[Boatanchors] 813 grid to filament short

Fri Oct 2 05:05:02 EDT 2015

Bob,

thanks.  I think the ERP got tossed in perhaps in error, but in defense of ERP on the broadcast bands, FM and TV stations have, in the past, used their ERP power levels in promotion material and nothing about their true transmitter power output.

I remember many years back WBBQ in Augusta, GA, used to claim 137,000 watts - 100kW in the horizontal plane and 37kW in the vertical, as I think they had a split antenna system to cover both polarizations.  This may have been before circular polarized FM antenna systems became the norm, and I believe they added the vertical polarization antenna years after they went on the air.

I note that Terman doesn't discuss asymmetrical modulation and his discussions appear to be based on sine wave modulation characteristics.

Yes, voice audio is very asymmetrical and negative modulation peaks and positive modulation peaks in AM can give very different peak readings.  

The AM broadcasting industry has been well aware of this for years, and  Leonard Kahn was one of the first, when he manufactured, and marketed to broadcasters his Symmetra-Peak device, taking advantage of this, changing audio phase the higher of the peaks was on the positive side of modulation.  Most modern audio processors designed for AM transmitters have this capability built in.

As we know, 100% negative modulation is a hard limit - carrier cut-off, but positive modulation on the AM band is limited by either the FCC (now 125%) or the modulation capability of the transmitter and perhaps how much distortion the listener was able to tolerate.   

As for monitoring, let me ramble here and if I am wrong, please correct me.  Assume a relatively accurate method of measuring 375 watts output, then use a scope to look at the transmitter output when 100% modulated with a sine wave, that peak level, top to bottom of the wave form, that would be your 1500 watt PEP level, right?  Any major audio greater than that peak to peak level, even on voice modulation, wouldn't that exceed the 1500 watts PEP?

73
Sheldon

________________________________________
From: rbethman <rbethman at comcast.net>
Sent: Thursday, October 1, 2015 11:37 PM
To: boatanchors at mailman.qth.net
Cc: Sheldon Daitch; wv2zow at gmail.com
Subject: Re: [Boatanchors] 813 grid to filament short

Hello Sheldon!

Long time since I've seen a post from you!

I don't have such a late edition of Terman's.  Mine is the 1943 edition.

In any event, there has been a whole lot of Bovine Scatology tossed around!

ERP has been tossed into this mess.  Obviously it is another red herring!

The 375W @ 100% modulation will indeed get you 1500W PEP *if* the 100%
modulation is symmetrical.

The human voice is anything but symmetrical.  This is where the majority
of folks start getting off the real path of meaningful numbers.

I only wish that there was a REAL way to measure PEP of any voice
modulated signal.

I do have a peak reading Bird, and the necessary slugs that are
compatible with it.

Then the real rub!  Whom owns an antenna system that is a pure 50 Ohm
resistive load?

I haven't found one of those either!  No 50 Ohm resistive load, no
realistic numbers!

This discussion started as more of a classroom exercise in lieu of any
realistic look at the tried and true real world.

The only place that can measure this PEP with any degree of accuracy is
up the road a bit from me, and that would be NIST, (National Institute
of Standards and Technology), and I don't think we will see them get
into this.

My Bird has a calibration sticker on it, but it is long out of its due date.

The entirety gets back to 813s and mounting them horizontally. Bad
move!  W3BYM uses 4 813s to modulate 4 813s.  Should one dig up the old
ER issue, you'll find that he mounted them vertically! The only
intelligent way to do so.

Simply because Heathkit used 572Bs and mounted them horizontally does
not indicate good engineering practice!  Yaesu also tried that back with
the FL-2100B.  Another wonder that was a problem child.

It is another exercise in futility to attempt to deal with Amateur
Radio.  What sort of feed line is used?  What band?  If it is coaxial
fed, then is it the proper length for THAT band?

The variables are vast, and there is less sense in the discussion.

Regards, Bob - N0DGN

On 10/1/2015 3:05 PM, Sheldon Daitch wrote:
>
> Bob,
>
> Terman is up to the challenge.
>
> My 1955 Terman's Electronic and Radio Engineering, Fourth Edition, page 530, chapter 15, section 2, in the miscellaneous aspects of plate-modulated Class C Amplifiers, and tubes typically used:
>
> "Plate-modulated Class C amplifiers normally employ the same triode, beam, and pentode tubes that are used for ordinary Class C amplifiers.  However, the rated d-c plate voltage and carrier power of a tube used for plate modulation are less than the corresponding ratings of the same tube when used as an unmodulated Class C amplifier.  This is because at the peak of a completely modulated wave, the equivalent plate-supply voltage applied to the tube is twice the d-c value, and the power output at modulation peaks is four times the carrier power.  However, the tube operates at these peak values for only a small fraction of the time, and the average power power output generated under fully modulated conditions is 1.5 times the carrier power."
>
> Also see:
>
> http://www.w8ji.com/amplitude_modulation.htm
>
> and
>
> http://www.pa2old.nl/files/am_fundamentals.pdf
>
>
> 73
> Sheldon