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Old 23rd May 2014, 01:11 PM   #1
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Default Rockola juke box amp - unusual features

I'm helping a friend to restore an old Rockola juke box amp. The schematic (attached) is mostly straightforward, but has a couple features which have got me puzzled.
There is a relay which seems to apply 10 volts to the heaters at start up and then, when it is energized when current flows through the cathode circuit of the 6L6s, switches the heaters down to 6.3 volts. (Seems crazy! Am I misunderstanding something here?)
There is a speaker plug which prevents the amp being energized if the speaker is unplugged (very sensible).
The same plug also seems to place a 5K 40W resistor across the DC rail when the speaker is plugged in. Could this be a substitute for the load
incurred by a speaker field coil in an alternative (or earlier) implementation?
Any comments appreciated.
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File Type: jpg Rockola Amp Schematic.jpg (964.2 KB, 127 views)
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Old 23rd May 2014, 02:17 PM   #2
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The 10v with the relay is for ''quick start'', no one wants to wait for tunes!
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Old 23rd May 2014, 05:12 PM   #3
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Thanks. That makes perfect sense. I wonder if it shortened the valve life?

The more I think about the PM (permanent magnet) speaker plug switching in a 5K resistor, I am coming to the conclusion that it is for 'plug compatibility' with older systems where the speaker had an electro-magnet field coil.
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Old 23rd May 2014, 08:32 PM   #4
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A: It most certainly had negative effects for the valves, but when these were in service you could buy tubes at the drug store for next to nothing.....

B: probably right. This was the kind of thing that a bar or restaurant bought and had for twenty years. The ability to easily service these machines would be important for the manufacturer.
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