I have to disagree with you on that point Jon. I witnessed first hand, VHF
spurs that were being generated by my friend's homebrew 2 X 813 amp.They
were horrendous and if the FCC had been monitoring, he probably might be
getting his ticket back today! After reading about the improvements for the
SB-220 in QST, (we all tend to suffer from "League Fatigue!"), he installed
the aforementioned Nichrome wire parasitic chokes and the spurs were
eliminated, (not tamed, or reduced but eliminated!!).
Say what you want, but actions speak louder than words, (or spurs!).
Cheers,
Mike(y)
W3SLK
----- Original Message -----
From: Jon Ogden <jono@enteract.com>
To: Brian Miller <brianmiller@xtra.co.nz>; <amps@contesting.com>
Sent: Tuesday, February 29, 2000 8:07 PM
Subject: Re: [AMPS] FAILURE OF PARASITIC RESISTORS IN AL-811X AMPLIFIER
>
> 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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>
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