[TenTec] Titan 425

[Rob Atkinson, K5UJ]

Hi Kelly,

Just out of curiosity, what's the amp you are interested in?  Henry 3K Premier?  (just kidding but it does seem to be an excellent amp.  You gotta love that 200 lb. p.s. as long as you don't have to move it around, hi).

I'm assuming the 425 is the amp using the 3cx800s with the p.s. and rf in separate cabinets.

I have no experience with that one however I have done some work on input impedence with the Centurion.  It is important to have the amp on a very short strap to a ground bus, and it helps if the ground bus is short, a foot or two.   these 8' long copper bars only lengthen the ground run.  the exciter and amp should be kind of close together so their separate straps to the bus are not very long but the amp's should be shortest--it should be right where the ground line comes in from outside and the transmatch should be right there with it if you use one.  The feedline from the exciter to the amp does seem to make a difference.  In my case I put a MFJ analyzer on the input, keyed the amp so the MFJ would see the tuned input to the cathodes, and experimented with different lengths.  I found that (except then the tuned circuit was not resonant which is the case with the Centurion up near the top of 75 meters) while the feedline R was always 50 ohms +- 2 or so, the reactance varied with length.  The problem of reducing this to get nothing but R was a moving target--a certain length that worked on one band, say 40 meters would, not surprisingly, be a problem on 20.  And so on.  I ran out of patience in swapping jumpers of different lengths in and out and finally concluded that the best thing to do was to shorten the length of the feed between the exciter and amp as much as possible.  It was originally around 15' before I discovered length matters, and is now down to around 3 not including a line isolator.  

I realize this does not matter to you now, but I thought it might help others, especially those who are driving amps with non-TenTec rigs that fold back if the swr goes up to 1.5.  TT exciters seem to be robust in output even with relatively high swr for solid state rigs.  perhaps that's one reason why their amps don't have inputs you can tune.   I briefly considered a small matching network between my exciter and amp and rejected it because I didn't want one more thing to fool with when changing bands (however it's been pointed out to me that a LDG autotuner would take care of this for me).

The Centurion drives to full rated output (1.2 kw pep on ssb) easily on some bands--on others it needs a bit more wattage than what comes out of a 100w. exciter, and on the WARC bands it seems to top out at around 900 watts (which is plenty for me).  Getting grounds and the line between the amp and exciter as short and sweet as possible can be a cheap passive way of solving some drive problems.
73,
Rob
K5UJ