Thanks Joel and Bill.  This proves out what I thought.   I know Lenny had an array of some kind for 80 at least, but I don’t know if he had a separate receive antenna.   I wish I could ask him how he used the preamp.    He may have indeed used it with some old tube rigs on the higher bands.  My thought was to see if it would help with very low gain receive antennas.  I have noticed that on 160, I often get a better S/N ratio when using my HF yagi for receive.  However, the signal level is very low.  The preamp boosted both the desired signal and the noise level, but didn’t provide better copy.  Other antennas, like small loops, have very low output on lower bands.    

Joel, I know your high Z array uses amps of some kind, but they may mainly be for impedance conversion.   Other circle arrays seem to have plenty of gain to produce a usable signal without help.  I suspect the pennant, flag, etc antennas will be OK with just the preamp in the rig.  (<>10 db??).     A receive antenna with gain of -30 or more might need some help.

Bill, the PT-3 has a sensitivity adjustment and it didn’t seem to make much difference in S/N at most levels.  I suspect S/N deteriorated at the highest settings due to distortion of various types but through most of the range, it just amplified what was there, noise and all.   Gain is supposed to be variable between 0 and 20 db.  

  I will probably try it on a Drake 2-B on 10M but the Drake may be OK by itself.      I think my 75S-1 is a little deaf on 10, so might try that also.  Then the preamp will go back on the shelf.    Maybe if I ever succumb to another beautiful Hallicrafters, it might come in handy.  However, if you see me considering one, please intervene! :) 

73,
Dennis/RZ

On Thu, Jan 23, 2025 at 4:18 AM <w5znjoel@gmail.com> wrote:

ADXA Folks,

 

Second to antennas, a preamp is my most favorite topic to discuss and Bill FUV states a very valid point we must all pay attention to. I have no direct experience with the PT-3 but I do have considerable experience with preamps in general.

 

A question I’m asked frequently is “Do I need a preamp?” to which I always reply “I don’t know, do you?” That’s not meant to be a derogatory response but a valiant attempt to get the person to learn what a preamp does, and does not, do and determine whether one is appropriate for your specific situation.

 

A preamp is simply that, an amplifier prior to the receiver that will amplify noise along with the desired signal. You can add high pass, low pass, and bandpass filters that will help reduce unwanted out-of-band signals being amplified but remember noise is generic, its just like Santa Claus – IT’S EVERWHERE!! The preamp isn’t intelligent, all it knows to do is AMPLIFY so it does so equally to everything! Thus the need to have a preamp with a low noise figure. A preamp with a noise figure of 0.5 dB and 10 dB of gain is exponentially more effective than a preamp with a noise figure of 2 dB and 25 dB gain. “Mo gain is not necessarily mo better!” I won’t go into the math but can provide it if you want to fry your brain.

 

BUT WAIT!!!!! We’re talking 1.8 MHz so you really can’t achieve a low noise figure in the real world, like you would in the VHF and above region, because atmospheric and other noise limits the effective noise figure you can achieve at 1.8 or 3.5 MHz. I believe Dennis RZ has proven this when he notes the PT-3 seems to do better as he advances higher in frequency.

 

As I noted in my paper on vertical array comparison I don’t use a preamp on my RX antennas however, you WILL need one if you’re using a loop type RX antenna because that type of antenna’s gain is low. Yes, I know “gain” is not our objective for a low band RX antenna but there is a point of diminished return. The only way you can determine if an improvement is realized with a preamp is to do exactly what Dennis RZ is doing – try and see what the result is. If it is not what you expected. Investigate why and try something else. I don’t recall if the PT3 has a variable gain control or if it is fixed. There was one similar preamp design that provided an adjustment to vary the gain. If so, give that a try.

 

Randy ZJ is using the same approach in his evolution with loop antennas. Try it, document the result, try something else, document the result, then compare!

 

These are all great topics to discuss, and projects to work on, to get us all ready to GET IN THERE !!!!!!

 

73 Joel W5ZN

 

From: adxa-bounces@mailman.qth.net <adxa-bounces@mailman.qth.net> On Behalf Of BILL KENNAMER
Sent: Wednesday, January 22, 2025 10:52 PM
To: Dennis Schaefer <dennisw5rz@gmail.com>; ADXA <ADXA@mailman.qth.net>
Subject: Re: [ADXA] Ameco PT-3 preamp

 

The more gain it has, the worse the s/n ratio. It amplifies noise too. Probably 10-20 db is enough.

 

FUV

 

On Wednesday, January 22, 2025, 9:52 PM, Dennis Schaefer <dennisw5rz@gmail.com> wrote:

I bought Lenny's PT-3 preamp years ago but recently connected it  for the first time.   I thought even though it wouldn’t be at the antenna feedpoint, it might be helpful with low band receive antennas with low output.  

 

I tried it and it has gain, but it doesn’t really seem to help the S/N ratio.  I haven’t found a lot of online info yet, but it seems that it is mainly helpful on higher bands.  It probably would have helpful on one of my old tube rigs that lost sensitivity on 10M.  

 

I just wondered if anyone else had any experience or tips for this piece of equipment, especially for use on 160 or 80 meters.

 

73,

Dennis/RZ

______________________________________________________________
ADXA mailing list
Home: http://mailman.qth.net/mailman/listinfo/adxa
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ADXA@mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html