test and conditions, so roughly a 110V drop from one side of R4 to the other.  If I'm correct in my understanding, if the tubes are supposed to be showing 100V on pin 4, then the near side of R4 should be 100V, a drop of 70V from what is supposed to be on the far side, 170V, and a drop of 72-76V from what I'm getting for my readings.

The resistor is in fact a 22K resistor, I have swapped it for two 10K resistors in series since I don't have a spare 22K on hand.  Both the pair and the original test close to their anticipated values.  The diagrams/schematics specify 22K, so it seems like it is the intended part and is functioning as it should.  

MrFixr55 provided current values for the tubes that suggested to get 100V at pin 4 across a 22K resistor, we expect 200V give or take at R14/15 which to me says, everywhere it's presently reading 175 or so, which doesn't seem to add up either.  In saying that, I'm not suggesting he or his information is wrong either, but this is a case where I'd say the math isn't mathing.  Changing the resistor didn't change the readings, neither did swapping tubes from another set, though I could just be lucky and have two sets of equivalent tubes that are both bad in the same way, though I think it's unlikely as well.

Perhaps not a silly question, but not knowing how it would affect anything else along the way, if we assume that the current that MrFixr55 found/calculated is correct, that it's 0.0044 amps, would a 16K (or thereabout) resistor work to drop the voltage from approximately 170 volts on the far side of R4 to about 100 on the near side and pins 4 on both the 6A8 and 6K7?  If so, that would get us within spitting distance of the anticipated value, but would be a deviation from the design and components actually in place.

Is there anything else in this part of the circuit that would be impacting what I guess is the screen grid voltage (pins 4)?  Everything else is, from what I've heard so far, reasonably close to where it's supposed to be.  Capacitors are new.  Resistors test within spec of have been replaced, including the entire voltage dividing resistor.

The radio runs, but if it can run better, I'm game, but I'm way out of my depth with this rabbit hole.

Best wishes,
Roy


Messages In This Thread
RE: RCA 6K3 Preliminary Capacitor/Resistor Research - by glyphmon - 05-12-2026, 01:36 PM



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