[Hallicrafters] What kind of resistors?

[Bill Gerhold]

Your are misinterpreting the ratings.  Resistors do not have a "Overload"
rating unless referring to the wattage rating.  However,they do have a
maximum working voltage and what that means is the voltage drop when
measured across the resistor body.

When sufficient voltage drop occurs across the resistor body, there is a
danger of flashover from one end of the resistor body to the other.  In a
typical radio circuit, this is normally not a problem.

In HV circuits however, it posses a problem.  For instance.  If I needed a
50 megohm resistance to drop 50,000 volts to read it on a 50 microamp meter,
I would not want to use a single 50 megohm resistor simply because most of
the 50,000 volts would appear across the resistor body and flash over its
length.

However, now place 50 1 megohm resistors in series for the same voltage
drop, you now have only about 1,000 volts dropped across each resistor body
and no change of flash over.

K2WH

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Bill Barfield
Sent: Sunday, February 05, 2006 8:31 AM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] What kind of resistors?

I have a couple of old units to restore that 600 to 700 volts running around
in them and I may need to replace a few resistors. I have no trouble finding
resistors with the correct wattage and resistance.  But I want to be sure
that they could handle the working voltage. I noticed some talk on the list
concerning a resistor's maximum rated voltage. Does it really matter that
much?  If so, what type of resistor do I use?

I always assumed that carbon composition resistors were a good catch all,
unless you needed really tight tolerances or high temperatures. But the
carbon composition units from AES are only rated at 350v working voltage.
They can handle 700v in overload. But 350v is kind of puny. Metal film
resistors and wirewound resistors are also 350v working and 700v overload.
Carbon film ristors are 500v working and 1000v overload. Metal oxide power
resistors only have 350v working and 600v overload.

One good option might be the resistor kits sold by AES.  A kit of 1 watt
carbon film resistors are 500v working and 1000v overload. That is still
short of the 700v working voltage in many tube transmitters, but certainly
better than 350v.

What did folks use back in the "Good Old Days"? What do I use now?

Bill Barfield
KD4AL


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