The MagnaClone -- a self-split push-pull guitar amp

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.
 

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Wow! What a sick circuit! How cheap can you get, really? Must be good.

I have a couple varistor Magnatones but I'm always interested in offbeat ghetto circuits. This one shows a lot of promise.
 
Very cool! How is it at playing Psyco Billy? Sorry, I was in a mood for some Reverend H. H. Today.
 
Ooooh,I like that!! Beautiful workmanship and a great look.I'll bet that lop-sided output stage sounds great too:)
Is that a Silvertone guitar? I noticed it in your avatar,but keep forgetting to ask.
 
Ooooh,I like that!! Beautiful workmanship and a great look.I'll bet that lop-sided output stage sounds great too:)
Is that a Silvertone guitar? I noticed it in your avatar,but keep forgetting to ask.

Thanks! I had to pay careful attention to the finger joinery of this one because it was going to be visible :) Yes! That guitar is a 1969 Silvertone 1445, the Mosrite copy made by Teisco. Probably one of the nicest samples of Teisco-made guitars of the era, laminated neck and all. Some of the hottest single-coil pickups around, which has its drawbacks (holy-crap-noisy, mostly). Previous owner rewired middle pickup to be out of phase to provide some noise cancellation and a VERY interesting tone (especially when all 3 are engaged). My other guitar is an '81 Les Paul XR1 sunburst with the zebra 'buckers and these Teisco single-coils provide easily as much output ... crazy.
 
Man, look at how empty that thing is inside. Who designed it, the Magnatone accounting department?

Really nice job on the build.
 
Kind of interesting how guitar amps differ from hifi. Things that are specifically avoided on a hifi amp might be done on purpose to add a specific distortion to the sound. I remember reading about that phase inversion setup somewhere, and one of the weaknesses of it was that it wasn't quite 180 out of phase, which is part of why the harmonic content is considerably higher than with other designs.
 
I'd love to see a breakdown of that designs distortion components.Got a spectrum analyzer handy? Ha ha!

Big +1 on the finger-jointed corners.All of the custom amps I've built over the years are Baltic birch plywood with this type of construction. I originally used dovetails,but this didn't work out as well with the rounded over corners.

Those oddball Mosrite's and their clones are really special sounding guitars,and I have always loved them.The filthy canoe-paddle in the photo with the missing volume knob is my Aria 1803T,and it has the same problem as your guitar;the hottest,noisiest single coils I've ever had! This thing is easily twice as loud as any other guitar,and the amount of ''honk'' in the midrange is astonishing.
I had to retire it from testing duty on my repairs/restorations/rebuilds as it just wasn't representative of a ''normal'' guitar.Now I use an Epiphone Les Paul:(
Although I did try out a (cranked) Hayseed 30 repair with it a while ago.As Canadian Maritimer's say, ''Lord sufferin' Jesus boy!!''......

DSCF2321.JPG DSCF2320.JPG

Again,beautiful work and congratulations:beerchug:

Art
 
Kind of interesting how guitar amps differ from hifi. Things that are specifically avoided on a hifi amp might be done on purpose to add a specific distortion to the sound. I remember reading about that phase inversion setup somewhere, and one of the weaknesses of it was that it wasn't quite 180 out of phase, which is part of why the harmonic content is considerably higher than with other designs.

Totally true... like, for example, purposefully building an unbalanced long-tailed pair PI (or simply not caring if it's balanced or not). Amps targeted towards different applications will be build differently, though... I have a non-working (but OK transformers) Challenger HF8 sitting in my "eventually" box that I plan to turn into a clean amp for jazz guitar, with the intent of plugging a semi-hollow arch-top into it. It will keep many of it's hi-fi circuit features.
 
Very nice work! As a fellow musician, i truly appreciate the craftsmanship. I would love to hear that amp and guitar!
 
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