[Amps] Standby tubes (was: Even more power solid-state amp's)

[Dave Haupt]

> From: "Joe Subich, K4IK" <k4ik at subich.com>
> Subject: Re: [Amps] Even more power solid-state
> amp's
> To: <craxd1 at ezwv.com>, <amps at contesting.com>
> Message-ID: <000d01c536ca$a1aab4a0$c080a8c0 at laptop>
> Content-Type: text/plain;	charset="us-ascii"
> 
> 
> Will writes: 
> 
> > What if you had two final tubes, one idle until
> the other 
> > gave up the ghost. It's easy to change over the
> same way with 
> > way less circuitry. 
> 
> The cost of a "hot standby" transmitter using vacuum
> tubes is 
> far higher than just the tube alone.  It really is a
> completely 
> separate transmitter with the exception of, perhaps,
> the low-
> level stages.  In television, since pre-correction
> adjustments 
> (linearity) generally need to be set up based on
> amplifier 
> tuning, it is far better to 'marry' a low level
> stage and 
> final amplifier. 
> 
> When one considers the relatively small premium for
> solid-state 
> TV transmitters at low/high VHF and in some cases
> lower UHF, 
> tubes did not make a lot of sense in the last
> equipment replacement 
> cycle. 
> 
> Now, with HDTV's (digital TV) 12 to 16 dB peak to
> average ratio 
> and the need to make up to 60 KW average output in
> some cases, 
> specialized tubes have reasserted themselves at UHF
> (though I doubt 
> any ham rig will ever use a multi-depressed
> collector Inductive 
> Output Tube - MDCIOT). 
> 
> 73, 
> 
>    ... Joe, K4IK 

As Joe says, a standby tube is a monumental task. 
Broadcast-sized rigs have fractional to multiple
horsepower blowers, and it's a daunting task to
auto-switch anodes, cathodes, etc.  So you'd end up
having a completely redundant PA stage with its own
cooling.  If the tube involved is an indirectly heated
type, you'd also keep the heater running, which means
a separate heater supply.  And that's a dubious
notion, as the wearout of an indirectly-heated tube is
dependent on heater time, not cathode emission time.

Further, who says it's going to be the PA that fails? 
Drivers fail, also, so you may as well have a
redundant one of those.  In the solid state designs,
you often have one solid state module driving eight
more, then another 64, all identical.  Therefore, one
module suffices as a quickly replaceable unit for any
of PA, driver, pre-driver.

In my days in broadcasting, stations would simply
acquire the replacement tubes when the present PA and
driver tubes reached a certain number of hours.   Then
they'd watch the PA currents and so forth, looking for
signs of degradation, so that they could schedule the
tube replacement at a time when the cost per minute
off-air was less (2am).  The major market stations I
was familiar with (all in Denver, CO), had entire
backup sites - a 50kW station might have a 5kW station
on an adjacent mountain top, on hot standby.  It would
go on the air when anything failed at the main site. 
Stations just as often lose the audio feed as a PA. 
Typically, the smaller TX was the station's earlier
rig, prior to their approval for higher power.  But
that was 20 years ago.

Nowadays, most projects I work on are intended for
production volumes of millions per month.  You'd think
a standby Pentium would be a good idea, but I suppose
they don't fail very often!

73,

Dave W8NF


		
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