Hi Frank,

Nice surplus components site , sadly no credit card payments, hence only
for local (USA) customers.

I'm in Canada and we deal all the time :-)

He accepts money orders and CC through PayPal.

Cheers,
__
Gregg

K7ITM wrote:

Adding to Ian's good words, the larger resistors (1W, 2W and sometimes
more) are commonly metal-oxide. They also work well, and have the same
few-turn spiral structure that the metal film ones do.

I should have made it clearer that metal-oxide resistors exist, and are
generally similar to metal-film in their construction and low
inductance.

However, we can only say "generally". If the very lowest inductance is
important, you need to remove the coating and check for yourself.

Several years ago I measured a little dummy load I made from four 200
ohm 2W metal-oxide parts in parallel. The construction was "tight" so
that the leads didn't contribute more inductance than the resistors
themselves. I don't have the numbers with me at the moment, but I
recall a return loss measurement equivalent to about 1.1 or 1.15:1 at
150MHz, and around 1.5:1 at 450MHz.

The metal-oxide resistors have another interesting characteristic. They
can dissipate enough power to glow red and do a very respectable job
maintaining their resistance value--though expect some change if you
let them get that hot.
[Moved]
It's very unlikely that a carbon composition resistor, or a carbon
film, or even a normal metal film, will be able to hold its value as
well if it gets that hot.


Metal film resistors do seem to behave well in similar tests. Like Tom
(and Tim in another message) I have deliberately overloaded samples by a
factor of 10 to see what would happen. From glowing bright red, the
resistance returned to a few percent of the original value.

In parallel tests, modern carbon film resistors simply burned up - which
is exactly what you would expect carbon to do. I didn't waste time on
carbon comp, as any OT already knows they cannot tolerate even moderate
overloads without oozing organic binder materials, accompanied by smoke,
an awful smell and a large, permanent increase in resistance.

The other low-inductance power resistors that haven't been mentioned are
the flat-film types, designed to be bolted down to a heatsink. There's a
nice constructional example of a large RF power attenuator using these
devices at:
http://granta.digital-crocus.com/Attenuator.php3

What surprises me is that carbon comp resistors are still available as
specialist items because of their claimed performance in pulsed
applications. With so many alternatives available, that have almost
equally low inductance and vastly superior power handling, it would be
interesting to learn what the specific advantages of carbon comp might
be?


That's a double-edged sword. On the one hand, it's nice to know they
will be pretty stable, but if you mount one on a circuit board, you
need to make sure that it won't dissipate too much power, because it's
quite capable of burning a hole in the board.

These modern power resistors are also much smaller than traditional
types... but this too can be double-edged. I sell a kit that includes a
number of 1W, 2W and 3W metal film resistors, and occasionally receive
e-mails saying: "There are several small resistors left over - and where
are all the power resistors?"


--

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

Ian White GM3SEK wrote:
I have deliberately overloaded samples by a factor of 10 to see what
would happen. From glowing bright red, the resistance returned to a few
percent of the original value.

Sorry - that should say: From glowing bright red, the resistance
returned to WITHIN a few percent of the original value.


--

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

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

Nice surplus components site , sadly no credit card payments, hence only
for local (USA) customers.

I'm in Canada and we deal all the time :-)

He accepts money orders and CC through PayPal.
=================
Tnx Gregg , that's good to know.

Frank GM0CSZ / KN6WH

"K7ITM" wrote in message
ups.com...
On Aug 27, 11:17 am, K7ITM wrote:
On Aug 27, 2:09 am, Ian White GM3SEK wrote:



George Kavanagh wrote:
I'm in the process of removing parasitics in the final of a 6DQ5 75W
transmitter (1963 ARRL HB, pg 176), and am in need of source for 2+
watt non-inductive reistors for parasitic traps in grid & plate
circuits. Sources for higher wattage carbon composition resistors seem
to have dried up. Can ceramic composition resistors, such as Ohmite's
OX/OY series be used? They are touted as "non-inductive". Please
advise.

This one keeps coming around...

"Inductive" is not a yes/no quantity. Any component has stray
inductance, so you always have to ask how much.

Carbon composition resistors never were completely "non-inductive"...
except by comparison with wirewound resistors. That claim was always a
lie, and it shouldn't scare you away from more modern resistors where
the manufacturers are being more honest about how much (or how little)
inductance there really is.

There's a very good reason why carbon composition resistors are
becoming
hard to find. In professional equipment they have been replaced by
metal-film resistors, which are better in almost every way. MF
resistors
have better stability of the resistance value, and better thermal
performance in a smaller package, because the resistive element is on
the outside where the heat can get away more easily. MF resistors also
have vastly better tolerance of high operating temperatures - which are
guaranteed to exist at the anode cap of a transmitting tube.

