Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TVC15 on December 03, 2018, 03:32:10 pm
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I'm trying to identify a power transformer from 1940-42.
I need to replace it with a new/old stock power transformer or, find a matching replacement. It is for a 1942 National Dobro Concert No.400 amplifier. My
transformer is stamped WP 3137 on the bottom wax soaked paper. Does anyone know who, in the 1940's, marked their units with "WPxxxx"? I have looked at all of the online National and Valco schematics and 99% have WPxxxx listed for their transformers. (I have not yet found a schematic for my amp.) I've looked at old Stancor and Merrit catalogs, no transformers listed that correspond to my WP 3137.
If this transformer can not be identified, can I determine what it should be replaced with by the tubes in the amp? Or, is rewinding my only option?
Thanks
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http://judyboxamp.blogspot.com/2012/02/overhauling-national-dobro-mystery-amp.html
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Thanks for your reply.
I have seen that site. They say that amp had a Stancor transformer. Mine looks the same but is not identified as a Stancor transformer. And, that repair was done around 2012, don't know how well it held up. On the schematics, almost all of the vintage transformers from National/Valco are WP3xxx. The Judybox site had a P6xxx transformer. Judybox amp may have had a transformer replaced prior to being sent for repair. Hopefully more than 2 of these amps exist and I'll eventually get a confirmation on the correct transformer.
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Is your goal a vintage exact replacement, or looking for what PT will work?
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To spec a PT you need to produce voltage in terms of current draw. Presumably, preamp tubes draw nearly -0- current. So, checkout the tube charts for your power tubes @ idle & max signal: plate + screen draw. Vintage amps often woefully exceeded current draw specs for their iron, which may explain the PT failures for these amps.
If this is not clear, please post for more info.
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The tubes in the amp: 5U4GB - General Electric; two 6v6, two 6SC7, two 6J7 Radiotrons (all 6)
I have looked at spec sheets for two of the above but, don't know how to translate it to determine what PT to buy as a replacement.
I was going to have the existing transformer rewound but, very expensive. I'll hang on to it and rewind in a few years.
Thanks
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If you want to stay with the 5U4 then you need 5V @ 3A, 6.3V @ 3A, and 275V-0-275V @ 125mA or 150mA. If the transformer is a standup type this is about all the info you need to go shopping. But if the transformer is a laydown type (requiring a big rectangular cutout) then you also need the size of the cutout and the spacing between the mounting bolts. Some hi-rez pics of your amp would be very helpful.
I have a couple questions...
1. How do you know the PT is bad?
2. Is the speaker a field coil type? Many are from that era.
3. If it is a field coil speaker, is the coil connected as a series choke or is it connected as a shunt load?
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Thanks for the info.
The PT is mounted in a cut-out in the chassis, 4 holes for mounting bolts.
There are photos of an identical amp (except for the transformer number) at the website address in post #2.
1. How do you know the PT is bad? Had it checked at a transformer shop.
2. Is the speaker a field coil type? Many are from that era. Yes, speaker is field coil.
3. If it is a field coil speaker, is the coil connected as a series choke or is it connected as a shunt load? I'll check into that.
Thanks
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Some hi-rez pics of your amp would be very helpful.
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I got bored;
Here's a pic from JJ's link, and I re-drew the schematic from that, didn't put in values but you probably have a pencil :laugh:
If you find differences I can edit it pretty quick, including values, on to the PS :think1: