I'm trying to get my old Yaesu FT-101EE back into operation. During the process, I discovered that one of the finals had been broken. After a lot of web surfing, I reached the conclusion that I wasn't going to find either the original NEC 6JS6C or the maintenance manual recommended Toshiba 6JS6C. What can one use? I found something that said one should change C125 from 100pf 1000v mica capacitor to 10pf 1000v mica capacitor. Does that mod work with just any 6JS6C one can find? By that I mean, is there some specific manufacturer's tube I should look for? I'm also assuming that I need a matched pair of tubes? Is that correct?
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ft-101 finals
- Thread starter steve315
- Start date
That was the mod to the neutralizing circuit, that would work with American made 6JS6's.
Leave the neutralizing circuit alone until you find that it won't adjust properly. If TC27 sets properly on its own, it wasn't broke and didn't need fixing.
Besides, I think that bit of "add a capacitor to TC27" folklore is mostly not relevant.
I blame it on what happens with two tubes that are not mechanically matched on the inside are used as if they were a proper pair.
The internal layout and dimensions inside a 6JS6 will vary from one production line to the next. So long as the tube performed at 15.734 kHz in a color TV, it didn't matter how it worked at a frequency 1800 times higher, like 27 MHz. Wasn't part of the design choices.
One result of this is that from one factory to the next, the internal capacitors, "parasitic" capacitance will vary a lot from one assembly line to the next. Where those internal pieces get positioned is not the same for every manufacturer. Those "parasitic" capacitors in every tube will dictate the setting of TC27, the neutralization control.
TC27 serves to cancel out the feedback from input to output that the tube's internal structure will cause. Think about it. The input and output inside that tube are close together. Some of the tube's output energy will leak back into its output. That's called "positive" feedback and causes the tube to oscillate.
A tube from the GE plant in Owensboro, KY will tend to be stable at a setting of TC27 that's not the same as a tube made in Korea (for example) They may each be stable with TC27 45 degrees away from the other.
Now take that GE tube and a korean tube and put them in the FT101. Never mind that the red paint on each tube says "RCA", and each one came in a red carton bearing that name.
This is legit. The brand name on the tube only tells you who sold it. Not who made it.
Have a look at the visible features of the two tubes' internal parts and structure. The outer edge of the mica "shelf" at top and bottom inside will have a different shape for every different factory.
Using two tubes with different mechanical structure now creates a problem. There is no single setting of TC27 that makes both tubes stable. The first tube is stable with the shaft set to 10 o'clock, and the Korean tube is stable with the shaft at 1 o'clock.
Both tubes have to match in this respect for TC27 to provide a stable final stage.
There's a quick way to check this. Put the wattmeter on average mode, using a dummy load. Peak the Preselect, Tune and Load for max power. Now have a look at the "Ip" reading on the radio's meter. Rock the Plate Tune left and right. You'll see the Ip show a "dip", sorta the opposite of the peak you see on the wattmeter. THESE TWO METER READINGS SHOULD APPEAR AT THE SAME SETTING OF THE TUNE KNOB. Yes, I'm shouting. This is the heart of determining how "neutral" the 101's final stage is.
If the dip is too far from the peak, this eventually becomes a problem. And if they are really close together, I would leave TC27 alone. Nothing's broke that needs fixing,
How you get TC27 set when it's way out of whack will have to wait for another time.
73
Besides, I think that bit of "add a capacitor to TC27" folklore is mostly not relevant.
I blame it on what happens with two tubes that are not mechanically matched on the inside are used as if they were a proper pair.
The internal layout and dimensions inside a 6JS6 will vary from one production line to the next. So long as the tube performed at 15.734 kHz in a color TV, it didn't matter how it worked at a frequency 1800 times higher, like 27 MHz. Wasn't part of the design choices.
One result of this is that from one factory to the next, the internal capacitors, "parasitic" capacitance will vary a lot from one assembly line to the next. Where those internal pieces get positioned is not the same for every manufacturer. Those "parasitic" capacitors in every tube will dictate the setting of TC27, the neutralization control.
TC27 serves to cancel out the feedback from input to output that the tube's internal structure will cause. Think about it. The input and output inside that tube are close together. Some of the tube's output energy will leak back into its output. That's called "positive" feedback and causes the tube to oscillate.
A tube from the GE plant in Owensboro, KY will tend to be stable at a setting of TC27 that's not the same as a tube made in Korea (for example) They may each be stable with TC27 45 degrees away from the other.
Now take that GE tube and a korean tube and put them in the FT101. Never mind that the red paint on each tube says "RCA", and each one came in a red carton bearing that name.
This is legit. The brand name on the tube only tells you who sold it. Not who made it.
Have a look at the visible features of the two tubes' internal parts and structure. The outer edge of the mica "shelf" at top and bottom inside will have a different shape for every different factory.
Using two tubes with different mechanical structure now creates a problem. There is no single setting of TC27 that makes both tubes stable. The first tube is stable with the shaft set to 10 o'clock, and the Korean tube is stable with the shaft at 1 o'clock.
Both tubes have to match in this respect for TC27 to provide a stable final stage.
There's a quick way to check this. Put the wattmeter on average mode, using a dummy load. Peak the Preselect, Tune and Load for max power. Now have a look at the "Ip" reading on the radio's meter. Rock the Plate Tune left and right. You'll see the Ip show a "dip", sorta the opposite of the peak you see on the wattmeter. THESE TWO METER READINGS SHOULD APPEAR AT THE SAME SETTING OF THE TUNE KNOB. Yes, I'm shouting. This is the heart of determining how "neutral" the 101's final stage is.
If the dip is too far from the peak, this eventually becomes a problem. And if they are really close together, I would leave TC27 alone. Nothing's broke that needs fixing,
How you get TC27 set when it's way out of whack will have to wait for another time.
73
I don't know if this is relevant, but this shows some substitutions.