Hi Peter,
My "spam" filter made me miss most of this, but I should add a few
comments.
> Rich says:
>
> >Consider that the price of a 25w rheostat is under $6.
Of course Rich always leaves a few things out.
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.
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.
3.) If you get past the above you have given someone, who's skill at
reading meters, eyesight, and common sense are unknown, ability
at the mere tweek of a knob to RUIN a tube quickly. You can bet
that person, even if he did something wrong, would demand a free
tube...or it could be a V31BB who actually rewired his transformers
for 200 volts (on a 240 volt main ) so he could run more power!
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.
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.
> Yes. Which is why in a home brew amp, you'd fit one,
> without argument.
Not me. None of my amps have rheostats for filaments. The reason
is simple. My line voltage varies from 241 volts to 252 volts from the
highest demand periods of summer to the lowest demand times in
winter, and I am in a rural area many miles from generating plants
and even HV transmission lines. That's well within the acceptable
range of tube filaments. I simply set the transformer taps at the
correct mean voltage, and let it go at that.
Even if voltage stability of my mains was + - 5% or more, I'm sure
I'd never see the difference in failures because I've never had a
stripped cathode or open filament. By far the biggest stress in
amateur service is the constant thermal cycling of the filament, and
not the running time deterioration. If I wanted to make tubes last
longer, I'd quit turning the filaments off and on a few times every
day.
I don't do that because it cost me more in energy to run the
blowers and filaments than it costs in tube life reduction.
Most tube failures are due to manufacturing defects in the tubes, or
abuse. Virtually none are controllable and correctable failures.
It's easy to sit in a room parked in from of a computer, never build a
commercial product, and take unfounded pot-shots at the world. It's
quite another view when you spend time analyzing returns and
looking at the cost to reduce the few things that are controllable.
Factually, whether people like it or not, every single prevention
system ADDS failures in other areas while reducing failures in the
target area. It is easy for additional problems to offset the benefits,
and wind up costing more that leaving things alone.
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.
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.
> 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.
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).
> 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
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.
The out-the-door cost to correct that non-problem was about $100
back in 1988.
73, Tom W8JI
w8ji@contesting.com
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