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[Amps] Why people don't build amps.

Subject: [Amps] Why people don't build amps.
From: Roger <sub1@rogerhalstead.com>
Date: Thu, 25 Mar 2010 15:56:47 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
The reason for homebrewing is a slight, but valid turn from the original 
direction of the "Good Engineering" thread,

There are many reasons for people not home brewing and the fear of 
failure is just one and I doubt the major reason.
Today's crop of hams coming up are not (in general) as technical minded 
as those of earlier days.

In days gone by we built often from necessity. There were lots of 
sources for parts, not the least was surplus and cheap/inexpensive 
surplus at that.

My first transmitter was the pair of 6L6's with a 6AG7 Oscillator built 
on the corner of an old TV chassis. It was big, ugly, and it 
worked...well...after I discovered you don't use wire wound resistors in 
those circuits. Oh! and it was cheap as IIRC it was all from used parts.

Since that time I've build a number of amps, Yagi antennas for 20 and 
15, but the source for parts is drying up and what's left is getting 
expensive. When I became a ham I had a stack of large and heavy catalogs 
with almost anything I needed. 

If I want to build something, I don't want to spend 5 years (or more) 
hunting through swaps looking for transformers, tubes, and band 
switches.  What are the sources for inexpensive transformers that will 
do the job?  Not microwave ovens, those things get hot and are not 
designed for continuous operation. OTOH if one is willing to put up with 
their short comings they can be paralleled to provide power for a medium 
size amp.

Another reason for lack of building, at least for people like me, is 
when it's finished I don't want it to look like something some one 
cobbled together. In that vein I'm a bit of a perfectionist.  Plus if 
you have to purchase the parts you can likely go out and buy the whole 
amp for less money AND have something with a reasonable resale value.

Now I see the legal limit brought up when tubes of substantial ratings 
are brought up, but it really has nothing to do with legal limit.  For 
instance, those rugged triodes should not be written off, and the more I 
think about it the more I think if I built another amp that's what I'd 
use and why not?  Well other than the power it takes to light the fire 
inside that is.  Look at the characteristics of those tubes. See what 
they take for drive?  IOW most of todays rigs with only a couple 
exceptions are not capable of driving those tubes any more beyond the 
legal limit than the AL1200 and 1500 can go. Besides, when run at that 
power level they don't take a lot of cooling.  Some of them don't even 
require a socket making the construction of the amp more simple.

People don't build because they have no desire to do so, or they see 
they can purchase an amp ready to go for the same or even less money 
than building one. Then there is the lack of metal working skills. I've 
seen very few amps that were home brew that didn't look home brew and 
I'm being very courteous referring to the appearance. Want to change or 
upgrade later on? You'll be lucky to get the value of the parts out of 
it, but then again, you might actually find some one who wants it. 

However I do think there is some fear when some one looks at spending 3 
or 4 thousand dollars to build a rugged, legal limit amp capable of TRUE 
"brick on the key" 24X7 such as digital, or long winded AM operation. 
None of this brick on the key sending a series of dots for 24 hours. 
Depending on the weighting, that could be any where from 50% duty cycle 
to 30% or even less.

But lets say you have access to one of those ultra linear 3CX or 
3CPX5000 pulls with fantastic IM in the minus 50 db range,  that can 
comfortably run the legal limit using all of the drive from your 100 
watt exciter. You only need a plate supply capable of about 3000 watts 
if you figure 50% efficiency AND want the key down 24X7 CCS , or you are 
going to be operating...<gasp> AM.  There is absolutely nothing illegal 
or unethical about this approach.  Although big, this could be built 
into a desk top amp maybe slightly larger than the the EMTRON DX-3SP 
although squeezing a 3KVA plate transformer and 350 watt filament 
transformer in there along with the rectifier and capacitor banks  may 
take some doing.

Winding the tank coils is easy, but may require sacrificing some copper 
tubing while learning, or you can find some one who is good at winding 
coils.  Given the plate impedance, you should be able to find a design 
to copy which makes placing the taps on the coils much easier.  Also 
good hefty toroids for the 160 and 75 meter bands is now the way to go. 
I've lost track of how many coils I've wound, but that used to be part 
of my job with those induction heating generators so it's many thousands 
at least and no, I don't want the job.

So if the builder is like me and wants the thing to look well 
constructed in addition to working well, figure in at least redoing the 
tank coils and cabinet about twice. But even neglecting that and getting 
everything right the first time, the power supply alone is going to cost 
more than most entry level amps. I'd guess by the times it's finished 
and working the amp is going to cost about as much as the top end 
Ameritron, or maybe Palstar but it will be rugged and something to be 
proud of. 

Now, how many people do you know that would spend as much as an AL1500 
costs to build an amp when they can purchase one for the same price. 
Granted they can build a better amp, but still...

Now lets go back to that legal limit issue. I'm not condoning illegal 
actions, but what is really more ethical, running a 2 or 3 KW amp with a 
very clean signal, or some one trying to push an amp way beyond it's 
capabilities you can hear 10 to 20 KHz up and down the band. That over 
driving along with the speech compressor turned to max is a common 
practice on virtually all HF and 6-meters. There is a faction that will 
crank every thing up for just a few watts that were it clean would never 
be detected on the receiving end. Instead of getting louder they just 
get broader.  In my case my voice actually benefits from a fair amount 
of compression, BUT when out in the shop all any one would hear between 
words would be the fans and equipment running in the shop. It basically 
sounds like a huge turbine running in the background if I us much if any 
compression.

Many times when the bands are open I often think we are lucky that more 
hams do not run QRO, or try to push a few more watts out of what they have.

73

Roger (K8RI)
 
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