Vern has it just about right in my book. Yes the Johnson KW matchbox was
rated for legal limit AM in those days. Now just what was legal limit AM
way back then. Basically stated it was 1KW DC input to the final amplifier
stage. So what was the real output power? Typically class C rigs operate
at between 60 and 75% efficiency, thus 1 KW DC input translates to some 600
to 750 watts of carrier power. This carrier modulated 100 %, being a power
factor of 4, is some 2400 to 3000 watts PEP. Today's SSB amps, legally, are
rated at 1500 watts PEP.
Keep in mind too the duty cycle of AM rated tuners vs. SSB/CW rated tuners.
Without doing a lot of math, 750 watts of carrier is 750 watts of carrier at
100% duty cycle for as long as the carrier is on. Then add the long winded
AMers 10 to 15 minute transmissions (yea we are noted for being old
windbags) and one can see that unless things are really up to snuff,
something gets hot and often lets the smoke out of the box. Conversely in
SSB today, the normal speech pause between every word and every syllable,
the output is actually near 0 watts. Duty cycle of SSB voice is in the order
of some 20%. From a voltage flash-over point, 1500 PEP watts of SSB and
1500 PEP watts of AM produce the same peak voltage values. In many cases in
today's tuners, the current rating of components is very marginal while
voltage rating is safe at the 1500 watt PEP level. (Legally today's AM
transmitters under current rules may operate at 375 watts carrier output,
thus 1500 watts PEP.)
In the case of the old vs. the new one can now see the difference in the new
tuners and their ratings.
73
Bob, K4TAX
----- Original Message -----
From: "Vern Kunes" <vern@mantlelamp.com>
To: <tentec@contesting.com>
Sent: Tuesday, July 01, 2003 9:20 PM
Subject: Re: [TenTec] T tuner (transmatch) losses?
> As I have been told, the Johnson Matchbox was rated for AM, double
sideband,
> full modulation. I would think that it would be able to used with
moderately
> more power for SSB, but since I am not an engineer, I can't say that with
> final authority! For balanced lines and balanced antennas, they are great,
> although their matching range is a hair on the narrow side. Not lossy like
> tuners with baluns (it's usually the balun that gives you alot of loss,
and
> the balance in a balun is not as perfect as the old link coupled tuner).
For
> coax, ther "L" is king. VERY, VERY low loss and with enough inductance and
> capacitance, you can match about anything. But you will definately need
alot
> of capacitance (that is why Ten Tec has a space on the back of their
tuners.
> You can add more capacitance externally if you need to. Especially on
160).
> Just remember, when talking tuners, look at what the QRP guys use. They
need
> the lowest loss possible so they use the lowest loss components. What
works
> well for QRP works well for QRO (it just takes BIGGER components). Vern
> N2YZS
> ----- Original Message -----
> From: "Carter Grabarczyk" <k8vt@ameritech.net>
> To: <tentec@contesting.com>
> Sent: Tuesday, July 01, 2003 9:36 PM
> Subject: Re: [TenTec] T tuner (transmatch) losses?
>
>
> >
> >
> > denton wrote:
> > > Too bad no one is making the old link coupled transmatches anymore...I
> have
> > > a Johnson Matchbox..the 250 watt version...that I use for a basis of
> > > efficiency testing.
> > > Nothing I have ever homebrewed or anything commercialy available
> nowadays I
> > > have tried can really match the balance or efficiency ability of the
old
> > > Matchbox, as far as matching to either one of my delta loops fed with
> 450
> > > ohm ladder line.
> >
> > I also have a Johnson (well OK, a Nye Viking *identical* clone) 250 watt
> > matchbox with SWR bridge. As these were fairly beefy and built in the AM
> > days, does anyone know what power it can take on SSB, RTTY and CW or is
> > 250 w really it?
> >
> > Thanks,
> > 73/Carter/K8VT
> >
> > P.S.
> > It followed me home from a (non-ham) flea market. A CB'er was selling it
> > and the CB rig together for $15. I asked him if he would take $10 for
> > the tuner and he could keep the rig. We both walked away happy! <g>
> >
> > _______________________________________________
> > TenTec mailing list
> > TenTec@contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
>
>
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