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Old 26th December 2004, 03:56 AM   #1
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Question Tube Nube - need advice on Scott LK150

I was given a H.H. Scott LK-150 "Stereo Master" amplifier. When it was working it sounded awsome (but I could heat my house with it). The Power transformer smoked and needs to be replaced. I have no idea what transformer to buy or where to get one. I have the schematic for the amp and parts list but there is no mention of the secondary voltage or current in the schematic.
Can anyone help me with the power supply?
Can someone explain what voltages should be on the un-labled transformer outputs? I do not see voltage marked for the 5AR4 heaters, I assume it should be 5V 1.9A. It looks like both plates are tied to 405V and the Filament and cathode are tied together to the heater output of the transformer. This leaves a unknown center tap voltage coming from the heater section of the tranny. Can someone help me with the unknown voltages on this beast?
The Scott part number for the tranny is TR-30-4-2 if that is any help. I have never been able to cross reference the tranny part number to anything.
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Old 28th December 2004, 04:15 PM   #2
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You need 405-0-405V at 300 mA or more, tapped for bias at 36V or more, two 6.3 V windings at 3.5A or more, and 5V @ 4A.

The Hammond 378CX is close - a bit short of filament capacity though, and no 5V winding. You could use solid state rectifiers, and filament overload would be OK, since overall rating is higher (465 mA vs. 300 mA required). If the Hammond's 4 3/8 x 5 1/2" core size is the same, the end bells could be used from the old transformer to keep original mounting.

You may be able to get it rewound or get a custom made replacement - I've heard the name Heyboer transformer in this regard. Heyboer
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Old 30th December 2004, 11:52 AM   #3
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I think the original voltage for the 5AR4 heaters would have been 10V since they are in series. Not that it matters now that the orginial transformer has smoked...

The "unknown" heater center tap voltage is the unfiltered B+
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Old 30th December 2004, 03:00 PM   #4
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Oops! make that 10V @ 2A, not 5 @ 4A, I should have looked closer.
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Old 31st December 2004, 07:28 PM   #5
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Default Secondary current requirments

Quote:
Originally posted by Tom Bavis
You need 405-0-405V at 300 mA or more, tapped for bias at 36V or more, two 6.3 V windings at 3.5A or more, and 5V @ 4A.

The Hammond 378CX is close - a bit short of filament capacity though, and no 5V winding. You could use solid state rectifiers, and filament overload would be OK, since overall rating is higher (465 mA vs. 300 mA required). If the Hammond's 4 3/8 x 5 1/2" core size is the same, the end bells could be used from the old transformer to keep original mounting.

You may be able to get it rewound or get a custom made replacement - I've heard the name Heyboer transformer in this regard. Heyboer

Since there are 4, 6550 tubes in the output wouldn't the current requirment be 600Ma? And the heater current be 10V @ 4A or more?
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Old 31st December 2004, 09:49 PM   #6
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150 mA per pair of 6550s is the data sheet value - it will increase momentarily on music peaks, but you don't have to consider that for transformer rating. Two 5AR4s in series will be 10V @ 1.9 A.
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