[TenTec] Centurion Amplifier draws negative grid current-help requested

Fri Feb 12 10:45:26 EST 2021

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 at 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 at 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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