Rockola PSU and amplifier. - UK Vintage Radio Repair and Restoration Discussion Forum

Biggles · 23rd Aug 2018, 6:11 pm

Just a quick question for the juke box experts out there. Do juke boxes with valve amps switch off the HT or even LT to the amp when they are idle? A PSU I have appears to have a relay to disconnect the HT to the amplifier. I would have thought that this could be a bad idea due to the possibility of cathode poisoning, but the alternative is having the amplifier running for long periods of time when idle, wearing out the valves anyway. Did they run with maybe reduced HT when idle? I'm not very well up on juke boxes and just have the PSU and the remains of an amplifier.
Alan.

KeithsTV · 23rd Aug 2018, 6:50 pm

Based on my experience of Seeburg and Wurlitzer valve amplfiers

Seeburg amplifiers run with the heaters powered whenever the jukebox is on and idle, applying the HT via a directly heated HT rectifier, typically a 5U4, whenever a selection is made.

Wurlitzer amplifiers run the heaters at a lower voltage when idling, applying full heater voltage and HT via a directly heated rectifier when a selection is made.

I suspect cathode poisoning would not be a problem when installed in a busy pub or café.

Jukeboxes are designed for ease of service. If an amplifier has failed it would be a few minutes work to unplug the failed amplifier and fit a replacement. After all their primary function is to make money for the operator and if it's not working it's not earning its keep.

Keith

Lucien Nunes · 23rd Aug 2018, 7:08 pm

There is another method used e.g. on the earlier Wurlis, where the whole amp is switched off when idle but the heaters are overrun at first (by about a factor of two) to warm them up quickly. A relay switches them to normal voltage once the output stage emission comes up.

The important factor is to have the amplifier completely ready to play at full output the moment the mute contact releases, which could be a matter of a few seconds after selection depending on the position of the record rack. Jukeboxes in commercial use worked hard and received regular maintenance. Many parts, including all the records, were expendable in a way that we don't think of in preservation. Valves are now seen as semi-permanent but someone probably worked out the relative merits of leaving the amp full on and cutting the HT and found the latter preferable given the relatively short life the they had to provide at that time.

We may also be hyper-critical of the possibility of cathode poisoning. An example is the Miller individual-oscillator organ, in which all oscillators (174 ECC82s) run with their heaters on but no HT until keyed. Some less-popular notes probably play for a few seconds a year, maybe only during tuning, out of many hundreds of heater-hours. Admittedly these do not require the heavy emission of an audio output valve but most have survived with satisfactory performance after 50 years of this treatment.

Biggles · 23rd Aug 2018, 7:17 pm

Thanks for the input. As I say, I am a jukebox novice, my main experience has been feeding them with coins in pubs when I was in my teens. I have no real interest in them but since finding the PSU and looking at the layout I realised there must be some switching going on.
Alan.

23rd Aug 2018, 6:11 pm	#1
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Rockola PSU and amplifier. Just a quick question for the juke box experts out there. Do juke boxes with valve amps switch off the HT or even LT to the amp when they are idle? A PSU I have appears to have a relay to disconnect the HT to the amplifier. I would have thought that this could be a bad idea due to the possibility of cathode poisoning, but the alternative is having the amplifier running for long periods of time when idle, wearing out the valves anyway. Did they run with maybe reduced HT when idle? I'm not very well up on juke boxes and just have the PSU and the remains of an amplifier. Alan.

23rd Aug 2018, 6:50 pm	#2
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,573	Re: Rockola PSU and amplifier. Based on my experience of Seeburg and Wurlitzer valve amplfiers Seeburg amplifiers run with the heaters powered whenever the jukebox is on and idle, applying the HT via a directly heated HT rectifier, typically a 5U4, whenever a selection is made. Wurlitzer amplifiers run the heaters at a lower voltage when idling, applying full heater voltage and HT via a directly heated rectifier when a selection is made. I suspect cathode poisoning would not be a problem when installed in a busy pub or café. Jukeboxes are designed for ease of service. If an amplifier has failed it would be a few minutes work to unplug the failed amplifier and fit a replacement. After all their primary function is to make money for the operator and if it's not working it's not earning its keep. Keith

23rd Aug 2018, 7:08 pm	#3
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: Rockola PSU and amplifier. There is another method used e.g. on the earlier Wurlis, where the whole amp is switched off when idle but the heaters are overrun at first (by about a factor of two) to warm them up quickly. A relay switches them to normal voltage once the output stage emission comes up. The important factor is to have the amplifier completely ready to play at full output the moment the mute contact releases, which could be a matter of a few seconds after selection depending on the position of the record rack. Jukeboxes in commercial use worked hard and received regular maintenance. Many parts, including all the records, were expendable in a way that we don't think of in preservation. Valves are now seen as semi-permanent but someone probably worked out the relative merits of leaving the amp full on and cutting the HT and found the latter preferable given the relatively short life the they had to provide at that time. We may also be hyper-critical of the possibility of cathode poisoning. An example is the Miller individual-oscillator organ, in which all oscillators (174 ECC82s) run with their heaters on but no HT until keyed. Some less-popular notes probably play for a few seconds a year, maybe only during tuning, out of many hundreds of heater-hours. Admittedly these do not require the heavy emission of an audio output valve but most have survived with satisfactory performance after 50 years of this treatment. Last edited by Lucien Nunes; 23rd Aug 2018 at 7:14 pm.

23rd Aug 2018, 7:17 pm	#4
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Rockola PSU and amplifier. Thanks for the input. As I say, I am a jukebox novice, my main experience has been feeding them with coins in pubs when I was in my teens. I have no real interest in them but since finding the PSU and looking at the layout I realised there must be some switching going on. Alan.