Hi all
I have previously posted a mail to this reflector concerning
poor efficiency of an Ameritron AL1500 amp. (8877), But after
all discussions on parasitics I decided to make some additional
measurements, to see if maybe this could be the cause to my problems.
The "poor efficient" was must pronounced on the higher bands
(20, 15 and 10m).
First let me say that I have experienced no arcs the tank circuit
in either bandswitch or capacitors.
The AL1500 uses a 8877 that runs in GG. Ameritron does NOT use any
parasitic suppressor in the anode. Rich put my attention on this
after my first mail, and I made up my own suppressor little do I know
about it, just looking how others did. I've got somewhat wiser during
the past few months after reading the numerous number of mails
concerning the topic.
Back to the measurements
I checked for resonance with a GDO coupled between the anode and DC-blocking
capacitor and found several (very sharp) from 100MHz and up!
I then made a test setup with the AL1500 driven by a TS-680 and connected
to a dummy load and used a spectrum analyzer to "sense" the RF.
Would have used a directional coupler to get exact values, but the one
available covered from 0.1-2GHz, and I was short of power attenuators.
My RF probe consisted of 10cm wire. I terminated the end of the cable
at the probe end with a 6db attenuator to reduce reflections.
I tuned up on 20m, and was I surprised. The AL1500 produced signals up to
350MHz!!! No wonder the neighbor had complained about TVI. Parasitics or
not, I don't know. The broad band spectrum only exists when the amp. is
excited. Keying the bias on/off can't produce any unwanted behavior.
And yes, I checked the TS-680 for harmonic suppression, and the signals
was definitely not coming from the driver, though it wasn't the best I've
seen.
15m and 20m are the worst bands, 10m nearly the same. 40m better
and 80m only random oscillations at high speed keying (tuning pulser).
I removed the HV connection and connection to the DC-blocking capacitor.
Inserted a little VHF ferrite suppressor between the tube and DC-blocking
capacitor. Resonance around 100-120MHz were gone however I could still
see some resonance at app. 140-150MHz using the GDO.
I then removed all connections to the anode and resonance at 140-150MHz
was still present!.
The suppressor I made was a copper strap 6mm wide 3 turns OD. of 10mm and
a 22 ohm 3-5W resistor in parallel. I know this is far off!!!.
Despite the "high-Q" suppressor, I could see that the 22 ohm resistor had
been VERY warm, however it was still 22 ohms.
Looking at the grid circuit, Ameritron have made the input filter in
a separate box away from the grid/cathode compartment. The input filter
(pi) is connected to the cathode with a coax cable. Could this cable
cause any problems since it will act as a 1/4 wave shorted stub at VHF!
Now I would like to cure this problem. I need some directions to design
the suppressor for the anode and to put some damping resistors in the
grid/cathode. Would also appreciate comments from owners of AL1500.
73 de Soeren, OZ1FTU
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