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Neutralization adjustments de-mystified. Yeah.

nomadradio

Analog Retentive
Apr 3, 2005
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www.nomadradio.com
So here is a file I have used as a crib sheet to neutralize grid-driven tetrode RF amplifiers for decades. The first half is how Collins said to adjust neutralization for the final stage in their transceivers. You just have to have a way to see grid current rise and fall to do it that way. The one amplifier we see that's built this way is the Johnson Thunderbolt with two neutralized grid-driven 4-400 tetrodes.

The second half is for radios that have no way to read final grid current, like the "hybrid" ham HF transceivers of the mid 1960s to early 1980s.

But first: A word about neutralizing a HF transceiver with "sweep" tube finals. A lot of lip service is paid to the need for "matched" tubes for a final amplifier stage. Very little is said about what does or does not "match" them. Yes, a similar gain/current reading in a tube tester is a good thing. A weak tube and a strong one will never share the power load equally. The stronger tube will overheat and fail prematurely while the weak tube just loafs along. But this is not the "match" in question here.

What really matters is how the two tubes "match" for that pesky neutralizing adjustment. Sweep tubes were built for frequencies way, way below the HF bands in a FT101 radio. The neutralizing circuit serves to cancel out the positive feedback that happens inside the tube. Mostly this is caused by accidental or "parasitic" capacitors that form inside the tubes' structures. Put two pieces of metal near each other in a vacuum and Mother Nature gives you a capacitor whether you wanted one or not. The tube's internal layout dictates how much of this unwanted positive feedback needs to be canceled out. The design of a 6JS6 tube or 6KD6 tube doesn't control this part of its internal dimensions tightly from one factory to the next. At the low frequency it amplified inside a color TV, those capacitance values had almost no impact on performance. There was no practical reason to control how much or how little of this feedback capacitance the tube may have. Wouldn't affect how a color TV performed. Small differences in the size and placement of the metal parts inside the tube make a big difference in how much unwanted RF feedback needs to be canceled out. Hence the variable "NEUTRALIZING" capacitor seen inside the final stage of a FT-101, or Kenwood TS520/530/820/830 radio, or any Swan radio. This adjustment compensates for these mechanical tolerances inside the tube.

Just one problem. Two tubes each from a different factory may each require that adjustment to be at a different shaft position. Let's say one tube came from Japan and will be stable with the adjustment at, say, 1 o'clock. The other tube came from the old Sylvania plant in Pennsylvania. It requires the neutralizing capacitor's shaft set at 10 o'clock to be stable.

Right away, it's clear that with both tubes tied together in parallel there is no single setting of the neutralizing cap that makes both of them stable. Lost track of how many frustrated hams would pull out their hair trying to get a FT101 neutralized. Their "matched" pair of tubes each said (for example) "RCA" on the carton, and painted on the side of the tube.

But one tube was made in Korea, the other one in Owensboro KY. No way they will "match" for neutralization. The name on a retail carton has nothing to do with the factory that made a tube. It might, but it may not. What matters is the internal structures. If the mica insulating washers and color/shape of the anode structure look the same, both tubes probably came from the same assembly line. And if they are different, this is not a "matched" pair the way a radio needs for them to match.

Lost track of how many frustrated hams threw up their hands after trying every little increment of adjustment, but with a radio that had feedback problems no matter where that adjustment was set.

It's not the adjustment, it's the tubes.

Maybe I should have inserted the rant after the first section.




FILENAME: TETNEUT.DOC 7/2/01 C.R.

PROCEDURE FOR NEUTRALIZING GRID-DRIVEN TETRODES:

GRID CURRENT SHOULD PEAK AT PLATE CIRCUIT RESONANCE, aND NOT ON ONE SIDE OR THE OTHER. Resonance means a "dip", or minimum reading on the plate-current meter.

IF GRID CURRENT PEAKS ON HIGH-FREQUENCY SIDE OF PLATE CIRCUIT RESONANCE: MORE NEGATIVE FEEDBACK IS NEEDED.

-> IF THE VARIABLE ELEMENT IS IN THE HIGH SIDE OF THE CIRCUIT, INCREASE THE VALUE. ````

-> IF THE VARIABLE ELEMENT IS IN THE SHUNT SIDE, DECREASE IT.


IF GRID CURRENT PEAKS ON THE LOW-FREQUENCY SIDE OF PLATE RESONANCE: LESS NEGATIVE FEEDBACK IS NEEDED.

-> IF THE VARIABLE ELEMENT IS IN THE HIGH SIDE OF THE CIRCUIT, DECREASE THE VALUE.

-> IF THE VARIABLE ELEMENT IS IN THE SHUNT SIDE, INCREASE THE VALUE.


----------------------------------------------------------------
ADDENDUM FOR FT-101 AND SIMILAR TUBE-FINAL RADIOS:

SHOULD APPLY TO ANY SIMILAR NEUTRALIZED FINAL WITH THE VARIABLE ELEMENT IN THE HIGH SIDE, FIXED CAP ON THE COLD SIDE OF DRIVER OUTPUT COIL.

-->> IC DIPS TO THE LEFT (LOW-FREQ SIDE) OF THE OUTPUT PEAK: NEEDS MORE NEGATIVE FEEDBACK: TURN THE TRIMMER FOR HIGHER CAPACITANCE.


-->> IC DIPS TO THE RIGHT (HIGH-FREQ SIDE) OF THE OUTPUT PEAK: NEEDS LESS NEGATIVE FEEDBACK: TURN THE TRIMMER FOR LESS CAPACITANCE.

The "dip" of the plate current meter may become quite shallow when the neutralizing cap is correct. Should still be able to see it clearly.

This probably deserves some added detail, but that will have to wait for another round tuit.

Oh, and the all-caps is an artifact from the 8-bit bad old days. Not worth the effort to convert to lower case.

73
 

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