[AMRadio] would like to build a little homebrew project


John Coleman jc at pctechref.com
Mon Aug 6 11:47:25 EDT 2007


Very well said Brett:
	And learning what's what about the individual chunks will take you
into the nitty gritty of circuit design and experiments which is what I like
most.  
	If you don't mind my rambling, I once experimented with
neutralization circuits.  The most fun was the inductor type where an
inductor is place in series with a DC blocking cap and the two pieces are
placed from grid to plate of the RF triode.  K5MZH (Koby) used this.  The
inductor resonates with the grid to plate capacitance of the tube. Causing
Xl to cancel Xc.  Great for a folks that don't band hop much because you
need to change the neutralizing coil to change bands or even if you change
freq very much.  The grid to plate capacitance is small so at 3.8 MHZ it
took a lot of coil.  A variable cap with very wide spacing and small
capacitance placed across the neutralizing coil will enable the find tuning
of the circuit.  

	This is when I learned about dielectric heating of PVC.  I thought I
would save money and wind the coil on a 6 X 1.5 inch PVC pipe it worked
great.  That is until I poured the voltage to it.  The RF voltage across the
coil heated the PVC and caused it to shrink and the circuit would go out of
resonates.  It took me a while to figure it out because I could apply drive
all day and checked the neutralization from time to time and it was perfect
but when I put the plate voltage on for just a few minutes the neutralizing
would start changing.  The longer the plate voltage was on the worse it got.
Output loading or heat in the cabinet made little difference.  That is how I
determined that it was dielectric heating.  Winding the coil on a cardboard
paper towel roll fixed it all.

John, WA5BXO          

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Brett gazdzinski
Sent: Monday, August 06, 2007 9:42 AM
To: 'Discussion of AM Radio in the Amateur Service'
Subject: RE: [AMRadio] would like to build a little homebrew project

Look at fests and ebay for handbooks from the
early 50's to the early 70's, both Bill Orr and
arrl.  I like the red and yellow Bill Orr handbooks,
forget what year they were exactly.

You don't need to get a lot of them, say one every
4 years or so, as they did not change a lot from
year to year.

Then you take parts of designs and put them together,
no need or reason to build something just like any book says,
you just need a basic circuit and experiment from there.

Like the 3x4D32 rig, there IS no design anywhere for that rig,
but its just your standard beam power tube rig.
Take a 4D32 tube and multiply everything by 3, 3x screen current,
3x grid drive, 3x power input, etc.
For the tank circuit I just wound copper tubing then shorted turns
till I had a good Q, good plate current dip, good efficiency, etc.

Then you think about the modulator....how much power do you need?
In the 3x3D32 rig, its about 300 watts...my rule of thumb is to have
at LEAST as much audio drive as power output (for a clean signal).

What gives 300+ watts of audio at 1000 to 1250 volts?
Why 811A's do nicely, zero bias to!

You can do the same thing when building receivers, mine have circuits
out of Bill Orr, ARRL, the detector came off some web page, some design
was taken from the old Scott SLRM I used to have, etc.

Everything gets broken down into little chunks:
RF only deck,
Bias supply,
Screen supply,
Metering and lights,
Protection circuits,
Keying circuits.

You can mix and match those to get what you want/like.
Adjustable bias? Variac? Grid leak resistor fixed or big pot?
Meter for voltage and current? Decouple grid leak bias from fixed
bias with a diode?

You can then build exactly what YOU want, complexity gives
more options, and maybe more safety, but gets bigger and more expensive.

It sure is FUN to homebrew!


Brett
N2DTS



  

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jason Buchanan
> Sent: Thursday, August 02, 2007 9:58 AM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] would like to build a little homebrew project
> 
> 
> On Jul 31, 2007, at 9:11 PM, crawfish wrote:
> 
> > I just received a copy of the 12th Radio handbook(1949). After  
> > WWII, 160m
> > was given to LORAN for the most part, and you had to go to the pre- 
> > war Radio
> > Handbooks to find amps for 160m.
> 
> I may scrounge around for some of those books - do you know what  
> years might be good books to look for?  I probably won't find them  
> for sale so probably will have to borrow...  If you know what 
> year of  
> publication had good ones i'll try to find those years.
> 
> thanks a ton................
> 
> --
> 73 Jason N1SU
> http://n1su.com/
> 
> 
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