[AMRadio] Broadcast Processors

Tue Jun 14 16:48:32 EDT 2011

----- Original Message ----- 
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 1:47 PM
Subject: Re: [AMRadio] Broadcast Processors


> 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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