[Amps] Can I cool too much the tubes or ants?

[Jim Thomson]

Date: Mon, 23 Aug 2010 23:06:46 -0400
From: Roger <sub1 at rogerhalstead.com>
Subject: Re: [Amps] Can I cool too much the tubes or ants?

Unlikely it'd be the antenna heating, but OTOH the typical balun sold to 
hams is quite susceptible even at less than the legal limit. On low 
bands like 160 and 75 it's easy to fracture the ferrite inside.  A few 
weeks back when we were having temps in the lower 90's and bright 
sunshine I was operating on 40 meters using the AV640.  The bands needed 
a bit of readjusting since I rebuilt the matching unit, but I'd not 
gotten to that yet.  The AV640 uses a current balun for isolation 
followed by a 4:1 balun with both using 2 cores each IIRC.  Although not 
recommended when using baluns I was keeping the SWR down to the amp 
using the Palstar AT5K tuner.

## I just read the Hy-gain AV-640 manual.  It's a 40-6m vertical..that's
supposed to be  3/8 wave length on each band.   How you get 3/8 wave lengths
on a 26' vertical on 40m is beyond me.  It's less than a 1/4 wave long. 

## The manual states the Z is several hundred ohms. They use   72"  long spoke
radials  to recuce the Z a bit..down to 200 ohms.  The  200 ohms is reduced to
50 ohms with a 4: 1  xfmr.     Then they use a 1:1  current choke balun.

## The ant is only rated for 500 W  RTTY on  40-10m.... and only 100 W rtty on 6M. 
Dunno if the limitation is  the  4:1  xfmr... or the  1:1 choke balun.. or both.  The power
limitation may also be the  decoupling stubs and sky high V.    





I was running the legal limit and had been on for a while when the amp 
(Emtron DX-2sp) kicked off due to high SWR.  And it was high...on the 
order of roughly 6:1.  I was mulling this over wondering what had gone 
wrong when I hit the key to check the SWR again.  It was showing 1.0:1!
I switched and made a short transmission at the legal limit on SSB 
before the amp kicked off due to high SWR *again*. So, this time I 
waited 5 minutes and again the SWR had dropped back to 1.0:1.  This time 
I managed about a 30 second transmission before the SWR again went high.

I have the AV640 down, but I've not had a chance to open the tuning unit 
yet.

##  maybe what u need is a real  4:1 balun..like the AS units.  You can't blow 
them up.    Then follow it up with a real  1: 1 current balun.   You can't blow up
type 43 material on 40-6m either. 





With the designs listed in Jim Brown's (K9YC) RFI tutorial I use an 
antenna tuner at the legal limit and very high SWR at the band edges on 
both 160 and 75 with no problems of overheating. 6 turns of LMR-400 or 
Davis BuryFlex(TM) on 5 or 6 2.4" cores seems to work quite well.
I had some misgivings about not having both sides of the wire antennas 
at DC ground for precipitation static build up, but "so far" it's not 
been a problem.

##  Either a 45 uh choke directly across the feed-point would work.   Also,
a  45 uh choke... at grnd level/shack/ spg  will also work...or just use a 
ICE  lightning arrestor.  




OTOH I think I need to add a remote tuner at the feed points to keep the 
voltages down on the feed lines to the 6-pack and the remote antenna 
switches.

## A small nema box with just ONE coil in either leg and 1 x spst relay,
[relay used to short out the entire coil on ssb]... will work better than
any auto tuner.  Allen Bond has loads of latching vac relays  [spst] that
will easily do the job. 

##  perhaps what you really need for 40m is a rotary dipole.  I have 3
of the F-12   EF-140B's.    These are  44'  long, weigh 12 lbs..and are about
1.4 sq foot.  Triple walled in the middle [.175" thick]... then double walled,
then single walled. Cover's 240 khz.  And 300 khz with the switch box which has
1 x relay and 2 x coils, one per leg.   Good for 100+ mph.   You can mount it directly
to the mast.  Normal deal is to orient the 40m dipole so it's parallel to any boom
below it.    IE: it radiates 90 deg   from the yagi's below it.   It won't screw up a 
15m yagi either.     I have 3 of em that I won't be using, if anybody is interested. 
14 db F/S..which is nothing to sneeze at.   

later... Jim   VE7RF 





73

Roger (K8RI)
>   I