Jon, you stated previously that you were in the resistor business.
Why don't you provide a mini lecture on the inductance in helical wound
"metal film" resistors of the type that are probably going to be
encountered in the 5-10 Watt class on RF suppressors?
Here in the third world, even 1/4 Watt "metal film" resistors turn out
to be helical wound jobs, readily proven by burning the paint off them
and inspecting the remains...
Surely if the function of a resistor is to emulate R only, then inherent
inductance must be kept low, and carbon composition types are the
preferred option?
Another thing that worries me about this parasitics debate:
1. If certain tubes have a tendency to take off at low VHF freqs, ~110
MHz, what about 28 x 4 = 112 MHz, 21 x 5 = 105 MHz, 14 x 8 = 112 MHz,
etc? The point is, if there is a tendency to oscillation out of band in
a linear, a sniff of unwanted harmonics would drive it over the edge.
This could explain why there are thousands of unmodified, stock SB 220's
out there, which have never hiccupped for years - the drive has low
harmonics.
2. Another question, does the input circuit on a linear such as the SB
220 have sufficient low pass attenuation to prevent direct coupling into
the tube? Most input circuits are tuned for max power output, minimum
SWR as seen by the exciter, with nary a thought given to harmonic
attenuation, anyway. Never mind RF feeding into the input from poor
local conditions or high SWR on the output line and so on.
I'm asking these questions following the expertise you have built up on
the home-brew amp.
Cheers,
Ian ZS6BTE
Jon Ogden wrote:
>
> 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.
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!
>
>
> 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.
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