[Amps] My AL80-A Project

[WYsixK]

Ok, I will chime in here as the token rabid Dxer.  1.4 db can definately make a difference in
certain situations.  So consider me a cheerleader with respect to your "soup up the amp" project. 


Where does it stand now?  How well does it work.

Michael

--- Richard <2@mail.vcnet.com> wrote:
> 
> 
> >Hi all,
> >
> >A few weeks ago, I mentioned that I had an AL80-A that I had modified for a 
> >higher B+ voltage, mainly for the educational value.  After putting it back 
> >into service after a 7 yr hiatus, I had some fireworks and asked for 
> >advice.  I thank everyone who responded. That aspect has now been tamed. I 
> >did some other mods, such as added a DC "brushless" fan in place of the AC 
> >fan, which moves more air and is much quieter, but must be shielded from 
> >the RF deck, lest it stop when I transmit.
> >
> €  Did you try bypassing + to ­  at the DC input?
> 
> >I hadn't had much chance to use the amp immediately after my mod, because I 
> >relocated to Oklahoma from Colorado.  So, I was fuzzy on its operating 
> >characteristics.  About all I recall is that, after the mod, I wasn't very 
> >happy with it on the low ends of 160 m and 80 m, but that 75 m and all the 
> >other bands seemed fine. The move, new job, and fresh biological harmonics 
> >left little time for radio, and it, along with all my other 
> >equipmentcollected dust .
> >
> >When I started this, about 7 years ago, that I fully realized that 1) 
> >modifying perfectly fine, working commercial equipment is often a fool's 
> >gambit, 2) raising the B+ from 2700 V to 3700 V (key-down) is pushing the 
> >components to their limits, and 3) an extra 1000 V nets me only a nominal 
> >1.4 dB increase in power output, which is of no practical 
> >significance. 
> 
> €  amen to that, but don't mention this to a rabid DXer or he/she is 
> likely to start frothing.    
> 
> > Yet, in the face of all that, I had the itch.  I wanted to 
> >*do* this, and see, hands-on, how all of this stuff works. I don't have the 
> >tools, and only barely the metalworking skills, to fabricate the hardware 
> >needed for a complete homebrew amplifier, 
> 
> €  The 20db, tetrode-with-handles amplifier on my Web site was built with 
> a vise, a coping saw, a drill, a square and other basic hand tools.  
> 
> >and I don't have the 
> >facilities.  So, I was left with fixing stuff that ain't (yet) broke.
> >
> >After the mod, I knew that the output impedance of the 3-500Z would be 
> >higher, but decided to "just see" how things went.  Sure enough, all the 
> >knob positions were different and I couldn't tune the amp well on the low 
> >end of 80 and 160 m.  It was as if I needed more tune and especially load 
> >capacitance.  Fiddling a bit with the equations and a simple program showed 
> >me that the change in output impedance could not be easily ignored on the 
> >low bands, especially.
> >
> >The output network (a Pi-L or L-L, take your choice) now had too high a 
> >loaded Q (probably around 16-18 or so) and needed far more overall 
> >capacitance because the inductance of the Pi part remained constant.  I 
> >guesstimated how much to move the taps to increase the inductance and bring 
> >the loaded Q (and so capacitance requirements) back to a reasonable range 
> >for the low bands (40 - 160) and everything worked great, except 
> >160.  There are no unused turns on the Pi inductor, so I had to add some. 
> >Along the way, I found that the resistor in my parasitic suppressor had 
> >been baked to a value of ~10 ohms, and so I replaced that.
> >
> €  more DC volts = more amplification.  The most destructive parasite I 
> ever encountered was at c.45MHz in a 12MHz 80kW PEP amp that ran enough 
> anode V to make S9+15db  X-rays. 
> 
> >...
> 
> cheers, Kim
> 
> -  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
> www.vcnet.com/measures.  
> end
> 
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