The 3-500Z type tubes were designed with tungsten-thoriated filament
material. This is a material much different than standard tungsten,
although, grant it, any material heated and cooled will exhibit a finite
life from expansion and contraction related to temperature. I've never seen
a filament burn out in a 3-500Z tube, although did encounter one the
wouldn't "light" and measured no resistance between the filament pins. I had
heated those pins with a soldering iron until the solder flowed, added a bit
of 2% silver solder and BINGO. That tube provided a decade more of reliable
service for the ham I did that. He figured he had nothing to lose and as he
was prepared to buy a new tube, I tried this method that I had read about
years back, as I would have if it were mine.
The resistors that you refer to are surge suppressor resistors, typically
installed on the primary of the filament transformer, to "step-start" the
filament voltage for a few seconds to aid in prevention of the large current
surge from distorting the filament by delaying full power from the filament
transformer until the filament gets warmed up slowly with partial power
first upon turning the amp on initially. There are various thoughts on this
mythology as When Eimac developed the 3-500Z as an instant ON tube, the
design criteria was that the transformer impedance was quite adequate in
itself to prevent the calculated high surge current from reaching the
filament/s as if someone was switching on a supply already powered on with
core saturation and impedance of the windings not included in those
calculations. My take is that while the step start circuitry may not be
necessary, it certainly doesn't do any harm either. Many early commercial
manufacturers of amplifiers using these tungsten thoriated filament tubes
did not include any filament surge suppression and they provided good
service life. In recent times, many manufacturers of 3-500Z tube type
amplifiers provide for the step start approach. The addition of this
band-aid makes many consumers feel like it is an added benefit and will pay
more for it. Seems to me that in time, all manufacturers jumped on the
bandwagon and now have step-start as an added feature and benefit. I came
from the old school of Eimac engineering thinking of keeping it simple. Of
course, Your opinions and your mileage may vary.
In all probability, this conversation would be best served on the AMPS
reflector. Although it started out as a TenTec Centurion Amp discussion, it
really now refers to all amps using tungsten-thoriated tubes.
The AMPS reflector was one of the first that I unsubscribed from a half
dozen years or so back as I got quickly tired attempting to educate folks
that did not really want an education, but more so a debate. I have no
desire or the time to waste my spare time on any reflector debating heresy.
If folks would just spend a bit of their time searching the internet and
paging through books, the truth can easily be found on most any subject. Not
that everything ever published is correct, but with enough research you can
make your own educated decision.
73,
·´¨¨)) -:¦:-
¸.·´.·´¨¨))
((¸¸.·´ ..·´ -:¦:- Rog-K9RB
-:¦:- ((¸¸.·´*
FCC First Class Radiotelephone licensee (obtained in 1968 at age 18). Since
changed to General Class.
A-1 Operator Club, ARRL Life Member, DXCC #1 Honor Roll (352) Mixed, Phone,
CW (1961 - 1992)
presently need 11 more on RTTY for H.R. Need (4) more Zones on 160M. for all
(9) HF band "Worked All Zones". At present 160 Meter DXCC - 208 + 36 zones.
Former member Northern IL.DX Assn., 9th area incoming QSL bureau sorter for
many years, Charter Member Metro DX Club, Life member / former Trustee W9AA
Hamfesters ARC., CP-40 in 1963 at 14 years of age, former ARRL OO, & NCS,
active 160M through V.H.F. / U.H.F. for 45 years. 1st place CQWPX-CW 15M in
1980. 1st place CQWW-CW 40M in both 1980 & 1988. (Ancient history now!) Also
KG4RB /GTMO Bay - Bio and photos available at www.qrz.com
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of John K Gotwals
Sent: Sunday, October 08, 2006 4:40 PM
To: tentec@contesting.com
Subject: Re: [TenTec] Centurion Amp Woes
I was intrigued by Rog's longevity recommendation for "Instant On" tubes,
since I use a Drake L-4B amplifier. For ordinary tungsten light bulbs the
bulb's life can be shortened by frequent turn-on/off due to the thermal
expansion/contraction of the tungsten filament. In addition to the two
3-500ZGs, my L-4B power supply has two series connected 50 watt bleeder
resistors which get exceedingly hot while the power is turned on, and I
would guess that thermal fatigue of these resistors might also be increased
by frequent turn-on/off.
I would be interested in other comments on the turn-on/off vs. leave it on
conundrum.
John, N9JG
-----Original Message-----
Message: 10
Date: Sun, 8 Oct 2006 15:26:45 -0400
From: "RB" <K9RB@bellsouth.net>
Subject: Re: [TenTec] Centurion Amp Woes
To: "'Discussion of Ten-Tec Equipment'" <tentec@contesting.com>
Message-ID: <070501c6eb0f$b1224dd0$0201a8c0@k9rb8c670f7e81>
Content-Type: text/plain; charset="us-ascii"
[snip]
I also might suggest that with an amplifier using 3-500Z type "Instant On"
tubes, you will increase longevity of the tubes, as well as save on your
electric bill by turning the amplifier off whenever you do not anticipate it
use during the upcoming 10-15 minutes. Since the tubes require virtually no
warm up time, turning them off when not in use is a preferred mode of
operation. Just the 5 volts at 15 Amps of current to each tube calculates to
be 150 watts of wasted energy. Add the fan and HV transformer and you're
likely looking as 250 watts just in the standby mode that you have been
paying for, expended mostly in generating heat and contributing to shortened
component life.
[snip]
73, & Good Luck!, Rog-K9RB
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