On Sun, 2006-10-08 at 17:42 -0400, Carter, K8VT wrote:
> RB wrote:
> > 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.
>
> I believe he said it was a NEW amplifier...although even "new" tubes can
> develop a short.
>
> > It could be accelerated by higher than 5.0 filament voltage
>
> This could be a possibility. I am the second owner of a ca. 1994
> Centurion and my filament voltage was above 5.0 volts with "normal" line
> voltage. My best guess was the filament windings should have had a turn
> or two removed. As I was the second owner and did not want to invest in
> a new transformer (with no assurances that the new one would be any
> better), I modified the rig by putting a resistor in series. This was
> done 8 or 9 years ago and I still have my notes (but not right in front
> of me) and it has been working well for all those years.
That resistor also helps limit inrush current.
Measuring 5 volts AC accurately to a couple percent is not a trivial
problem, many multimeters have poor AC accuracy that gets worse at low
voltages. To make things worse there is considerable sensitivity to
waveform distortion in the meters which often read peak and display RMS
based on RMS being 70.7% of peak. Then a half volt or so of rectifier
drop in the meter rectifier which can be a volt or more with a bridge
rectifier and is more than a little dependent on current gets in the way
of precise AC voltage measurement.
One test for PA tubes is to lower the filament voltage until the peak
power output is limited by filament emission, then raise that filament
voltage until the peak power output isn't limited. That optimum voltage
will probably changer with tube life and there is no guarantee that a
pair of "matched for gain" tubes will track for filament emission.
Running a cathode type tube at lowered temperature is not so successful.
<SNIP>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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