> 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.
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. 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. 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.
> 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 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.
> 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?
> 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. The addition of a simple rheostat to the power
tube's filament circuit makes good engineering and economic sense.
-Paul, W9AC
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