1963-1965 M15's came with either three 8" speakers, or two as pictured here.
The distinction between the two speaker configurations is discussed below in M15 Reverb.
|Power||4x 7189A (two pair for stereo)|
|Power Bias||fixed bias|
|Reverb||transistor, 2x 6GW8, 300:1.2ohm Transformer|
|Vibrato||3x12AU7, 12DW7, 8 varistors|
|Phase Inverter||2x 12AU7|
|Speaker||2x 8" Oxford 16ohm -or- 2x 5" 16ohm speakers|
|Reverb Speaker||8" 3.2ohm -or- 5" 3.2ohm|
|Output||2x 38 watts peak|
As you go through the 1963 Custom Series catalog from M2 to M15 each amp gets bigger and better. The M15 continues with that series motif and take the stereo M14 circuit and add reverb. Like the M14, the M15 is a two input channel true stereo amplifier with Stereo F.M.Vibrato.
At right is the control panel. From left to right: the two sets inputs with a stereo input jack between them. Each input has volume, treble, and bass controls, a contour switch and a pair of input jacks, followed by the four control vibrato section (see "Vibrato Control" below) and foot switch jack. Next is a single reverb control and foot switch jack, and last is the power switch and output speaker switch with left and right speaker output jacks.
The M15 and M14 are continuations of the magnificent
1961-1963 480. the only change to the circuit
came with Magnatone's move toward 7189A power
tubes and an ultralinear transformers. The M14, like the 480
and the M15, is a true stereo amplifier.
With twin 8" Oxford Alnico speakers, two output transformers,
four 7189As, and two phase inverter 12AU7A tubes, The M15 had stereo output channels (not just two input channels).
This amp can be confusing to talk about because there are two input channels and two output channels, but its not a single 1:1 relationship (like an Ampeg stereo amp, for example). Each input channel has its own pre-amp, and both of these signals are equally mixed together on both output sections. There is Stereo vibrato, but it only applies to the signal from Channel No.1 input. That signal is send to two different vibrato units, which each go to one of the two output sections.
The Stereo vibrato switch, in mono mode makes the two vibrato units modulate at the same frequency, and in "out of phase" frequencies in stereo mode. If vibrato was simply ON or OFF, the two vibratos, in stereo mode, might cancel each other out, however here in lies the magic. The Magnatone vibrato is the varistor. These magic varistors rise and fall at a much slow rate, so the rise and falls of the two output of phase vibratos never match up perfectly enough to cause complete cancellation.
The stereo vibrato was designed by Don L. Bonham. Magnatone used it under license and called it Stereo F.M.Vibrato.
The early M15 has a unique set of vibrato controls. Like all F.M. Vibrato equipped Magnatones, the M15 has speed and intensity controls. It also has a "Stereo Vibrato" two position rotary switch. There is a fourth two position switch that is not present on either the M14 or later M15's. This fourth switch is labeled "Vibrato Control" and allows the musician to turn vibrato ON/OFF with either a foot-switch or an ON/OFF switch on the instrument. If the instrument switch is selected, the musician must plug a stereo cable into the "high impedence" channel one jack. The "ring" on the instrument cable is switched between ground ("sleeve") to turn the effect on and off.
This neat feature was dropped on later M15's. It was probably a source of confusion for the musician because if the switch is in the "instrument" position and a regular mono instrument cable is used, the musician, unless he carefully read the manual, will not know why the foot switch doesn't work and why there is no vibrato effect when plugged into that one jack! Additionally, this was a non-standard instrument feature, probably only available on a few accordions.
You can find a schematic of this M15 in the 4.1 revision of the Aspen Pittman book (see schematics).
The Early M15's had a dedicated speaker and output section for reverb. As the original grill cloth fades, the 3rd 8" speaker hole begins to show through (compare it with the M15 picture at the top of the page).
See the M15 and M15A reverb comparison for a complete review of reverbs used on these amps.
Like many aspects of the M15, this reverb would be again redesigned in 1965 with M15's replacement, the M15A.
The name "Dual Imperial" was used on some M15's. While the M13 was the "Imperial", an M15 was more like a dual M10 than it was a dual M13. the M13 used an 8ohm 15" speaker with 6CA7 power tubes while the M10 was closer to half a M15 with a 16ohm 8" Oxford and a 3" tweeter with 7189 power tubes and an ultra-linear output transformer.