Magnatone Guitars!

New for Fall 2013, a complete guide to Magnatone guitars and the stories behind them!

1938-1960 Steel Guitars

1956-1963 Bigsby/Barth Era

1964-1966 Starstream Era

 

Magnatone Custom M7

Years 1963½-1966
Series Custom Series
Channels one
Power 2x 7189A
Power Bias fixed
Preamp 7025
Phase Inverter 12AU7
Speaker 15" 16ohm Oxford 15L5N-1 Alnico
Speaker 15" 16ohm Jensen C15P
Speaker 3" Oaktron or McGregor
Rectifier solid state
Output 38 watts peak
Weight 46 lbs. / 42 lbs.
Height 23.25"
List Price $260 (1963) $270 (1966)

The M7 was a single channel combo amplifier designed for bass guitar. With only a single tone control and a volume knob, the M7 was one of the most basic amplifiers Estey included in the new Custom Series amplifier line.

Speaker

The M7 switched from an Oxford 14oz. Alnico speaker to a Jensen C15P ceramic speaker sometime between late 1964 and early 1965. The Oxford speaker was also used in the M9, and it was the speaker used in the RS3 extension cabinet. (The RS3 was available for any Custom amp that had an extension speaker jack, however when paired with the M13, the "Imperial M13" became the Dual Imperial M13).

Like many models in the Custom Series amp line, early production M7's were voiced with a 3" tweeter in addition to the 15" drvier. The tweeter handled high frequencies via a high-pass capacitor.

Circuit

The M7 circuit is very basic, and highlights the basic circuit building block for many of the other Custom Series models including the M9, M10, M14, and M15. All of these amplifiers used the same power supply, and the same woodward-schumacher 10KΩ:16Ω ultra-linear output transformer. Power tubes were fixed bias General-Electric 7189A's, and the phase inverter was a cathodyne 12AU7. To this basic power amp, the M7 adds a single input preamp channel.

Color Change

Sometime in the spring of 1965, the gold motif of the M7 was changed to silver (as were all Custom Series amps that remained in the catalog). The 3" tweeter speaker was also dropped by spring 1965. Internally, the M7 became known as the M7A.

Besides being designed for bass guitar, the M7 was versatile enough for other market segments, including accordion and guitar. Magnatone also made a small marketing effort to suggest the M7 is well suited for vocals as a public address (PA) system. The M7, despite being so basic in terms of amplifier controls, was one of the more popular Magnatone amps at the time. M7 production figures for any given Custom Series model might have only surpassed by the M10/M10A. Orders for this amp continued into late 1966.

This M7 was rescued from the dump. It was in good condition except that someone had installed carpet as grill cloth! Despite the fact all the electrolytics are original, the amp is incredibly quiet.

There are several tip offs to this being a very early Custom: the flat M7 badge, and the bare metal unfinished handle.

Running Changes

The Magnatone assembly lines were in close proximity to the warranty repair area (depending on staffing levels, they might have been the same people!). This meant that these amplifiers were continually improved. In modern times, people get frustrated that schematics in the amplifiers don't always match the actual circuit. So if your amp doesn't match the schematic that it came with, the amp's differences are probably engineering improvements.

In any case, like all Custom Amps, the M7 had a few running changes to the circuit. These might have have M7 vs M7A changes, I'm not sure. The two schematics we have available for download highlights these changes.

Presence Added

An .047uf cathode by-pass capacitor was added to the 12AU7 gain stage to adjust the brightness/darkness of the amp. Early ones didn't have it, later ones did (See the "C" notation on the image below). This served to brightness the amp up with presence.

Coupling Cap Changes

The coupling caps were all changed slightly. Earlier amps were darker and not as suited for guitar as the later version was. The final coupling caps on the original M7 design were .22uf caps, and later versions used .047uf caps.

Hot Rodding the M7 for Guitar

The M7 is prime for hot rodding. In original form, the M7 is a very clean amp. With a Gibson style humbucker at max volume, the amp is clean at full volume with a light pick attack, but when you dig in, it growls a bit. If you want more overdrive, there is a bit to be had.

Negative Feedback

The M7's original negative feedback resistor (NFB) is a 22KΩ. Removing this almost makes the amp too unstable, but increasing it to 47KΩ or even 100KΩ will increase the over all gain and open the amp up a bit. The downside is you lose the flat response that the original NFB circuit provided (especially useful for bass guitar).

Here's a way to mount extra controls and avoid drilling holes in the control panel.

Voltage Divider Network

Since the M7 is single channel amp instead of a two channel amp, Estey engineers compensated for what would have been a 50% load of the second channel. If you look at an M10 circuit, the two channels mix together with 1MΩ mix resistors. This attenuates the either channel's signal by 50%. For the M7, the engineers installed a voltage divider network between gain stages with the 680KΩ and 470KΩ resistors.

These two convienently added up to almost 1MΩ. I suspect that the original test mule had a 1MΩ pot in this location and after some tweaking, they settled on a 60%/40% split. Replacing these two resistors with a 1MΩ pot is an option.

I prefer to leave the 460KΩ grid leak resistor as-is, and reduce the value of the 680KΩ series resistor. On mine, I installed a three position switch that allows this resistor to be 680KΩ ("stock" 60% att.), 280KΩ (increased gain, 62% att.), and 0Ω (max gain, no att.).

"A" is the NFB resistor, "B" is the 680KΩ attenuation resistor, and "C" is cathode resistor for the 12AU7 gain stage. Later M7's put a .047uf capacitor in parallel with "C" to add presence.

Brighter Tone

That 680KΩ resistor discussed above can be strapped with a parallel capacitor to allow higher frequencies to bypass the attenuation of that voltage divider. A .001uf would be brighter than a .01uf cap in this location.

Other Bass Amplifiers

The M7 and the M12 were both bass combos offered in the new Custom Series as replacements for the 425 and 435 of 1962. The M12 was more powerful (80W vs 38W peak), added an another channel to the amplifier, and provided bass and treble controls for both input channels. While the M7 is closely related to the M10 and M15, the M12's circuit sibling was the M13.

 

This is a dealer brochure from sometime in 1966 showing what the dealer paid, and what was suggested for a retail price. The nickname, "The Crusher" was light-hearted and aimed at a youth market.

 

DA/MagnatoneAmps.com

 

 

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