[1000mp] MPV Driving Amp

[Steve Fraasch]

In most cases, you'll be fine using the tuner.

The issue that some hams allude to is the fact that most tuners are in a "T" or "PI" configuration that induce approximately 90 degrees lead or lag; therefore, the tuner's phase-shift may result in an instability where feedback from the antenna might reach a low level stage in the transceiver and cause the system to "take off" and oscillate.

The worst case is an amplifier that already has a pi tuned input, inducing the 90 phase shift lag, proceded by the a pi antenna tuner inducing another 90 degrees lag.  The approximately 180 deg of shift is combined with the normal 180 shift of the amplifier: an oscillator results.

The Collins 30S-1 manual recommends (and supplied) a quarter wave line between the transmitter and the amplfier to get the phase shift right on 10 and 15m for extra stability, and to "stub" RF flowing on the shield back to the transmitter.

Another misconception is that the tuner will reduce IMD.  Not a chance.  IMD occurs over the relatively narrow voice channel (3rd and possibly 5th order) which will spread at the most +/- 15 KHz from your carrier location.  The tuner will do nothing to attenuate these entering the amplifier.

The only advantage of using a tuner to provide a little more drive power to the amplifier which will be minimal, an extra dB at the most.  Besides, most transceivers have more than enough power, and amplifiers have more than enough gain that providing a perfect match between rig and amplifier is unnecessary.  Also consider that a matched tuner has loss too.

Having said all of that, if you experience no problem running your tuner; fine, go for it.

What I believe is much more important is to check the SWR of your amplifier when it is off or bypassed.  This can a problem especially with the older Henrys that had frame relays and long, inductive wire connections between the contacts.  I installed a pi network for 10m on a Henry recently which improved the SWR when the amplifier was bypassed.  The better match on 10m helps to reduce the power fold-back caused by SWR, and the match also reduces the mismatch loss (about 1 dB for 2:1 of SWR, plus extra for lossy line).

Design the Pi (or T) for 10m, it will essentially fix the relay mismatch for all the lower bands.

73,

Steve, K0SF

-----Original Message-----
>From: Win <w0lz at winlyn.us>
>Sent: Oct 22, 2006 10:41 AM
>To: 1000mp at mailman.qth.net
>Subject: [1000mp] MPV Driving Amp 
>
>I have heard, or read on this list, that you should not drive an amplifier with the tuner in line. I am sure that there is a good reason for this, but I can not, for the life of me, figure out what is is.  
>
>Yesterday, when a DX station I needed appeared on 15 meters, I switchewd the amp over and started tuning.  The SWR going into the amp was 2 to 1, limiting my drive.  This is probably because of some mods I made in the amp.  In any case I turned on the tuner and matched to the amp, and made the contact.  
>
>Correcting the problem with the amp is nothing more than adding a cap to the input circuit the nexty time I am into it..  I still ponder, however,  why I should not use the tuner into the amp.  I have done this before many times with other rigs using passive external tuners.
>
>Win
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