Amplifier Head Switcher

[boedonaldson]

Ok heres my situation. I have a two Heads (Marshall JCM800 50watt and Mesa Boogie Lonestar), but only one cabinet (Marshall JCM800 series 4x12). What I want to do is to be able to footswitch the two heads into the cabinets. This of course means having within the same unit that routes the signal into the appropriate head also. What my concern is though is when say one head is switched off the cabinet how will I match the impedance. I thought within the box i could just have a 16ohm resistor for each of the heads when they are switched off to substitute for the cabinet loading. Would this work, be stuff my amps completely or is the a better DIY solution to this.

Heres the idea with a 3PDT switch           Amp Input 1            Amp Output 1                 Blank
                                                                   ----                         ----                          ----

                                                             Guitar Input            Cabinet Input                  Blank
                                                                   ----                         ----                          ----

                                                            Amp Input 2            Amp Output 2                 Blank
                                                                   ----                         ----                          ----

Could someone help me with an idea on where to put the resisitor to substitute for the Cabinet loading (16ohm in my case).
Any ideas would be greatley appreciated, Cheers.
Boe

[boedonaldson]

Just same extra added info. Im running the JCM through a Hotplate. Dunno if this affects anything. I thought id just add the extra info incase.

[Pushtone]

A load resistor is the easy part.
The hard part is timing the switching so the amps aren't damaged.

Check out this pic of the insides of a Tonebone Headbone.
That big white thing looks like a 10 watt ceramic resistor.
I bet'cha a dozen donuts thats the load resistor for the standby amp.






I've looked into doing a head switcher. In the end I though it was unrealistic as a DIY project.

The two commercial units, the Headbone and the Switchblade,
use a PICmicro processor to tightly time and control the switching of

1. AMP-1 guitar input
2. Amp-1 speaker output
3. AMP-2 guitar input
4. AMP-2 speaker output



To build one yourself,
You will need an electronic switch solution whereby you can control the sequence and timing of the switching.
A micro processor in other words.

Then you will need a strategy for making the physical connections.
I believe in the above mentioned products, opto-couplers are used to switch the instrument level signals while micro relays are used for the speaker level switching.

Finally, add in that load resistor thing and your done!

If your up for all that, then it should be a breeze to whip one up.
Oh by the way, it can in no way sound different from the amp connected directly to the cabinet.

You would however get the bragging rights for being the first to DIY-tackle this one.

Another draw back that you may be aware of is the fact that you can not use a delay unit in either amps effects loop.
A long regeneration (feedback) delay would continue to drive the amps power section after you have switched to the other head.
That would burn up that 10 watt idle load resistor. This killed it for me as I must have the delay in the amps efx loop.

For me, the solution was to re-wire my 4x12 cab so each amp drives two speakers. Works for two head and less complicated.

[boedonaldson]

Lol, bragging rights would be nice as Im pretty much a noob. That system is probably a little out of my reach though. Do the head bone and swithcblade route the signal into a single cabinet? Like I mean of course it would be easier to just have a signal router with a dedicated cab for each head but that is a pain the the backside to lug around and find room for at gigs. Its just that these heads are just so great and although the Lonestar being so diverse in its sounds its still cant get the JCM800 roar. I actually didnt think of that small signal delay when switching that would momentatrily unload the amp and in the end after lots of use probably destroy it. Thanks for pointing that out. Though it would most prefereably be a single switching unit so it souldnt produce much of a delay. Delay pedals wouldnt be a problem though as i dont run them in my effects loop.

[Pushtone]

Well I haven't tested it but I would think a 3PDT switch ain't gonna do it.
I don't know if there is a way to implement the idle load resistor with this switch.

I think tube amps could be damaged by this switching.
Transistor amps could be damaged too depending on how they are designed.
I think there would be some crosstalk and popping would be a issue as it's likely to be quite loud.

Read all you can about electronic switching at www.geofex.com, you'll need it for this project.

[d95err]

You really should put it in a separate box near the amp and use a remote footswitch. Running the speaker signals to your pedalboard and back again is not such a good idea. Running the inputs that close to the amp outputs could cause all sorts of problems including noice, hum and oscillation.

You need to mute the input of the unused amp. This means the unused amp will produce little or no output, so very little power to handle for the load resistor.  You need to use relays for the speaker output signal and keep it a bit away from the input. It's probably easies to use relays for the input switching and muting as well, but JFETs or any other electronic switch would do. Keep the input signals very well shielded.

If you do this project, make sure you get it right. If something goes wrong, you could end up causing expensive damage to your amps and/or a fire...

[bancika]

Check out this pic of the insides of a Tonebone Headbone.
That big white thing looks like a 10 watt ceramic resistor.
I bet'cha a dozen donuts thats the load resistor for the standby amp.

hmmm, but that would limit amp power to 10W, anything above would fry that resistor.

[d95err]

Check out this pic of the insides of a Tonebone Headbone.
That big white thing looks like a 10 watt ceramic resistor.
I bet'cha a dozen donuts thats the load resistor for the standby amp.

hmmm, but that would limit amp power to 10W, anything above would fry that resistor.

Nope (see my post above). The trick is to mute the input of the unused amp. That limits the output power to the amplified background hum and noice generated inside the amp. Unless the amp is extremely noisy, 10W can probably handle most amps.

[bancika]

Cool

[d95err]

I think I've seen a schematic for a head switcher floating around the web somewhere. Possibly by Aiken (www.aikenamps.com). There is also a few pages on switching amp outputs in Kevin O'Connor's "The Ultimate Tone".

[DougH]

http://web.archive.org/web/20001003171545/www.aikenamps.com/TI_diy.htm