Its sounds like the amp is marginally stable, and the input signal
is pumping the oscillation. I have seen this kind of behavior in a
marginally stable solid state IF amplifier strip that one of our
contracters at work put together. The amp would produce a
normal looking set of harmonics at low RF inputs levels. At
some point as the RF input level was raised, the second
harmonic would suddenly jump in level and get really big. My
guess is that IF strip was breaking into oscillation at this point,
and injection locking to the input signal. Sounds like this
AL-1500 is doing something similar. I don't get the 1.5
relationship however. I would expect an integer relationship
between the drive signal and the injection locked oscillation.
Have you checked how far you can pull the 15 meter spur
with the 14 MHz input? Does the 15 meter signal move exactly
1.5 times the amount of the chang in the frequency of the 20
meters signal (e.g. if you move the 20 meter transmit frequency
1 KHz, does the 15 meter spur move exactly 1.5 KHz)?
The 15 meter key clicks would be consistent with a pumped
instability which appears rapidly at some threshold point of the
input signal. The oscillation would in effect, square up the
shaped enveloped of the input signal.
BTW, are you sure the oscillation is occuring in the amplifier
and not the exciter? Have you tried driving the AL-1500
with a different exciter (the load impedance seen by the
exciter will vary with drive level applied to the amp). Also,
you mentioned antenna spacing. Do you get the same results
driving a dummy load with the AL-1500?
Good luck - it sounds like an annoying, but interesting RF
problem.
73 de Mike, W4EF..................
----- Original Message -----
From: "Richard" <2@mail.vcnet.com>
To: "David Robbins" <k1ttt@arrl.net>; "Amps" <amps@contesting.com>
Sent: Monday, May 13, 2002 8:01 PM
Subject: Re: [Amps] al-1500 parasitic?? or some other problem?
>
>
> >This problem has developed sometime in the last 6 months or so, but has
> >taken some time to pin down as it seemed intermittent and was being
> >blamed on other things. It did not exist when I first installed the
> >amp.
> >
> >First, the amp is on the 20m position of my station, there is an ice
> >bandpass filter between it and the ft-1000mp exciter. There is a pair
> >of stubs after it. I have bypassed the stubs with no change in the
> >symptoms.
> >
> >What I am seeing is this... with the radio set for 14024.5 I tune the
> >amp and all appears normal. Except when going above about 800w a strong
> >spur suddenly appears on 21036.9, almost exactly 1.5 times the
> >transmitter frequency. The spur can be moved by tuning the 20m radio.
> >It is not moved by adjusting the tune or load controls on the amp. With
> >the amp tuned to 1500w output on 20m the spur can be turned on and off
> >with fairly small adjustments of the amp tune control (5-10 degrees
> >max), it does not change over fairly large adjustments of the amp load
> >control. The onset point of the spur is not stable, sometimes it
> >doesn't start until the amp is up to almost full power, sometimes when
> >turning the tune control the start and stop points are not the same.
> >But once started the spur is very stable and its frequency is not
> >affected by the tune or load controls, not what I would expect from a
> >parasitic. When keying the radio with CW it creates large clicks on
> >15m, probably because of some instability when suddenly increasing
> >through 800w. There is also no discernable change in grid current when
> >the spur starts and stops as I would expect from a parasitic.
> >
> >Maybe unrelated.. in one test I had my 6m radio turned on and when I was
> >turning the spur on and off by adjusting the amp tune control. When the
> >spur stopped on 15m I could hear it sweep through 6m, with some practice
> >I could tune it to the 6m frequency using the amp tune control. This is
> >more what I would expect of a parasitic. This was very weak and
> >probably only detected because the 6m antenna is only about 5' above the
> >20m one.
> >
> >Any ideas where to start with this one???
> >
> ? The AL-1500 is the only mfg 8877 MF/HF amp on this planet that does
> not use a parasitic oscillation suppressor. A friend modeled the 8877 on
> a computer program for determining amp stability. In a typical HF layout
> with c. 120nH between the anode and C-tune, the program predicted
> marginal stability above 100MHz. My guess is that this has something to
> do wth the 8877's high VHF gain. An autopsied 8877 from an AL-1500 is
> shown in figure 24 on my Web site. This tube was damaged by
> gold-sputtering from the gold plating on the grid. Although it doesn't
> show up in the photo, most of the gold loss took place at the grounded
> end of the grid cage. To me, this suggests that the parasitic was in the
> UHF region. We sell a parasitic suppressor retrofit-kit for 8877
> amplifiers. In the interest of R and D, I will send you a retrofit-kit
> gratis if you will install it and report back to the group.
> >
> cheers, David.
>
> - R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
> www.vcnet.com/measures.
> end
>
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