Charley, It sounds to me likely to be a plate to grid short in one of the
3-500Z tubes rears it ugliness only intermittently, sitting idle. This is a
common mode of failure with old 3-500Z tubes. It could be accelerated by
higher than 5.0 filament voltage and/or poor cooling due to a sluggish fan /
blower and / or dirt obstructing the air flow path.
I suggest that you obtain and install a new pair of Premium Grade matched
Taylor Brand 3-500ZG-RFP tubes from RF Parts after getting the power supply
back in working order and you should be good to go for many years. (Best
bang for the buck on replacement 3-500Z's) Also insure that the filament
voltage is between 4.85 and 5.0 volts with no input/output load on the
tubes. This is a critical factor to 3-500Z tube longevity and if your line
voltage is a bit on the high side, the filament voltage can rise well above
5.0 Volts. ANY filament voltage above 5.0 Volts is indirectly proportional
to tube emission life due to a proportional increase of internal cathode
heat, even sitting idle in the standby mode. Insure that your air flow to
cool the tubes is not impeded by a slow running fan / blower, or
restrictions from dirt build up.
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.
By the way, I've not heard of any significant quantity of HV transformer
failures on the Centurion but if there is no secondary protection resistor
after the filter capacitors, and prior to the RF Choke feeding the tube
plates, there should be one. Usually a 10 ohm, 5 Watt wirewound ceramic unit
will do the job. Of course, another thing to check, and replace if they are
over 15 years old, is the electrolytic capacitors in the HV power supply.
They should have equalization resistors across each with a value of around
270K ohm at 10 watts.
73, & Good Luck!, Rog-K9RB
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of Charles Delaune
Sent: Sunday, October 08, 2006 11:42 AM
To: TenTec@contesting.com
Subject: [TenTec] Centurion Amp Woes
I'm having an on going problem with my NEW Centurion. Several weeks ago the
line fuses blew for no apparent reason. The amp had been on for several
hours, but in Standby mode. All of a sudden the amp bit the dust as both
line fuses blew. The line voltage is 240v and checked O.K. The tubes
checked-out O.K., as well. That is to say, no shorts between elements
(checked with ohmmeter) or plate arcing (removed tubes and replaced line
fuses then turned amp back on - fuses blew, again). After I made these
initial checks I sent the amp back for repair. It was repaired then returned
with the following list of items which were replaced:
Relay and cover (relay contacts were welded)
Q1 (transistor from HV-AC board)
Rectifiers (looks like all of them) in HV rectifier board
Power Transformer (Hi impedance short)
I've never heard of a Hi impedance short, by the way! In electronics school
I was taught that a short, if anything, is zero or near zero resistance.
Anyway, it looks as if the power transformer was defective and took all
those other components with it when it bit the dust!
Since the amp was repaired (about two weeks ago), it has been operating
flawlessly. Then, last night, after making a few contacts and being on for
approximately six hours the line fuses blew AGAIN!!! The fuses blew about
two hours after I had made my last QSO on 40 meters with the amp. Like
before, the amp was on but not being used. Then, out of nowhere the line
fuses blew. I'm going to assume that, once again, I have a defective power
transformer! Looks like, Monday morning, I'll be on the phone, once again,
with Ten Tec! It's a shame because when the damn thing is working it's one
of the finest amplifiers I've ever operated, but I'm getting close to the
point where I'm ready to tell them to either replace the amp or refund my
$$$!
My question is...have any of you other Centurion owners experienced this
problem? Is there a power transformer issue with the Centurion. In other
words...is it possible that there was a run of defective power transformers
that "fell through the cracks", sort of speak? I understand Ten Tec builds
their own power transformers for the Centurion. If there is an "issue" with
those transformers I feel the problem will only happen again if the
transformer is replaced or I opt for a new Centurion.
Charlie - N5BLY
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