Thursday, February 9, 2012

Overhauling a National Dobro Mystery Amp-Mystery Solved.

UPDATE: The clip from the 1942 National Dobro catalog identifies this amp as a Concert Amplifier No. 400.

This amp arrived with a seriously cooked power transformer spewing goop, and mesne other complaints. A little research revealed.....nothing.

A lot more research revealed.....more nothing.

As yet I do not know what the model number of this amp is, officially, so I'm calling it a model 100-B. It seems to be a modified National Dobro model 100, for which the schematic can be found in the John Rider Public Address Manual.

The 100 schematic bears a date of 8-28-39, and I have this mental picture of some poor schlub at the National Dobro design office having finished his master work three days before the most destructive war in history broke out, but nevermind. 1939 must have been a helluva year.

There are significant component value differences, some of which are related to the use of 6SC7s for the second preamp and phase inverter functions. I have documented these from actual observation.

The original Stancor P6013 power transformer had breathed its last, expiring in a puddle of smelly wax. Looking around my laboratory I latched on to a Midwest Coil and Transformer unit from a Hammond organ, the part number being A-024110-1. The voltages were just right, but unlike the Stancor, the 5v winding was not center tapped. National Dobro chose to use the center tap for B+, no doubt in an effort to reduce hum in the Jensen G12RS field coil speaker. I pulled my B+ off one leg as is more customary and nobody's the wiser.

In addition, National Dobro chose to ground the high voltage center tap through a .05/600 capacitor, which I thought was surplusage and so it got left out.

The output transformer, as yet unidentified bears a part number of C-3569 but no other marks.

I also took the opportunity to map the voltages present in this amp which are used in my hand drawn schematic. I have not yet figured out how to use Express SCH, which is a freeware schematic drawing program.

How's it sound? The fact that the amp has two different preamp schemes is a gift. The instrument input is sweet and clean, whereas the grid biased mic input is louder and grittier. The speaker's in good condition, which has to help.

I'm going to hate to give this back to the owner. Maybe he'll trade for something....hmmmmmmmmmm.

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