on 2/29/00 5:06 AM, Brian Miller at brianmiller@xtra.co.nz wrote:
> We have recently purchased a second-hand Ameritron AL-811X amplifier. It
> works fine on all bands except 10/12m where it is difficult to output more
> than 300W without turning the anode parasitic resistors into smoke!
>
> I have tried replacing the original carbon composition resistors with carbon
> film types but the problem persists. It looks like this is a design flaw.
>
> Has anyone else encountered this problem? If so did you find a solution? Any
> advice would be appreciated.
A lot could be happening. For one, Rich is correct in that the metal film
resistors are better than carbons. But forget the ballyhoo about the
nichrome. You don't need it. Trust me. Nichrome is meant for toasters and
model rocket igniters - NOT power amps.
One of the reasons for failure of parasitic resistors is the high amount of
circulating current that these receive on the higher bands. As the
frequency increases, the output capacitance of the tube becomes more and
more a significant factor as it is in parallel with the tuning cap. On 10
meters, the output C and tune C may be approximately equal or at least of
the same order of magnitude. As a result a significant portion of the
circulating current in the tank will actually flow through the tube, out the
anode and thru the parasitic suppression network. In an ideal situation,
the coil would look like a short circuit at the frequency of interest, but
in reality it and the resistor provide some sort of complex reactance. Rich
is right in that you need the correct balance of inductance to resistance.
So as the circulating current begins to flow through the network, things
heat up as power is dissipated. As the ambient temperature of a resistor
rises so does the film temperature of the resistor. The power rating of a
resistor is typically spec'd at room temperature or 25 degrees C. At some
point higher than that, the resistor will begin to derate, usually linearly
to some point where the resistor will not handle ANY power at all. The
higher the temperature, the lower the power handling capability of the
resistor.
Now obviously, Ameritron isn't able to stay in business by making amplifiers
that are not tunable on 12/10 meters to only 300 Watts. So there is likely
some other problem there that won't be cured by just the addition of metal
film resistors or nichrome. Trust me, I've gotten anode circuits hot enough
that the nichrome unsolders and the metal film resistors burn up!
I see you are from ZL land. If the amp was originally owned by a US ham,
the 10/12 meter operation was done as a modification AFTER it left the
factory. In the US, manufacturers cannot sell a 10/12 meter capable
amplifier, but it is legal to modify them to do so. Perhaps something is
wrong in the modification.
Secondly, there could be something amiss with your tuning. It could be
possible that a possible setting of the tune C could cause more circulating
current to flow through the tube's output C than you would want creating
even more heat and dissipation in the tank than it is designed for. I've
not thought it through all the way so it's just a hypothesis.
Replacing carbon type resistors with metal film types IS a good idea. One
reason why I believe carbon resistors fail after a long time in use in an
amplifier (and then a subsequent oscillation can occur) is that they are
exposed to a high temperature environment due to the conducted and radiated
heat of the tube as well heat generated due to the RF currents. High
temperature exposure of any electronic component greatly decreases the MTBF
and shortens the lifetime of the component considerably. So after a time,
the carbon resistors fail due to the temperatures involved. The metal films
will certainly hold up better.
But just replacing a component with a better component that *may* appear to
solve your problem (because you don't get an instant failure) does not
really help matters. It would be better to figure out what the problem is
that is causing the premature resistor failure (be it too much suppressor L
or whatever) than to just put in resistors that may work for a while but
eventually fail. You want to cure the wound, not just put a band aid on it.
Good luck!
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
Member: ARRL, AMSAT, DXCC, NRA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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