The M15 went through several changes during its production run of 1963 to 1967. The Reverb design changed several times. Hopefully, your M15 has a schematic inside, and hopefully, it is accurate to that particular version! If not, hopefully this page will lend some guidance.
For what had to be the most advanced reverb circuit of any guitar amp, in 1963, Magnatone installed a newly designed reverb in the M15 complete with it's own dedicated output section. Besides the stereo pair of 7189A's and output transformers, Magnatone installed a third 8" speaker and a third output transformer with its own pair of 6GW8 output tubes to take the signal from the Hammond tank and provide a dedicated reverb output sound!
The input to the reverb tank is a mix of signals from both stereo speaker outputs and the mixed signal coming from preamps sections. Recovery from the tank is handled by a 2N2614 transistor. Following a depth control, the signal is sent to the pair of 6GW8's, which make up the phase inverter and push/pull power section, and on to the dedicated transformer and 8" speaker.
The two primary 8" speakers where large Alnico 16ohm Oxfords and the reverb speaker was a smaller 3.2ohm 8" speakr.
This design might have been associated with the Dual Imperial label assigned to some M15s of this era.
Below is the schematic for a M15 with 2 8" speakers. The dedicated reverb section is not used.
It is very close to the early M15A schematic (see below) with the only significant difference, besides a few capactor and resistor values , is the 12" speakers being 8" speakers in this earlier M15. This M15 still has the "instrument/foot switch" control 2 position knob that was used on earlier M15's, but was abandoned with the M15A.
In this circuit, the reverb takes its input from the channel No.1 circuit, and sends the output to the channel No.2 output section. Both sides of a 12AU7 drive a Hammond stlye reverb tank with two consecutive 7025 gain stages providing recovery. Reverb control is a single depth knob.
Because this circuit is so close to the M15A circuit (discussed below), It most likely that this was produced between the 3x8" M15 and the M15A (I had previously listed this as the earliest M15 circuit, and I now believe that to be incorrect.
If you have a 2x8" M15, please contact me!.
The input of the reverb comes from the signal destined for the channel one stereo output section and feeds both sections of a 12AX7 which drives th reverb tank. Recovery is handled by two gain stages of a 12AX7. From there, the signal goes to a single reverb depth control and finally, the reverb singal is mixed with the signal destined for the channel two stereo output section.
I think there was only one more major reverb design change for the M15A. This came sometime after the initial M15A reverb design discussed above (which appeared in the spring of 1965), but before 1966. This change added two extra reverb controls to the control panel: tone and echo.
Rather than get the input from the pre-amp section of the circuit, the reverb tank's input comes from the speaker output of the channel one stereo output section. Tank recovery is handled by a 12AX7 and then proceeds through depth and tone controls and its ultimately fed to the power section of the channel two stereo output section.
The "Echo" control places a tremolo effect on the reverb signal via an LDR that is lit by a neon bulb oscillator.
If the musician is only using the Channel No.1 input, The stereo output will be instrument signal in Channel No.1 output speaker, and reverb signal and some instrument signal in the Channel No.2 speaker.
There were running production changes made to the complex reverb circuits. As design issues became known via reports of issues and warranty repairs, the engineering team did not delay corrective production line circuit changes. Once such variation occured on this "mid-1965" M15A. If you have an M15A built after Jan 1966. Take note of the following running production change that occured sometime in mid to late 1965:
(Thanks to Tom Fogarty for pointing out the change).
M15As were built into 1967, if not 1968. Mostlikely, this was not the last circuit change. If you have schematics (for any of these) or circuit changes for M15's, please contact us.