Thanks, Don!
The Globe uses 2 Amperex AX9909/6083 pentodes in parallel for the PA. These
tubes use indirectly heated cathodes. I have no idea how many hours are on
them, but I know that Dad has a replacement pair. However, I think that
indirectly heated cathodes had longer lifetimes than directly heated
cathodes, as in the 809's used in push-pull for the modulators. The 809's
are also original and seem to have plenty of emission. I have no idea what
the expected lifetime of their thoriated tungsten filament is, but if it's
anything like the filament of a 3-500Z, it's something like 20,000 to
30,000 hours. The downward swings are probably around 5% of the total I_p
(about 10-15 mA).
I suspect that the rig has always done this. The B+ supply is common to
both the modulators and the PA. Properly tuned, the I_p for the PA is set
for 300 to 325 mA and the peak current for the modulators is 200 mA. So,
peak demand from the B+ supply is at least 500 mA. It's a full-wave CT PS
that uses two 3B28 rectifiers producing about 900 vDC. The B+ supply uses a
choke-input filter; Dad has replaced the original 2 uF oil capacitor with
an electrolytic string providing about 22 uF of filtering. The transformer
isn't all that beefy, so a sagging B+ supply may well be exactly what we're
seeing. The 120 VAC mains seem to be pretty stiff (no dimming lights), so
I'd suspect the B+ supply first, tube emission second. I've also learned
that AX9909/6083 tubes may well be unobtainable, so we'll take good care of
what we have!
Aside: we couldn't find a set of 866A's that didn't develop arcing
problems, acting as if they were poisoned. I'd dearly love to see the glow
of those mercury vapor rectifiers again!
Next time I'm there, we'll stick a scope probe in the air and take a look
at the output wave form, just to make sure nothing obnoxious is going on.
Thanks for your advice and suggestions!
73,
Kim Elmore, N5OP
At 09:11 PM 2/25/2003 +0000, you wrote:
From: "Kim Elmore" <[EMAIL PROTECTED]>
I want to understand why we're seeing the negative deflections under
modulation. I'd expect positive deflections, not negative ones. What is
this telling me about the transmitter's operation?
That is not at all unusual, even though theoretically the plate meter
should stand completely still during modulation. The most likely cause,
if the rig uses a common power supply for modulator and final, is that
modulator plate current is dragging down the plate voltage. Sometimes,
even poor regulation at the 117 v. mains can cause the problem. The other
likely cause is nonlinearity in the modulated stage. The final tube may
be a little low on emission. Also, with screen grid tubes, the screen has
to be modulated along with the plate. If this modulation is not 100%
optimum, some nonlinearity will result, and the most common method of
screen modulation, thru a dropping resistor off the modulated plate
voltage, is rarely optimum. Sometimes the screen bypass caps will reduce
the screen modulation at higher frequencies so that it doesn't follow
exactly the modulation of the plate.
I wouldn't worry about it unless the downward swing in plate current
exceeds 5% or so of the total plate voltage.
If you have a monitor scope, look at the envelope pattern to see if
everything looks ok. Better still, try a RF trapezoid pattern (this
requires going into the transmitter and sampling audio from the
modulator), so you might not want to bother with it. The trapezoid will
give the ultimate indication if there is any nonlinearity, but the
envelope pattern is sufficient to tell if anything is wrong, such as
flat-topping or severe waveform distortion.
73, Don K4KYV
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Kim Elmore, Ph.D.
University of Oklahoma
Cooperative Institute for Mesoscale Meteorological Studies
"All of weather is divided into three parts: Yes, No, and Maybe. The
greatest of these is Maybe" The original Latin appears to be garbled.
