yaesu: FT-101, finals replacement

From www.mods.dk, author not known:

There will come a time when the finals in your FT101E will need replacing. These transceivers were originally equipped with 6JS6C tubes manufactured by NEC.
This tube's properties are slightly different from the 6JS6C tubes available today from American tube manufacturers. By the way, don't bother looking for tubes made by NEC. They got out of the business several years ago.

In order to use the "American" variety 6JS6's, a simple modification to the neutralization circuit must be made to the final section of the transceiver.

The modification consists of replacing the fixed value 100 pf 1000 VDC mica capacitor with a 10 pf 1000 VDC mica capacitor. This capacitor, C125, is in series with the 10 pf variable neutralizing capacitor off of the plate circuit.

If this modification has not already been completed on your rig, be sure to use a mica or silver mica of at least 1000 VDC. Do not substitute a different type, because the heat in the final compartment will change the value, and your tubes will fail prematurely. Also, be very careful to keep all leads short and in exactly the same orientation as the original capacitor.

Before reneutralizing, open the variable neutralizing capacitor all the way to minimum engagement and follow the neutralizing instructions in the manual. While dipping the plate, remember to adjust the neutralizing capacitor for equal value meter reading peaks (IC position) on both sides of the dip when tuning the "Plate" control.

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DR OM Eilert, I think this information has also been published in England, in the RSGB Technical Topics book in around 1978.I also seem to recall an article a few years ago, pre-internet, in the RSGB magazine Radio Communication,giving details of using 6146B valves as replacements, with suitable valveholders, as these are proper transmitting valves, instead of over-run colour TV line-o/p valves, which were never used in the UK anyway, except in amateur transceivers. Then more recently, Svetlana producing an EL509/EL519 type which may be useable.

So, it looks like there are a few ways to keep these rigs ("FT101- the rig that Yaesu made, but also the rig which made Yaesu") going for anothe 3 decades! 

73,Martin(G8UWM)

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Hello Martin,

after more than 20 years I would finally like to answer your post. In principle, the conversion to 6146(B) is feasible and functional. This was also done ex factory in the later FT-101Z(D) and FT-901/2 models. The conversion to EL509/519, however, is not - it does not fit mechanically into the PA compartment and the electrical characteristics are different. In addition, these tubes are almost as expensive now as the original 6JS6C. It is also a huge mess and a botched job!

I categorically refuse all these conversions of such a classic and influential transceiver in amateur radio history! The FT-101 series was the Heathkit "killer" according to Chuck Penson, WA7ZZE. You don't have to use such a rig in every day job. Also, the original tubes were mainly ruined by incompetent "LID" operators, who are or were not able to tune a PI filter of a tube PA stage quickly and correctly. Since 1976, my FT-101E still has the factory built in tubes from Toshiba, which provides still an output of around 80 to 100 watts to the antenna or driving various tube type amplifiers in my HAM shack.

What is often overlooked is the fact that instead of the PA tubes, the driver tube 12BY7A is often the one that becomes weak, which then causes the same loss of output performance.

Have a nice day and enjoy The Radio!

73

Eilert - DL9BDM

EM: Edit typos

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I've had an FT101ZD MkII (from about 2002) then an FT101ZD MkIII till earlier this year when I passed it on to friend.

I can confirm that the 12BY7A wears out faster than the 6146. Both are still available. The 6JS6C is still available though maybe a bit expensive.

Aiming for 80W is more reasonable than 100W on any of the FT101 series.

I'd keep these rigs as original as possible and they are better performance and more robust than the first PLL all solid state models. It's not till DDS is added that PLL synth is low noise for SSB (FM is not such a problem without DDS due to 12.5 kHz or 25 kHz steps). A new all mode HF 100W rig is about €500 to €1500 now, so even $200 for a pair of 6JS6C is better than a conversion.

The EL509/EL519, IMO, are only to be considered for a home-brew push-pull Audio or RF PA, for a tube enthusist. Even cheap switching FETS can do 80W up to 30 MHz.

Conclusion: The FT101 series is best kept as-is with the 6JS6C or 6146B tubes (which ever was fitted).

I replaced my FT101ZD MkIII a couple of years ago with an FTdx10, the "budget" version of the deliberately named FTdx101.

At lower drive I was able to do PSK31, RTTY, SSTV etc on the FT101ZD MKIII with my laptop (used April 2002 till November 2016). The FT101ZD MkIII outlived it and only the 12BY7A has been replaced since 2006. It's ideal for my firend who now can't use menus on modern rigs due to failing eyesight.

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