Mike,
I wonder if you have changed the jumper between the transceiver and 
the amp.  Perhaps it is some odd length that is causing trouble on 
twenty.  I seriously doubt that you are having tube problems with the 
amp - if so, as has been pointed out, output on ten and fifteen would 
be low as well.  And I think that your friend is making a good faith 
effort on your behalf and perhaps has just missed the actual 
problem.  I have a pretty good handle on at least the older gear but 
have made some incorrect diagnoses in my ham lifetime.  Doesn't make 
me an idiot - makes me a bit more careful the next time!
Good luck.
Pat, K9JAUAt 09:34 PM 4/6/2014, you wrote:
>Hi all:
>
>I know this discussion has been posted to the list many times but I 
>wanted to gauge an opinion again.
>Does anyone know if an Unun fed antenna, mainly a vertical, induce 
>high currents and voltages back through a coax feed line?
>I'm getting an indication that my 43 foot vertical is now the cause 
>of my apparent tube failure and if don't stop using it, I'll blow 
>the crap out of the rest of my station.
>My own tuner, after a period of a couple of months of use, started 
>having trouble tuning out a 75M 6/1SWR spike but behaved perfectly 
>on 40/10, including the DX contest with my amp running 600W CW with 
>50W of drive.
>The tuner I have now, handles this antenna from 75M to 10, didn't 
>try 160, with my amps 600W CW no problem. My own tuner, is currently 
>on a wire antenna, handling the SSB output from a HeathKit SB-220, 
>about 1200W, 75/10M with no problems.
>Put my tuner on my antenna, it will tune perfectly and handle 200W 
>on 75/10M but put 500W CW through it, the amp hissed. This may have 
>lead to the 20M issue a couple of weeks ago but I don't think my amp 
>is made of cardboard. LOL
>Now how would a trapped vertical be different?
>I'm very confused, discouraged and most of all scared.
>I don't know what's to gain by BSing me but its starting to look 
>like the gentleman who's been a helping friend since I got my 
>license is doing just that or is simply wrong.
>
>73:
>Mike VO1AX