Paul and company:
Are we overlooking the fact that as a tube approaches it max useable
freq., that the back bombardment of the cathode from electrons knocked
loose from the anode does raise the cathode temperature. Hence the
reason for reducing the heater voltage.......as I understand it. In
this case, it's reasonable to reduce the heater voltage. It also makes
for current limiting during startup.
In 24/7 service the cycle on and off times are nil. In amateur service,
and others, the cycle on and off times can be several times a day.
F Y I: My 2M 8877 amp now has 8058 hrs on the tube. (I've put them all
on there too.) The fil xfmr is a constant voltage, saturable core
reactor rated at 5V at 10A CCS. No adjustment needed as the voltage
stays well within the 5% of the required value.
I realize that this may not be an acceptable method with higher power
tubes, hence much higher currents for the heater. But for those
applications, I much more prefer to use an adjustable autoformer on the
primary side in place of a rheostat.
73
Bob K4TAX
Paul Christensen wrote:
>
> > 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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