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 In professional equipment they have been replaced by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.
Cheers,
Tom