[AMRadio] Broadcast Processors

Tue Jun 14 13:47:17 EDT 2011

Ron and I looked this over carefully.  No airflow problems, and the mod
trans was OK, a oil-filled Electro job.  The duty cycle was tough, with the
mod current meters just never moving off max except total silence.  Positive
peaks were around 120%.  The compression ratio was really high, and the
program density and bottom end correction was right on the edge of negative
overmod.  This TX even used the iron-free 'transparent' cathode-follower mod
driver setup.

Some combinations of parts called transmitters are not meant to
have asymmetrical positive peak ability.

Years later I saw the same characteristics on a CCA 5kW.  It had the same
tube compliment, 2X4-1000 modding a single 4CX5,000.  It couldn't handle
+peaks without tripping the OL relays.  They tried previouly, and had to buy
all new mod iron.

73DG

On Tue, Jun 14, 2011 at 10:30 AM, Bernie Doran <
qedconsultants at embarqmail.com> wrote:

> curious about what killed the 4-1000s, did they have propper airflow and
> normal plate current?  AB2 can develope 3800 w of audio at 6kv  that should
> be a good match for 5 kw out.
> did you ever ask Eimac about this?
> From: "Dennis Gilliam" <dennisgilliam at gmail.com>
> To: "Discussion of AM Radio in the Amateur Service"
> <amradio at mailman.qth.net>
> Sent: Tuesday, June 14, 2011 1:04 PM
> Subject: Re: [AMRadio] Broadcast Processors
>
>
> >A short story about an AM transmitter:
> >
> > Years ago I was CE for KZZP-AM in Mesa, AZ.  The station had a Bauer 5kW,
> > and management wanted it to be 'louder'.
> >
> > Replacing the old, single-band limiter, I installed a CRL set of three
> > units
> > (AGC, EQ, and limiter), the usual setup.
> >
> > Wow, what a difference that made!  It still didn't please the program
> > director, so I got Ron Jones (the owner of Circuit Research Labs) to come
> > out and take a look.  He said 'This transmitter has the worst bottom end
> > tilt I've ever seen'.
> >
> > He went back and cooked up an experimental box we installed the next
> week,
> > and boy, did it sound really good & loud.  Lots of mod plate glow, BTW.
>  A
> > week later, the 4-000 modulators were flat.  I didn't think that was too
> > bad, as they had been in service for some time.
> >
> > I put in a new set, things were good again.  Boy, those Eimacs were
> really
> > working for a living, I thought.
> >
> > Two weeks later, they looked like old coffee pots, brown from ion burns
> > and
> > totally without emission--again.  I mentioned this to the PD, and that
> > this
> > will cost some $.  He said, 'Don't worry about it, we are going to make a
> > bundle!'
> >
> > New tubes were ordered up, and in went my last spares.  The same drill
> > went
> > down, those tubes died in two weeks.  More tubes installed.  More great
> > glow....
> >
> > Then the office manager asked me about the invoices she was getting from
> > Richardson Electronics for all those 4-1000's.  I told her the story.
>  She
> > went to the boss, and that started a loud argument between her and the PD
> > about the $$ out vs. the $$ in.
> >
> > The big boss found for her, and the 'magic box' from CRL was retired and
> > mod
> > tubes went back to lasting about a year or so.
> >
> > The lesson of TANSTAAFL applied to transmitters.
> >
> > 73DG
> >
> > On Tue, Jun 14, 2011 at 7:45 AM, Jim Tonne <Tonne at comcast.net> wrote:
> >
> >>
> >> Regarding commercial broadcast processors for ham use.
> >>
> >> > The transmitter "Frequency response must be less than
> >> > 3dB down at 0.15Hz, and less than 0.1dB down at 9.5 kHz."
> >>
> >> That figure of 0.15 Hz may sound odd and arbitrary but is
> >> based on a direct-coupled modulator (i.e., flat down to DC)
> >> with a one-second time-constant on an input blocking
> >> capacitor and resistor.
> >>
> >> That figure of 0.15 Hz is sometimes specified in the contract
> >> between the manufacturer and the purchaser.
> >>
> >> Another way the low end is sometimes specified is that if
> >> the transmitter is modulated with a 50 Hz square wave that
> >> the tilt on the resultant modulation is to be less than 2%.
> >>
> >> Why this "nonsense" you may ask.   It is because the
> >> commercial broadcast processors  generally deliver an
> >> output waveform that is somewhat clipped, although it
> >> may be clipped only on transients.   For them to control
> >> the modulation requires that the following audlio stages
> >> (the modulator) have such a flat response.
> >>
> >> Another area that is of concern in the transmitters is the
> >> insistence by the purchaser that the response be flat on
> >> the high end and yet have no overshoot on a 2 kHz
> >> squarewave.   Can't do that and when the manufacturer
> >> sits down with the purchaser and explains the math,
> >> the purchaser comes away enlightened and happy.
> >>
> >> - Jim Tonne   used to design BIG PWM rigs at CEC
> >>
> >>
> >>
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