Magnatone 109

The first guitar amplifier I ever tweaked on was a Magnatone Melodier 109. I bought from a kid across the street along with a no-name (probably Silvertone or Teisco) electric guitar. It cost me the princely sum of $25.00 in 1973. I played it as is (was) for a number of years until I blew out the eight-inch speakers trying to use it as an electric bass practice amp. Then I built it into a cabinet with a fifteen-inch speaker. Later, when it developed a hum, I tore it apart to recap it. Sometime during that process, we moved, the amp bits got misplaced or trashed. Today, all that remains of this amp is its power transformer.

The amp has an unusual screen-driven self-split topology, as you can see in this schematic:

If you are thinking of replicating this amp, you'll need a power transformer. I measured the original Magnatone P50-4401 power transformer with 1.026 VAC on the primary leads and noted the voltages produced on the secondaries. The following table shows the measured voltages along with extrapolations to 115 and 120 VAC. I've also included estimated currents these secondaries will need to be able to deliver, given the above circuit.

Assuming a 2x uplift in current capability, A Hammond 272DX should be more than adequate for the power requirements of this amplifier.

Unfortunately, I no longer have the output transformer to measure. The folks at Vibroworld suggested their model #TF107, intended for the Fender Princeton, and old-style Deluxe. Likely, any transformer intended for push-pull 6V6s would be fine. Look for the following approximate 6500:8 or 6500:16 ohms.

23-December-2006

I acquired an output transformer from a Leslie Model 101 power amplifier, the transformer is labled: "LESLIE 102-12". The original Leslie circuit used push-pull 7189s with a 6800 ohm load to drive a 16-ohm speaker cabinet. This OT is approximately the same size and wattage rating as the original Magnatone OT and is best-guess close to the original impedance. It likely offers a slightly broader frequency response than the original, but that shouldn't be too much of an issue. In the absense of more accuate information, it's close enough for rock-n-roll.

26-December-2006

I just made a chassis from nothing! Well, not exactly nothing, but close to it. A couple years ago, I purchased a Hammond chassis that came with a cover. I didn't use the cover and it simply lay under the work bench. I kept thinking it should be good for something. It occurred to me that the original Magnatone 109 chassis was nothing more than a bent sheet of steel. Why not use that old chassis cover? It'd need more stiffening because it's aluminum, but that would be okay. So I jury-rigged up a box brake from an old 2x4 and bent it to shape. Then I snipped up some angle braces from aluminum sheet (also a freebie) and pop-riveted them into place. Once I sort out the layout details, I'll add some aluminum angle stock to strengthen the chassis around the power-tranny Here are some photos of the chassis. I think it turned out okay for a few hours of effort:

18-January-2007

Below is a drill-guide I have designed that shows layout of the major bits. The oil capacitors are the three 20uF filter caps in the supply. They aren't really stock, but I got them at a good price and I've wanted to try a build with no electrolytics. Now if I can just find an affordable oil-paper cap the cathode bypass. Something not so huge as to be unworkable inside the chassis....

The astute observer will note that I'm deviating from the original on the number of inputs. Here is the schematic I'm using for the input jack:

10-February-2007

I've completed most of the metal work on the chassis. I still need to size the holes for the controls and drill the holes for the tube socket mounting tabs. I want to settle on the orientation of the tube sockets before I commit to drilling those. Here are photos of drilled chassis and a mockup of the major parts placement: