[AMRadio] Broadcast Processors

Tue Jun 14 16:59:13 EDT 2011

interesting!  Just does not seem logical. The mod transformer would probably 
go down to 30 cy or less and probably the audio low cut off was not much 
below that.   my Gates COG also has cathode follower drivers, a great way to 
drive class BE.   Wonder if there was some component( with all that 
processing)  that was near DC and ended up as dissipation. That should have 
been obvious from color
----- 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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