It's more than 14 years since I've delved into these beasts,so I might have forgotten some points,but here goes:-
I am most familiar with the NEC Transmitters at TVW7,which have Sync Tip Output Powers of 13kW to allow for various losses in the feed system.
In this generation of NEC's high power Transmitters,the Sound (“Aural” in “NEC- speak”) & the Vision RF signals do not pass through the same amplifier chain,but are passively combined at the output of the Transmitter.
Valve/Tube PA:-(Vision)
Big tetrodes like the NEC 8F76R can easily provide around 13dB of power gain in Television service,with a Sync Tip Output power of around 13kW.
13dB is a convenient figure,as it translates to a power gain of 20times.,so 13kW output requires 650W of drive to the valve PA.
TRPAs:-
Normally,there is one TRPA driving the Vision (Valve) PA,having in this case an output of 650W,& two,(in TVW7's case) or more TRPAs operating in parallel to provide the Aural Output .
The total output power of the sound carriers combined is approx-10dB w.r.t. Vision Sync Tip power.
The TRPAs are also designed to deliver around 13dB of gain,so in each case require around 32.5W of drive from the exciter.
NEC had extensive experience with BJT RF PAs in their lower power Transmitters,so these TRPAs are extremely reliable in service.
Modular Construction:-
NEC used modular construction extensively in this series of Transmitters.
An example is the TRPAs &.their power supplies..
These are situated “back to back”,so that if the power supply fails,it may be removed & replaced with a spare without delving deeply into the “innards: of the Transmitter.
If a TRPA fails,it can simply have the RF connections removed,& be pulled out of the mounting slot.
Note on Sync Tip Power:-
Analog TV systems,except for very early ones,notably the British 405 line system use so called “Negative Modulation”,where the most negative part of the Video signal,the Sync Tips,corresponds to maximum RF Output Power.
This is very convenient when measuring Transmitter Output Power,as a “peak hold” voltmeter may be used .
It is simple to produce such a device which is hardly affected by the 64uS time between Line Syncs.
The reading is really PEP,but it is much easier to perform in this case,than with SSB.
The relationship between Vision & Aural power can also be determined easily using the same meter,as PEP & Average power of an FM signal is the same.(It does get a little bit messier with the 2 carrier Stereo Sound system used in Australia,but is still pretty close!)
Average power can be determined with greater accuracy using a water cooled Load,with the Transmitter modulated with Blanking & Syncs.
To obtain an accurate figure for Sync Tip Power using this method,however,it is necessary to very precisely set the modulation,so correction factors may be applied.
I have used "around" & " approx" a fair bit,as no components are perfect,or perfectly stable in their characteristics over time,especially large Power tubes.
A tube may have more than 13 dB Power gain when brand new,& require less drive than it will 20000 hours later!
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