thorpej
Super Member
I know this isn't hi-fi, but it is tube... hope I'm not breaking any forum rules...
This is a project that I've been meaning to finish up for quite some time... I built it last spring (2015) and have been using it practically every day since then, but I finally made the final tweak to it, so I thought I'd share it with the group.
This is a clone of the venerable Magnatone Melodier 109 from the mid-50s. Those familiar with it will know that it's famous (or infamous) for it's self-split push-pull output stage, a trick borrowed from some 1930s Philco radios, as I recall. When I heard about this circuit, I was intrigued and decided I should build one.
The first half of the circuit is fairly typical for low-to-moderate power guitar amps of the era... a single 12AX7 provides the initial gain, followed by a simple tone control and a volume control and the second gain stage. From this, a Champ-style amp would simply drive a single 6V6. In the Melodier 109, it drives a push-pull 6V6 output stage, like a Fender Deluxe, but with the economy of a self-split output.
The output stage works by placing a load resistor on the upper 6V6's screen. The screen essentially produces the same (though lower amplitude) inverted signal as the plate, which is then fed into the grid of the lower 6V6. Clever!
Anyway, these amps sound pretty great. Because of the inherent imbalances of the self-split topology, they produce a lot of harmonic content (the upper screen sags more than the lower, and the upper also clips earlier than the lower), and there's plenty enough gain in the front-end to overdrive the output stage if desired. The tone is perfect for blues and punk-a-billy.
The topology has a significant drawback, however... it is VERY sensitive to plate supply ripple. Normally, plate ripple noise is going to be cancelled out the the OT of a push-pull output stage. However, in the 109's case, any ripple on the plate is also going to modulate that screen, which is then going to be amplified on the other output tube.
I initially built mine true to the original schematic, but had to make a few guesses... the original schematic did not specify the B+ voltage nor the OT primary impedance... so I guessed. I WAG'd and used a 300-0-300 B+ winding and a 7.6K primary OT. The original circuit has very little filtering of the power supply (20uF x 3), and while the screen and front-end supply ended up pancake-flat, the plate supply had quite a bit of ripple, which manifested as a 120Hz hum. I added another RC filter for the plate supply as a stop-gap, and ordered a 15H choke, which sat in its box until a few nights ago, when I finally made the RC plate supply filter a LC plate supply filter. It made a big difference, and so I decided to call it finished.
Attached is the schematic as built, and some photos of the amp. I used 2 8 inch speakers like the original Melodier 109. It has more bass response than you might expect, due to the cabinet I think. I decided to make this one more like a furniture piece and use nice poplar and birch w/ stain, rather than covering it in tolex. If I had to do it over again, I'd probably forego the porthole style speaker openings for a large grille, but I can live with it as-is... everyone who sees it in person seems to love it, and it's a favorite at jam sessions.
This is a project that I've been meaning to finish up for quite some time... I built it last spring (2015) and have been using it practically every day since then, but I finally made the final tweak to it, so I thought I'd share it with the group.
This is a clone of the venerable Magnatone Melodier 109 from the mid-50s. Those familiar with it will know that it's famous (or infamous) for it's self-split push-pull output stage, a trick borrowed from some 1930s Philco radios, as I recall. When I heard about this circuit, I was intrigued and decided I should build one.
The first half of the circuit is fairly typical for low-to-moderate power guitar amps of the era... a single 12AX7 provides the initial gain, followed by a simple tone control and a volume control and the second gain stage. From this, a Champ-style amp would simply drive a single 6V6. In the Melodier 109, it drives a push-pull 6V6 output stage, like a Fender Deluxe, but with the economy of a self-split output.
The output stage works by placing a load resistor on the upper 6V6's screen. The screen essentially produces the same (though lower amplitude) inverted signal as the plate, which is then fed into the grid of the lower 6V6. Clever!
Anyway, these amps sound pretty great. Because of the inherent imbalances of the self-split topology, they produce a lot of harmonic content (the upper screen sags more than the lower, and the upper also clips earlier than the lower), and there's plenty enough gain in the front-end to overdrive the output stage if desired. The tone is perfect for blues and punk-a-billy.
The topology has a significant drawback, however... it is VERY sensitive to plate supply ripple. Normally, plate ripple noise is going to be cancelled out the the OT of a push-pull output stage. However, in the 109's case, any ripple on the plate is also going to modulate that screen, which is then going to be amplified on the other output tube.
I initially built mine true to the original schematic, but had to make a few guesses... the original schematic did not specify the B+ voltage nor the OT primary impedance... so I guessed. I WAG'd and used a 300-0-300 B+ winding and a 7.6K primary OT. The original circuit has very little filtering of the power supply (20uF x 3), and while the screen and front-end supply ended up pancake-flat, the plate supply had quite a bit of ripple, which manifested as a 120Hz hum. I added another RC filter for the plate supply as a stop-gap, and ordered a 15H choke, which sat in its box until a few nights ago, when I finally made the RC plate supply filter a LC plate supply filter. It made a big difference, and so I decided to call it finished.
Attached is the schematic as built, and some photos of the amp. I used 2 8 inch speakers like the original Melodier 109. It has more bass response than you might expect, due to the cabinet I think. I decided to make this one more like a furniture piece and use nice poplar and birch w/ stain, rather than covering it in tolex. If I had to do it over again, I'd probably forego the porthole style speaker openings for a large grille, but I can live with it as-is... everyone who sees it in person seems to love it, and it's a favorite at jam sessions.
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MagnaClone 109 schematics.jpg37.9 KB · Views: 175
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MagnaClone 109 schematics.pdf38.7 KB · Views: 89
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