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

[Will Matney]

Dave your correct, the do still change them out on a time line. That's where a lot of the pulls on eBay come from. However, I have seen some large stations with a standby final stage, two cabinets side by side which are switched over in case of failure. I doubt they fire up the tubes heater unless it's needed. Matter of fact, the biggie here in Ohio which was Voice Of America had a standby PA. It wasnt near the power of the big monstter though. Of course they dont run that big amp any more and is now a tourist attraction. Actually, they had to shut it down over shutting Canada down. At its hay day, the Royal Family of England used to make music requests if I recall. This one is just outside of Cincinnatti. It was something like 500,000 watts if I recall or maybe a little more. The Blaw Knox tower still stands, and the cooling ponds are still outside, just grew up with cat tails. I think a group did overhaul it and fire it up for the 2000 shebang. They didn't run it very long though.

Best,

Will


*********** REPLY SEPARATOR  ***********

On 4/1/05 at 1:02 PM Dave Haupt wrote:

>> 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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