>
>Hi Peter,
>
>My "spam" filter made me miss most of this, but I should add a few
>comments.
>
Spam -- that which questions pontifications.
>> 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
[chortle]
>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.
>
The DTR2000 runs 5.9v on the 8877 heater. The max fil. V is 5.25.
>> 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.
I know folks who have wider fluctuations.
.....
>Most tube failures are due to manufacturing defects
NIH syndrome.
>in the tubes, or
>abuse. Virtually none are controllable and correctable failures.
>
gold-sputtering is a correctable problem. Use a parasitic suppressor
with less Rp. Use a glitch resistor.
>......
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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