[BC] The value of tube transmitters

[Phil Alexander]

On 28 Feb 2007 at 0:58, Robert Meuser wrote:

> Phil Alexander wrote:
> 
> > On 20 Feb 2007 at 11:57, Robert Meuser wrote:
> >
> >
> > > Now, please give me a logical reason to keep a 50 year old TX on air.
> >
> > On the air? No, that increases expenses, but as a back up Tx, or one to
> > run when the thunder rolls ... that is good economy. They are also useful
> > for serious tuning work on DA's because they are tolerant of loads that
> > would choke any SS box off the air.
> 
> Provided you maintain it properly run it and don't let the caps dry out. 

Only electrolytics dry out. Some of the best tube units had few or none.
SS boxes OTOH .....

> Also there is that small issue of finding the now expensive replacement 
> tubes and hoping some other hard to source item does not go south.

Both 833's and 4-400's are avail. for 1 kW, and rebuilt 3CX2500's and 3000's
are around for the 5 & 10 kW boxes. 

> Often 
> a TX that old once not in regular service goes south quickly. 

Only if electrolytics are present and not replaced, or modulation
density is beyond the design criteria. Both are easily managed.

> As for DA 
> work, all you need to do is roll back the power output of the main TX 
> and VSWR is not an issue. Besides, with modern design techniques a DA 
> should come up pretty close to being on.

Sure, if you are building a new one. More existing ones need work than
new ones need building. They kinda like to stay on the air in the 
process so they can pay the bills. Since one of those will be mine,
I'm very much in favor of an unbroken cash flow at the station. The real 
world is not a pristine clean sheet of drafting vellum. Often, all you 
have is FCC data, steel that was stacked 50 years ago and know-it-all 
nitwits playing with the phasor for the past 25 years. OIB's of the bases 
are essential, but meaningless until the old rig can be brought into 
(temporary) compliance. At times like these, I'm looking for the oldest 
box in the room, not the new shiny one in the rack. Modeling? I ran into
one a couple of weeks ago that would have taken over a week. Totally
dissimilar radiator structures compounded by modifications, etc. etc.


---------------------------------------------
Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037