[Hallicrafters] hum and impedence

[Glen Zook]

The headphone jack on the older receivers is a
different connection than the speaker.  Also, if the
output is transformer coupled like in the SX-101
series (and every other tube type receiver that I know
of) you can "play around" a lot more with the
impedance of the load.

Most older receivers not only have a high impedance
output for the headphones, but they also have some
sort of protection (like a series resistor) that adds
protection agains a low impedance load for the
headphones.  Most of the older receivers also have a
3.2 ohm output and not the 8 ohm output of the more
modern equipment.  However, you can use an 8 ohm
speaker without any problems on these older receivers.
 You just can't get as much volume.  But, virtually
all of the older receivers have gain to spare and thus
can "drive the pants off" of an 8 ohm speaker.  It is
just that you can get even more volume from a 3.2 ohm
speaker.

Also, many of the receivers from the 1950s have a 500
ohm output as well as the 3.2 ohm output.  Often, but
not always, the headphones come off of this "winding"
on the audio transformer.

In some of the "hi fi" amplifiers the output is not
via transformer but is a "direct" connection to the
audio output transistors.  Since the speakers
themselves are part of the audio output circuit the
impedance of these can cause problems.  However, most
of the time going higher in impedance doesn't usually
cause problems.  But, going lower can certainly cause
all sorts of problems.  There are exceptions, of
course, and some amplifiers must have pretty close to
the "proper" impedance to work correctly.

In addition, tubes are much more "forgiving" than
transistors.  Tubes can take all sorts of "abuse" and
not be hurt at all whereas transistors can be damaged
very quickly (sometimes in micro-seconds) if
everything is not just "right".

Thus, with a receiver like the SX-101 series you
aren't going to do it any harm by "playing around"
with the output impedances.

Glen, K9STH


--- jeff <jeffv at op.net> wrote:

I was always told that if you raise the impedence,
it's bad for tubes.
 
i.e. If I were to plug a 32ohm load into my guitar
amplifier's 16ohm output, it would do bad things.  If
I had the shocking lack of taste to own an amplifier
with [gasp] transistors, a lower impedence would be
bad.


=====
Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco

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