>
>> 1.) There would have to be some reasonable assurance lack of a
>> rheostat actually is causing a problem with tube life. That doesn't
>> seem to be the case since almost 80% of tube failures are G/K
>> shorts and the remaining percentage mostly due to loss of vacuum
>> or voltage breakdown failures.
>
With 8877s, most of the failures have been from gold-sputtering. please
see Figure 24 on Web site.
>However, most commercial amps, including every commercial transmitter in the
>AM/FM broadcast industry use a rheostat to control filament voltage. In
>commercial applications, it's common practice to run a new tube,
>(particularly metal/ceramic power tubes) at the manufacturer's specified
>filament voltage for 500 hours of operation, then reduce the filament
>voltage at or slightly above the point where the emission begins to drop
>off. Is this directly related to tube failure? Maybe. Maybe not. But,
>tube life can be greatly extended when using this method.
Especially so with filament-type tubes.
>It's a
>well-settled fact that power tube life is substantially shortened when
>operating a power tube above the manufacturer's recommended filament
>voltage. In 20 years of commercial broadcasting as a V.P. of Engineering, I
>can honestly say that I had never experienced an open filament due to
>filamant over-voltage, but premature emission reduction can be always be
>expected.
For heater-type tubes, the problem with excessive heater voltage is that
it causes barium migration to the grid. .
>Going to the station's G.M. with the news that a 4CX20,000D has
>to be replaced or rebuilt is never a pleasant experience.
>
>> 2.) If the rheostat was added, provisions would have to be made for
>> monitoring voltage accurately. Most expensive panel meters are
>> only 2% of FS anywhere on the scale, and to make that worse it
>> would be driven by a rectifier and true-RMS filtering system that
>> would be full of components with tolerances. By the time all is said
>> and done, the $6 rheostat would add $100 of cost to the PA if you
>> bought new parts (which commercially you have to do, unless you
>> are someone who foolishly mixes in surplus parts) and would have
>> to be hand calibrated.
>
>Forget the meter. Simply add two insulated/isolated test jacks to monitor
>filamant voltage with a hand-held true R.M.S. DVM, or iron vane voltmeter.
>Those of us that feel filamant voltage sampling is important will use it.
>Those who don't aren't stuck with a precision $100 movement.
>
good point
>> 4.) Even if you stopped some life reduction by allowing filament
>> adjustment, the end result would be to add a certain number of
>> hours to the tube life. It would not make the tube live forever, and
>> very likely would not improve it a measurable amount in Amateur
>> service.
>
>Perhaps, but nobody is asking the tube to live forever. I simply want to be
>able to control the voltage to the tube if I desire so that my elevated 255
>VDC AC mains
Whew
> doesn't translate to 6.5V on a tube rated for 6.0 V. Let me
>make the choice. A rheostat and front panel test points is all I need. I
>realize that amateur service is not a 24/7 operation like that of
>broadcasting, but the benefit can still be realized just like adding
>step-start inrush protection.
>
amen
>> 5.) Factually the easier you make it for someone to screw up or
>> abuse the equipment, the more likely it will happen. There would be
>> a reasonably large percentage of additional failure from component
>> failures and customer abuse or errors.
>
>Any more factual than the presence of the load and plate tune controls on
>the amp? I can find plently of positions to kill my amp with improper
>tuning technique and excessive overdrive. To protect against these
>conditions from the operater costs no less than a rheostat. Keeping the
>adjutsable limits reasonable on a filament rheostat is a simple matter.
>
>> That's why you NEVER, from an engineering standpoint, add
>> components that have negative impacts on other areas of the
>> system. The rheostat Rich "harps" about is a prime example of
>> something that can cause more problems than it cures, because it
>> "corrects" what is almost always a non-problem while adding
>> unreliablity and the potential for damaging human error.
>
>NEVER? Historically then, why has Harris, Gates, RCA, CSI, Brown-Boveri,
>Continental, BE all included a manual or automatically-driven rheostat in
>their final power tube designs? There's nothing special about how or where
>these companies decided to place the location of a simple rheostat and I've
>never seen (to my best recollection) a filamant rheostat cause other system
>problems. Yes, it's possible. But the risk is certaily worth the addition
>of the control.
>
>> The trend of amplifier design, in the low-tech world we are in, will be
>> to REMOVE customer controls...not add them. Especially when
>> they have the potential to do harm and are unlikely to do any good.
>
>Can't argue with you here. Look at the Alpha 87A, ACOM 2000, and
>solid-state HF amps. However, the cost of operator protection goes into
>relatively expensive protection circuits which can, in and of themselves
>fail.
>
>> > In the commercial world, $6 on a component is at least
>> > $12 on the ex-works price. In the case of a variable or
>> > preset control, where testing and adjustment is
>> > required, add another $5. These are the real costs you
>> > find in a production environment.
>
>Fine, call it $20 for the rheostat and a couple of high-quality Pomona
>Industries test point jacks and I'll still pay for it.
>
>> You forgot the meter and circuits needed, and underestimate the
>> time required to make sure the system is calibrated, as well as the
>> cost of failures in the additional parts (any one of which could
>> actually cause a tube failure).
>
>Again, let the outboard DVM or iron-vane meter be responsible for the
>accuracy. When I'm not setting filament voltage, I can use it for other ham
>radio domestic chores.
>
>> > So all of a sudden, our filament rheostat is costing $20
>> > plus sales tax to the end user. Similar sort of add ons
>> > happen for step start, increased cooling for the tank
>> > circuit and so on. Very soon, you're talking of the $5k
>> > plus amplifier. Which is part of the explanation of why
>
>Huh? How did we go from a $20 rheostat to a $5K amplifier?
>
any port in a storm.
>> I did a cost estimate on adjustable filament voltage at one time,
>> and asked for statistical data from Eimac on their tube returns.
>> They said other than amps that run the filaments at 10-20% extra
>> voltage (Dentron and two other manufacturers) they saw virtually no
>> preventable filament failures. Such failures were "in the noise floor"
>> of the statistics. Unlike Rich, I trust what Eimac tells me.
>
>I don't believe its an issue of filamant "failure" or tube returns but one
>of reduced emission life.
correctamundo - either from tungsten dicarbide loss (filament types) or
from barium migration (heater types))
>The addition of a simple rheostat to the power
>tube's filament circuit makes good engineering and economic sense.
>
For me, it indicates good engineering.
cheers, Paul
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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