The bugaboo about inductance will go away if you look hard at it.
Instead of running scared when you hear the word "inductive", find out
how much inductance there really is, and how much it's really going to
matter.

Scrape off the coating of a typical wire-ended MF resistor, and you
will
see that the grey metal film has a slow spiral groove cut into it - in
effect, the resistive element is a few spiral turns of flat ribbon. Do
the same for a range of resistance values, and you'll find that the
pitch of the spiral and number of turns will vary from one resistor to
the next. The more turns there are, the longer and narrower the ribbon
becomes, so the higher the resistance will be.

But it is quite rare to find more than about 10 turns, because the
manufacturing process becomes too difficult to control accurately. For
the next higher resistance value, the manufacturer will step up to a
higher-resistivity base material, and drop back to the lowest number of
turns. Then the whole cycle of gradually increasing number of turns can
repeat up to the next break-point. Incidentally, this also means that
even some very high resistance values can also have a very low
inductance.

There is no universal way to predict which resistance values will have
the lowest inductance, because the break-points between about 10 turns
and about 1.5 turns will be different from one manufacturer to the
next.
If you really want to find out, you have to scrape off the coating and
look for yourself.

If you do that, then measure the dimensions of these little 'coils',
count the numbers of turns, and plug the values into the standard
formula for inductance. You will find that typical values of inductance
for small MF resistors are only a few tens of nanohenries - in fact,
not
much more than the inductance of the wire leads! The values are in
exactly the same ballpark as carbon composition resistors. (I have
verified this by direct measurements with a network analyser; and it
wasn't easy, because the parasitic inductance values genuinely are so
small.)

So now you have to ask: will a few tens of nanohenries matter in my
circuit? At all frequencies up to about 100MHz, the answer is almost
invariably NO.

(The only exceptions are when you're trying to make a resistance
standard for use in measurements at high frequencies. However, you can
make an excellent low-VSWR dummy load by connecting a large number of
small MF resistors in parallel, as the parallel connection reduces the
effect of the stray inductance.)

For all of these reasons, most makers of big power amplifiers have
moved
to MF resistors for VHF parasitic suppressors - typically a bundle of
2-3 3W resistors in parallel. The critical factor for power dissipation
is the RF heating from normal operation at 24-28MHz, where a small
fraction of the RF power will be lost in the resistor.

With your baby 6DQ5, you can use 0.25W MF resistors whose inductance
will be tiny.

When you're building retro equipment from the old handbooks, wherever
you see 'carbon composition', remember that there weren't any other
choices back then. If the ODGs who wrote those books were still around
today, they'd all be using metal film.

--

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

Adding to Ian's good words, the larger resistors (1W, 2W and sometimes
more) are commonly metal-oxide. They also work well, and have the
same few-turn spiral structure that the metal film ones do. Several
years ago I measured a little dummy load I made from four 200 ohm 2W
metal-oxide parts in parallel. The construction was "tight" so that
the leads didn't contribute more inductance than the resistors
themselves. I don't have the numbers with me at the moment, but I
recall a return loss measurement equivalent to about 1.1 or 1.15:1 at
150MHz, and around 1.5:1 at 450MHz.

The metal-oxide resistors have another interesting characteristic.
They can dissipate enough power to glow red and do a very respectable
job maintaining their resistance value--though expect some change if
you let them get that hot. That's a double-edged sword. On the one
hand, it's nice to know they will be pretty stable, but if you mount
one on a circuit board, you need to make sure that it won't dissipate
too much power, because it's quite capable of burning a hole in the
board. It's very unlikely that a carbon composition resistor, or a
carbon film, or even a normal metal film, will be able to hold its
value as well if it gets that hot.

Cheers,
Tom

In addition, about carbon comps... I had been saving them for years,
and about three years ago now I went through my whole stash, from 1/4
watt (and even a few 1/10-1/8 watt) to 5 watt monsters, measuring them
all--a few thousand of them. (One might ask why I bothered, given the
results...) OVER HALF were out of tolerance, many by a LOT. Almost
all were high, but the occasional one was low. It wasn't uncommon to
measure them at twice the marked value and more. It didn't seem to
matter if it was parts salvaged from equipment or unused ones. It
didn't seem to matter what brand they were; I could recognize that
from the appearance (mainly IRC and Ohmite and some British ones from
a manufacturer whose name I've forgotten). I've saved a few of the in-
tolerance ones of particularly interesting values, but mostly they
were tossed in the trash (out-of-tolerance) and given away (in-
tolerance). All in all, they were pretty terrible parts by today's
standards. I wouldn't even think of designing one into a production
piece of equipment.


Even the milspec 10% INITIAL tolerance ones could drift something like 43%
over life.