Topband: 160m monoband amp (was strange propagation)

[Louis Parascondola]

I came to the group to try to pick up pointers on improving my 160 meter experience.  In the short two weeks I have been here I have so much information to digest.  Especially with receiving antennas, which in my short time here has proven to be "the secret weapon".  If you can't hear them you can't work them.  Since I know little to nothing about these antennas I can't offer any help to anyone.  I am usually one that likes to contribute but I certainly can't contribute a single thing about these types of antennas and feedlines.  So I can only do the next best thing.  I'd like to share my experience with taking a couple of flea market basket case amps that I converted to mono band 160 meter amps with some surprising results!!  I am one that usually looks for the stuff at hamfests that doesn't work rather than some expensive stuff that I would gamble my money on.  So in the course of many years of perusing hamfests I picked up some basket case amps.  Two specifically that come to mind are a well beat up and chopped up Heathkit SB-200 and a Dentron Clipperton L,  They had missing band switches and tank circuits chopped for CB or whatever. I know that many hams don't use amplifiers on the higher HF frequencies, but some many consider that the extra power could be of benefit on 160.    So I decided to do a conversion on the other end of the spectrum from 6 meters.  I started out with the SB-200 and gutted it out completely and started with a clean slate.  Cleaned up the years of crude and went to work on a mono band 160 meter amp.  I scrounged all the parts from other amps that had been taken apart for various reasons.  The first thing I did was to upsize the filament choke with a higher inductance, a unit from an old SB-1000 I believe fit nicely.  A typical unit found in an AL-80B can be bought direct from Ameritron pretty cheap.   One of the things I found in the junk box was an nice heavy duty toroid that came from a Viewstar PT-2500 amp.  I decided to use this to make a toroid  tank circuit inductor instead of a open air coil to reduce the overall size.  Using the guidelines in the handbook I started at a tank inductor value that was close to calculation.  I rewound a coil on the toroid core and measured it.  It was very close to calculation so I installed it.  I then worked up  pi-net input circuit using an adjustable coil like in the original SB-200 and some mica caps.  I used values for both inductor and caps (to begin with) from some of the commercially made amps.  I put this all together and gave it a whirl to see where I was at.  To my surprise I was pretty much in the ballpark.  A bit of tweaking on the input circuit and I soon had a nice flat input swr.  In amp building the input circuit should be just as important as the tank circuit.  The more efficient the input circuit the better the amp will perform power wise.  This seems to be a place where many do not spend enough time on.  Once I got the input circuit perfect I plotted the output across the band and adjusted the tank coil for best results on the lower end of 160 meters.  When I tweaked it all up I was rather surprised as to the performance.  The SB-200 runs a pair of 572B tubes and like most thoriated tungsten tubes the limiting factor is grid current, well that could be said for just about any transmitting tube, anyhow loading the amp up into a dummy load and using the "white area" on the SB-200 meter as a guideline which relates to 200ma, I drove the amp and topped off the power right on the edge of the white line.  The power output was an honest 900 watts.  You have to understand that when you remove all the switching and compromise from a multiband amp and create a mono band amp, you increase the efficiency quite a bit, as can easily be seen here.  I did a "key down" test for about 20 seconds straight and the tubes did not show any appreciable overheating of the anode.  This usually proves that the losses are minimal and the anode is giving off its power to the load all of which points to a good tank circuit.  Trying to load into an improper load is quickly indicated by rapid anode heat, hence very red anode color.  I neglected to mention that I did pad the load and tune caps using values from the handbook to begin with and then tweaking so that I had sufficient travel on each variable cap to compensate for varying loads.  Running this amp at 800 watts would be a snap and for simply calling in pile ups full power shouldn't be a problem.  Now as for the Clippetron L,  I did a 160 conversion on that one also.  On this one I wanted to try an air wound coil, so I followed the same procedure as the SB-200 for calculations and put together an input circuit as before, once again using a starting point of a similar amp running 4 572B tubes  The AL-572B would be what I probably used to start.  I wound an air wound coil using heavy duty #10 enamel wire with the inductance I got from the handbook and gave it a whirl.  With some tweaking I got the amp to respond very well to input power.  Of course as before, I did pad the load and tune caps using door knob caps.  And again adjusted those for enough variable cap to compensate for various loads.  Once I had it adjusted well enough I tested it with maximum drive I could get from my 100 watt driver radio.  At 100 watts of drive I was getting 1500 watts output.and the plate current was less than 4 tubes can safely handle.  4 of these 572B tubes can safely handle 1amp of current.  I was at 800 mills with the 100 watts of drive, I am certain if I had more drive I could get more and still be in limits.  Although the Clipperton does NOT have a grid meter I am certain that if the amp is properly loaded the grid current would not exceed 400 mills.  But suffice to say we now have a legal limit amp here at dirt cheap flea market prices.  I also gave this amp the "key down" test.  On this amp I did a key down test for 30 seconds and again no signs of overheating at all.  So the next time you see a distressed one of these amps and you are one that would like a bit extra power on 160 here is a cheap way to get it.  I'd be happy to give you the design parameters for each amp.


73 Lou