Charles, I reckon the best thing you can do to ensure tube longevity in your
amplifier is check with an accurate RMS calibrated AC voltmeter to see
whether or not the filament voltage measured right at the tube pins is
within the tube manufacturer's rating....many amp manufacturers don't pay
close attention to this important detail.
Also never exceed the Data Sheet's maximum rated plate current; and use
plenty of CFM air flow to cool the tube's anode heat exchanger and seals.
My personal policy is carefully selecting and adhering to the mid-point of
the Data Sheet's tightly specified filament voltage range. Irrespective of
whether it's a thoriated tungsten filament or an oxide coated heater.
The other policy I fervently believe in is careful control of cold inrush
current in the filament/heater structure. I believe this is particularly
important in amateur / ham radio service to offset and mitigate the thermal
shock of the large number of tube ON/OFF cycles. The novel, prudent and
efficacious soft-start technique used by amp manufacturer Emtron comprises a
slow 4 second controlled linear ramp-up of voltage produced by a phase
controlled Triac circuit on the HV and filament transformer AC primary; good
for the HV electrolytic capacitor longevity too and provides a complete
absence of any turn-on transformer "bong". There is no big inrush current
surge at initial switch-on and the tube filaments come up nice and slowly
that beneficially soft-starts the cold filament/ heater and reduces thermal
stress on the tube filament structure. This contributes to longer tube life
in a ham amplifier application that, unlike commercial broadcast Tx service,
has many hundreds of repetitive ON/OFF cycles over its many years of service
life.
In my TL922 amplifier I've wired a pair of GE CL-60 NTC thermistors in each
leg of the filament transformer primary winding to control cold inrush
current and a 10 Ohm Arcol HS25 power resistor to set the AC voltage
measured at the 3-500Z tube pins to be 5.0 Volts with a Fluke RMS voltmeter.
It drops to 4.95 when the amp is keyed to full rated Po. Without these
measures the filament voltage of the Kenwood stock amp was on the high side
at 5.3V with my local mains supply voltage of 245 Vac. Ideally Kenwood
should have used more taps on their primary winding to make better provision
for regional AC mains voltage levels and variations.
On a similar note and extension of this policy, when I built my house I
instructed the electrician to put dimmers in every light switch panel in the
house (uses 100% incandescent R80/60 reflector light bulbs sunken flush with
ceiling); with one or two rare exceptions over the past 14 years I've never
had to change a light bulb. The rare exception was the kid's bedrooms where
they would habitually turn the dimmer knob up to full bore.
Even well nurtured Tx tubes will eventually go soft after typically many
thousands of hours operation when the emission / electron supply runs out
and can no longer meet the peak plate current demand; the emission is being
depleted whenever the filament is lit / cathode is hot and streaming off the
finite number of electrons in the emissive surface.
Leigh
VK5KLT
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Charles Harpole
Sent: Thursday, 14 June 2012 4:47 PM
To: amps@contesting.com
Subject: [Amps] "babying" radios and tubes, can do or not?
I want to know if there are any REAL benefits from babying a radio or an
amp?
I hear lots of talk about running lower RF power to "save the finals" or
"save the tubes" and even hear people talk about turning radios off for
taking a one hour (or less) break.
We do know, probably, that high inrush current may stress parts inside
tubes. But, there must be some smart folks who test things to destruction
who can speak with scientific instead of parental certainty about this.
It seems to be true that one can run any of the usual amp tubes at rated
specs for hundreds of thousands of hours before they really go bad?
I guess tiny impurities in the metal and not a perfect vacuum does lead to
failure eventually, but what are the real reasons?
What is the real physics condition of a so called "soft" tube? Why?
What is really the cause of transistor failure if always operated within
specs?
And, if you run your 100 watt radio at 80 watts, are you "saving" the radio
or just your electric bill?
Inquiring minds want to know. Please.
73
Charles Harpole
k4vud@hotmail.com
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