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

[Radioal]

I think you are mixing memories of both VOA and WLW, which were both located 
north of Cincinnatti.

----- Original Message ----- 
From: "Will Matney" <craxd1 at ezwv.com>
To: <amps at contesting.com>
Sent: Friday, April 01, 2005 6:25 PM
Subject: Re: [Amps] Standby tubes (was: Even more power solid-state amp's)


> 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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>
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