>The AL1500 uses a 8877 that runs in GG. Ameritron does NOT use any
>parasitic suppressor in the anode.
Ouch! That would worry me!
>I checked for resonance with a GDO coupled between the anode and DC-blocking
>capacitor and found several (very sharp) from 100MHz and up!
Not surprising. The parasitic inductances and capacitances inside the
tube and in the rest of your circuit will cause this to happen. As Rich
says, you can't eliminate it but you can reduce the Q of the resonance.
>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.
Does the output of the amp have a low pass filter? How hard are you
driving the amp? Are you getting these spurious signals with SSB or CW?
The reason I ask is that if the amp is being driven non-linearly, you
will produce TONS of crap where you don't want it. Intermod is the
technical name. Any signal also has harmonics and the amplifier will
increase those too. The LPF will knock those down to size. However, I
would worry about the amp putting that stuff out anyhow.
If you get the crap using an SSB signal while driving the amp hard, I'd
suspect that what you are seeing is intermod. However, if you get the
crap from a single CW signal, it won't be intermod but possibly harmonics
or a dreaded oscillation.
I doubt though that harmonics would be that big, so I would rule those
out.
If the amp is oscillating that could explain your efficiency problems
since you are generating a lot of unwanted energy.
>
>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.
>
I would suspect that 3 turns of the strap is too much inductance. I
would guess that with that you would get somewhere around 100 nH of
inductance which is probably too much. IMHO, I would want to get around
50 nH or so.
Also, I would use a larger resistor value: between 50 and 75 Ohms or so
(100 Ohms max). And I would use a metalized film resistor instead of
carbon composition. Also, I'd prefer the 5 Watt variety.
>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!
It could. As Rich said, there is a way to swamp this out. However, I
think the biggest contributor to an oscillatory condition is quite likely
the anode circuit.
Good luck!
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@webspun.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|