Hi, Low output in the FT-101 series can be caused by many factors.
Needless to say TUBES come fist. Check them and make sure you are starting with a good set. Second in line is the frequency. Key down with a bran new pair of tubes you should expect to see some 160w briefly keyed down on 75, 40m and it will drop off as you go up in frequency. 20m / 15m you can usually expect to see 120w output. And on 10 meters you can expect about 110w, again all these are approximate assuming a FRESH bran new set of tubes.
When looking at power loss, frequency of operation becomes critical as the tubes get older with the power dropping off on the higher frequency first usually 10m is most noticeable. Quite often a used set of tubes will still give full power on all the lower bands with 90 to 100w on 10 meters. At the price of tubes these days, once can learn to live with this kind of power assuming the tubes will still get you by for a long time to come.
Neutralization of the 6JS6C finals is critical on the upper bands and power output can also drop off if the PA stage is not neutralized properly. When properly neutralized a transmitter will have maximum power output at minimum current, in theory. However when observing current in the FT-101 that will not always be the case by observing the current I.C. meter. Other factors come into play as the current is monitored in the cathode circuit also showing the user screen grid current. So neutralizing an FT-101 can become a juggling act. #1 thing is to make sure the PA is not running away, often when it does you can hear it in the receiver making one hell of a racket. ALWAYS FROM HERE ON OUT AND FOR EVER MORE MONITOR I.C. ON YOUR RIG!!! FOR EVER!!! I highly suggest when using an FT-101 don't use the P.O. power out put position or worry about ALC during neutralization process. Used an external watt meter to observe maximum power output while doing the neutralization procedure and again always watch I.C., should the rig run away on you and I.C. shoots up like a rock you will see it BELIEVE ME then immediately turn the rig off as not to destroy the tubes. You can safely turn it back on but if your having trouble getting the PA neutralized with some tubes you may just have to turn the neutralization cap slightly prior to keying the rig so it doesn't take off you again. These are the kind of problems I've seen when using American made tubes in the FT-101.
When tuned into a 50 ohm load your antenna loading will be around #4 on the front panel. If it is at one extreme or the other on the front panel indicator you have a problem. the Plate control should be near that indicated for the band in use. Again neutralization can often become a juggling act from what the service manual tells you to do and what you actually get on the power meter. Primary thing is to make sure the PA is not oscillating or running away. That is why I saw ALWAYS, ALWAYS WATCH I.C. when attempting this procedure. Once neutralized you should notice a slight increase in power on 10 meters.
Next also make sure the coupling cap from your 12BY7a is 80 pf. Well sell 82pf caps in the FT-101 Kap Kit. See candy store items. Some schematics show 100pf for this cap, and the later Ft-101E will show 80pf.
Always check your rig and make sure you pull out any modifications or parts that JUST LOOK TOO STRANGE TO BE ORIGINAL components. If something doesn't look right to you, there must be a reason. Double check as these rigs are getting old and I have seen so many fubar modifications that I had no choice but to remove them and put the rig back stock. Quite often this will help in your repairs. Believe me the engineers who designed these old rigs were no dummies! They knew far more about the design and how to make it work then some hammy who owned it in the past and tried to modify the rig.
Lastly right now I see a rig on EBay that has special mods!
Special my rear. Someone put in 6146 final in that rig. I HIGHLY RECOMMEND you avoid this mod. You can expect nothing but headaches and problems by doing a 6146 mod to an FT-101. Right off the top the plate voltage is too low for 6146 finals and I have yet to ever see this modification work correctly, yet some claim they have made it work.
When ever I get a rig that has been modified for 6146 finals, I pull it all out and put the rig back stock, by doing that I have yet to ever have one not work properly after doing that.
Got to run.