Tim Shoppa wrote:

When I've purposefully burnt low-ohm metal oxides (it's surprisingly
hard - the resistor literally has to glow red hot for it to happen!)
it is clear that while there is a spiral winding, that it's quite wide
and only a turn or two for the resistors in the low ohm range and that
the turns are extremely wide and fat. They could very well be lower
inductance than the original carbon comps.

In fact the original choke fire that took out the original parasitic
suppressors probably resulted from parasitic oscillation in the finals
(I always assumed so - drawing a couple hundred mA with no input, and a
bright blue glow from the final compartment, I always thought must be
some sort of parasitic!)


Parasitic oscillations are the subject of perpetual Holy Wars among
certain amplifier builders... but both sides do agree about carbon
composition resistors.

The resistors used in parasitic suppressors have to operate in a very
hot environment, and they are also subject to heating by the RF current
passing through them (especially on the higher HF bands). With carbon
composition, the resistance is virtually certain to increase over time.
This makes the parasitic suppressors less effective than they were when
the amplifier was new, so there is some small risk that parasitic
oscillations may reappear in older amplifiers.

If this happens, both sides agree that the original carbon comp
resistors should always be replaced by metal film or metal oxide
resistors - or preferably by a small bundle connected in parallel to
reduce the inductance. (However, if the amplifier is still stable, it's
usually better to leave well-enough alone until something happens, or
until the next major overhaul.)

The same applies to the much higher-value resistors that are connected
in parallel with the electrolytic capacitors in the power supply. These
resistors are intended to equalize the voltages across the capacitors,
but many old amplifiers used under-sized carbon composition resistors.
Once again, these are virtually certain to have increased in value by an
unpredictable amount, and instead of equalizing the voltages, some of
them may now be having the opposite effect! Replacement with 3W metal
film or metal oxide resistors is strongly advised (probably of a lower
value than original, for improved voltage equalization).

Once the carbon composition resistors have been replaced by metal film
or metal oxide, the problems of resistance change will be gone for good.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek