[Jukebox-list] Speaker ground
The Jukebox Junkyard
jukeboxjunkyard at cox.net
Fri Oct 6 07:12:36 PDT 2006
Dave, on tube types, without speakers, the windings may break down because of the excessive high voltage developed across the primary winding that should be reduced to a safe level with a load. Wes
----- Original Message -----
From: "Dave Halford" <Dave.Halford at telent.com>
To: "Jukebox mailing list" <jukebox-list at lists.netlojix.com>
Sent: Friday, October 06, 2006 2:58 AM
Subject: Re: [Jukebox-list] Speaker ground
>
>
>
> Never understood the problem with valves OC though.>
> regards>
> Dave H
>
>
>
> Jay Hennigan <jay at west.net>
> Sent by: To: Jukebox mailing list <jukebox-list at lists.netlojix.com>
> jukebox-list-bounces at lists.n cc:
> etlojix.com Subject: Re: [Jukebox-list] Speaker ground
>
>
> 06/10/2006 06:31
> Please respond to Jukebox
> mailing list
>
>
>
>
>
>
> 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
> NetLojix Communications, Inc. - http://www.netlojix.com/
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