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 Post subject: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sat 18, 2021 7:06 pm 
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Hi a Yaesu FT-101E had two OEM 6js6c tubes (one of them was NEC) and it worked ok with about 100W at all bands, until it oscillated and burnt out the tubes.

I found the problem and I replaced these with original Toshiba 6js6c tubes that were sold to me for around 80% good.
Indeed when I plugged these in the radio the output power on 20m was around 80-85w when whistling on the mic.
But on higher bands I get almost no power (1-4W max).
ALSO on lower bandsI get lower power (40m=50W, 80m=35W or so).

I did not change anything in the alignment of the radio when I replaced the tubes. I tried only to do the neutralization, but I could only do it on 20m, cause on 10m I get no output power.

My question is, can the output final tubes be bad so as to produce power only on 20m? This is weird, why on 20m I get max power and on higher and lower bands I get little or not at all? Is that a feature of used tubes, it does not make sense? Only alignment makes sense to me for such a problem. But if alignment is needed, why the older tubes worked?


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sat 18, 2021 10:01 pm 
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I would start by turning the bias to maximum (minimum plate idling current) and then with only one final tube installed set bias for about half of normal idle current. Remove this tube and install the other without changing the bias setting and see what the other tube indicates. For this test, do NOT just pull the plate cap but actually pull the tube to avoid damage.

If idle current is fairly equal, put both tubes back in and set bias to its proper level as per the manual. Have you tried a substitute driver tube? It is common for this driver tube type to lose nearly all gain at higher frequency as it goes bad. You may have an issue with lack of drive to the final.

When it oscillated, was anything else damaged besides the tubes themselves? If the safety choke to ground at the output is damaged, it will cause odd tuning and loss of output. Its purpose is to provide a DC path to ground if the plate coupling/blocking capacitor becomes leaky or shorts. I ran into an issue with an amp where only this choke was damaged after a feedline failure and it showed up as unusual tuning and low output only on certain bands where it was no longer operating as an effective RF choke.

Rodger WQ9E


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sun 19, 2021 12:41 am 
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Joined: Jun Wed 19, 2019 11:07 am
Posts: 68
rsingl wrote:
I would start by turning the bias to maximum (minimum plate idling current) and then with only one final tube installed set bias for about half of normal idle current. Remove this tube and install the other without changing the bias setting and see what the other tube indicates. For this test, do NOT just pull the plate cap but actually pull the tube to avoid damage.

If idle current is fairly equal, put both tubes back in and set bias to its proper level as per the manual. Have you tried a substitute driver tube? It is common for this driver tube type to lose nearly all gain at higher frequency as it goes bad. You may have an issue with lack of drive to the final.

When it oscillated, was anything else damaged besides the tubes themselves? If the safety choke to ground at the output is damaged, it will cause odd tuning and loss of output. Its purpose is to provide a DC path to ground if the plate coupling/blocking capacitor becomes leaky or shorts. I ran into an issue with an amp where only this choke was damaged after a feedline failure and it showed up as unusual tuning and low output only on certain bands where it was no longer operating as an effective RF choke.

Rodger WQ9E


Hi Rodger,

I measure L9 (the finals choke to the ground) to 320uH (desoldered one end) which is close to the value in the manual and it does not seem visually to be damaged.

When it oscillated the final tubes AND the driver were red hot.

I say "oscillated" but I am not sure. The failure was a leaked capacitor in the bias PSU PCB which shorted a nearby HV line to it. In fact the PCB at that point was burnt black. I cleaned this, replaced all the capacitors in the regulator/bias board, as well as the final tubes AND the driver tube and it seems ok now. I have checked the bias and high voltages and they seem within limits. I operated the set for quite a long and the tubes do not get hot, even on 20m where the power is 80W.

I cannot check individual tubes bias because the tubes filaments are wired in series, if I remove one tube the other will not work.

I wished for this choke you mention to be bad because this would be an easy fix, but it is in nearly perfect condition.


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sun 19, 2021 12:49 am 
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I forgot about the series/parallel scheme in the 101 series, common for transceivers designed for fixed and mobile use.

I suspect one of your tubes may be damaged. You could test them one at a time by disconnecting the screen lead from one socket and also leaving the plate cap off on that socket but with the tube still in place.

It is unusual for tubes to loose gain at both ends of the HF spectrum but anything is possible. I have seen very unusual behavior from a tube with a damaged screen grid and that will often show up when it is compared for idling current with another tube.

Good luck on sorting this out!

Rodger WQ9E


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sun 19, 2021 1:09 am 
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Joined: Jun Wed 19, 2019 11:07 am
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Hm...once I tweaked a bit TC5(alignment procedure) I can now get 60W on 30MHz whereas before I could only get 2W.

I have no idea why it worked with the old tubes and it needs realignment with the new ones.


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sun 19, 2021 1:15 am 
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The internal capacitance of the new tubes will be different from the old ones, even when you replace with the same brand.

Rodger WQ9E


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sun 19, 2021 1:17 am 
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Joined: Jun Wed 19, 2019 11:07 am
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rsingl wrote:
The internal capacitance of the new tubes will be different from the old ones, even when you replace with the same brand.

Rodger WQ9E


True!
But it is weird that TC5 is in the pre-driver tube stage, does really the finals affect it?


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sun 19, 2021 1:25 am 
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You might have done some damage to your driver tube changing the element geometry when it was overheated. Or if you replaced it, the new tube will be different.

Another possibility is it was slightly out of alignment all along but the newer finals are requiring additional drive.

Rodger WQ9E


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 Post subject: Re: Yaesu FT-101E 6js6c tubes low power on some bands?
PostPosted: Sep Sun 19, 2021 2:05 am 
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Joined: Jun Wed 19, 2019 11:07 am
Posts: 68
rsingl wrote:
You might have done some damage to your driver tube changing the element geometry when it was overheated. Or if you replaced it, the new tube will be different.

Another possibility is it was slightly out of alignment all along but the newer finals are requiring additional drive.

Rodger WQ9E

I think also,that when I unscrewed the PCB with the trimmers, so as to replace a capacitor below(damaged electrolytic) I accidentally moved these trimmer capacitors a bit. These are touchy as far as I see. at all bands now I can get at least 60W. I am still aligning it, but without touching the transformers, just the trimmers.


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