External MFX and GMD40 - MIDI programs

Hi, everyone. Quick dumb question:

I already own the FSM and use it to change programs on the head, almost as a MFX, saving them to the GMD40 128 MIDI presets.

Now I'm considering using the head with an external MFX processor, using MIDI to change channels on the head, and using the processor's FX pre and post FX Loop.

If I want to save presets this way, my guess is that the PC MIDI commands are stored on the MFX and not on the head. Or am I wrong, and I'll be saving OVER the amp's 128 MIDI presets, if I press "Store" when all FX are dialed in?

If I have it right in what you are thinking then I would agree with you, the H&K presets will not be overwritten.  If you are just changing channels via MIDI and everything else is down to your Fx Loops, then the MFX is only sending a channel change message to the amp and no info is being stored on the amp at any time.  (This is as always assuming the GM40D is the same as the GM36, remember I'm a Luddite! )

The GM36 channel change message is controller number 31.  Contrary to some belief, there are no specific values for each of the channels.  The range used in the H&K MIDI setup is usually 0-255, that is 256 discrete values which fits well with simple digital control circuitry (a byte).  (There are a couple of settings in the GM36 which share a range between them so have less range but that is not relevant here.)  For rotary settings like Treble, all values in the range will specify their own unique level in the amp so stepping through them appears continuous.  For settings like Channel, the range is split amongst the number of options.  So in our case for the channel, Clean will be any value between 0-63, Crunch is 64-127, Lead is 128-191 and Ultra is 192-255.  Whatever you choose to use in these ranges is fine and you can even use an expression pedal to control them if you want, though switches are more reliable of course.

For at least the GM36, you would need to set up the MFX to send out messages with these values in them to Controller Number 31.  (A full list of H&K's allocation of controller numbers should be in the GM40D manual. --- EDIT:  Yup they sure are, Section 8.3 Page 12.)  That way there is no change to the amp's presets at all, it only responds by changing channels.  I would assume it remembers the other settings associated with each channel as the GM36 does.  So when you change channels you can also trim the channel's levels and tone to suit what you want.  These settings will automatically be set up every time you then go to that channel.  Again that will not affect the saved presets in any way.

Thanks for your input on this, bordonbert. Sorry for the overall convolution of my my post, but I guessed it right after all!