[Jukebox-list] Speaker ground

[Jay Hennigan]

Bill Taylor wrote:
> Hey Guys,
> 
> Hey, with my wurlitzer 1900, when the speaker plug is
> plugged to the amp, the secondary winding of the
> transformer T1 for high voltage uses ground thru the
> plug for the center tap of the winding.
> 
> Ok, obviously they don't want you to start the high
> voltage without the speakers connected to the output
> amp. Ok, I understand that now, but... why?  What is
> the problem without the speakers connected?  When I
> work on my amp I use it on my bench.  I thought I
> could just jump the ground to start the voltage
> without the speakers.  What happens without using the
> speakers?

Tube type amplifiers don't like being run without a load.
Voltages can rise above safe limits.  For basic testing without
cranking it up, a clip lead to chassis will serve just fine.

The speaker plug jumper to enable high voltage is more critical
with field-coil speakers.  These typically had a 5,000 ohm or so
winding between HV and ground to serve as an electromagnet to
work against the voice coil.  Without the load of the field coil,
the HV would rise above safe limits.  So the manufacturers ensured
that the HV would be disabled unless the field coil was plugged
in.

Solid state amps, on the other hand, can be damaged by being run
into a short or too low resistance but generally aren't affected
by being run into an open circuit.

> And, if that is something I shouldn't do, then my juke
> has plenty of speakers.  On the bench, I can't put all
> the speakers on the bench?  What can I do instead to
> work on the amp?  Will just one small 8 ohm work?

Yes, but it can get kind of loud.  Parts Express sells 8-ohm
audio dummy loads for this purpose.  You can then connect a
small speaker in series with a 50 to 100-ohm resistor across
the dummy load in order to monitor the output without blowing
out your ears or the small speaker.


-- 
Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
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