All of this smells of: (a) excessive grid current, (b) gassey tubes.
GRID current is THE MOST IMPORTANT current to monitor on grounded grid
triode amplifiers such as the Centurion, SB-220, TL-922 and other triode
grounded grid amplifiers. I find that excessive drive, improper
tuning, and improper loading is the cause for excessive grid current.
The 3-500's have a very high vacuum and are prone to have pin-to-glass
seal leaks even when sitting on the shelf. To de-gas these tubes, the
only and correct way, is to run them with the anode showing red color.
The anode is coated with a material, when heated to the proper color,
absorbs the gas. Incorrectly, as many hams believe, leaving the
filaments on WILL NOT provide any benefit and WILL NOT serve to de-gas
these tubes. This applies only to direct heated cathodes such as the
3-500's, 872's and other like tubes. Tubes which have an indirectly
heated cathode, i.e. 4CX250's 4CX1000's, 4CX1500's and such are quite
different in this regard.
As to correct tuning..........many believe they first must dip the
plate. { I reserve dipping for those that chew tobacco. } Dipping the
plate is correct for a Class C amplifier. Linear amplifiers are Class
AB1 or Class AB2.
With a linear AB class amplifier the better way is to adjust the TUNE or
Plate control for maximum output as observed on the power meter, AND
observe a slight increase in grid current. This is due to the input
drive power, which does appear in the output power, as the input is in
phase with the output and thus is additive. When grid current peaks
this assures the path from the input to the output is in resonance.
Second step is to increase drive power to the point of rated grid
current or about 250 ma to 270 ma for a pair of 3-500's. Then adjust
LOAD control to reduce the grid current to a minimum value. Increase
the drive power a bit more until grid current is about 250 ma. Then
adjust LOAD control to reduce the grid current to a minimum value.
Until you are familiar with the process this may take 2 or 3 times.
The objective is to arrive at a point where grid current is at maximum
rated current with the correct amount of drive power and the LOAD
control does not reduce grid current when increased loading is applied.
You'll notice I have not suggested adjusting the TUNE or PLATE control,
other than at the initial adjustment. The TUNE was correctly set during
the first step where one tunes for maximum output AND a slight peak in
grid current. The amplifier is now correctly tuned and loaded and the
drive is at the correct and maximum level. Never exceed this drive
power level!!!, nor exceed the recommended maximum grid current
value!!!!. In all of this, I've not mentioned plate current. In fact,
one can correctly tune and load the amplifier with the plate current
meter covered up. Plate current is not important, unless you need to
determine DC input power. Volts X Amps = DC input power.
A very IMPORTANT point. If one wishes to operate at less than rated
and/or maximum output, after adjusting for maximum output, then
correctly one should reduce the drive power and DO NOT touch the TUNE or
LOAD controls. Thus it was correctly tuned for maximum output and
maximum efficiency. This is the way the amplifier was designed to operate.
I've used this process with my Centurion of 30+ years old with original
tubes and my later Ameritron AL-80B with a single 3-500.
These amplifiers are designed to operate into a load exhibiting a 3:1
SWR or less. Excessive SWR can or will likely cause a flash over of the
plate tuning capacitor or the band switch. Either can be very
destructive. If your antenna SWR exceeds 3:1, then fix your antenna.
It will likely save your amplifier. Likewise with antenna tuners, they
should be adjusted at 100 watts or less and usually with the amplifier
in STANDBY mode. Never adjust the ATU with the amplifier producing
output.
Using the correct procedure, and not believing old ham lore, will save
your amp and your tubes and produce a clean signal.
73
Bob, K4TAX
On 2/12/2021 8:29 AM, W3HKK@roadrunner.com wrote:
This may or may not help, but here's my story. My 23 yo Centurion
worked like a charm for 5 years after I bought it used, 10 yrs ago.
Then one day the grid started going negative. I had a local tech look
at it...twice. The first time we changed out the original tubes. A few
months later a similar problem began and he repaired/replaced the bias
transistor...and I believe a grid resistor. (The amp warmed up ok then
the grid meter would slowly begin to crawl its way negative as the amp
idled.) But because he was a good tech with good prices, but took 3-4
months to fit ham gear in between his normal police/fire department
repair work, I bought a new ACOM1000 for my birthday, so as not not to
miss a couple of big contests.
FWIW the new amp had problems a year later with reduced output. Turns
out the finals in it were defective. The factory rep replaced the
finals and sent it back. Soon it began tripping out the safety
circuit. Sent it back in and he replaced the finals he had put in at
no cost. Since then Ive had full output and all is well. My point:
Finals have caused 3 of my 4 amp problems - two on the Centurion and 2
on the ACOM. Apparently it is not unusual for the ACOM1000 finals to
be damaged in shipment...even to or from their tech repair center.!)
Meanwhile I eventually got the Centurion amp back, working well. It is
now my backup.
I will say this. Not many amps are built better, are more attractive,
and have smoother tuning than the Centurion. Had it not been for those
two issues and the slow local repair service, Id still be using it
today. Had no other complaints.
-----------------------------------------From: "Dave’s"
To: tentec@contesting.com
Cc:
Sent: Friday February 12 2021 8:44:43AM
Subject: [TenTec] Centurion Amplifier draws negative grid current-help
requested
Hi all,
My Centurion amp is drawing negative grid current and has no power
output. I have checked the power transformer, diode string and filter
caps and verified with an external VOM that I have 3000vdc on the
plates. I have also changed the tubes, no improvement.
Any thoughts as to the root cause? I had replaced the pesky bias
transistor 5 years ago when the unit would not key in transmit. The
symptoms are different now, since the unit will go into transmit when
keyed by my xcvr.
Appreciate any thoughts. My Centurion is 27 years old.
Thanks and 73,
Dave, K2DP
Sent from my iPad
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